Implemented most of the new observable algorithm.

core/src/commonMain/kotlin/world/phantasmal/core/Assertions.kt

@@ -5,3 +5,7 @@ inline fun assert(value: () -> Boolean) {
 }
 
 expect inline fun assert(value: () -> Boolean, lazyMessage: () -> Any)
+
+inline fun assertUnreachable(lazyMessage: () -> Any) {
+    assert({ true }, lazyMessage)
+}

core/src/commonMain/kotlin/world/phantasmal/core/disposable/DisposableTracking.kt

@@ -0,0 +1,107 @@
+package world.phantasmal.core.disposable
+
+private const val DISPOSABLE_PRINT_COUNT = 10
+
+/**
+ * A global count is kept of all undisposed, tracked disposables. This count is used to find memory
+ * leaks.
+ *
+ * Tracking is not thread-safe.
+ */
+object DisposableTracking {
+    var globalDisposableTracker: DisposableTracker? = null
+
+    inline fun checkNoLeaks(block: () -> Unit) {
+        // Remember the old tracker to make this function reentrant.
+        val initialTracker = globalDisposableTracker
+
+        try {
+            val tracker = DisposableTracker()
+            globalDisposableTracker = tracker
+
+            block()
+
+            check(globalDisposableTracker === tracker) {
+                "Tracker was changed."
+            }
+
+            tracker.checkLeakCountZero()
+        } finally {
+            globalDisposableTracker = initialTracker
+        }
+    }
+
+    fun track(disposable: Disposable) {
+        globalDisposableTracker?.track(disposable)
+    }
+
+    fun disposed(disposable: Disposable) {
+        globalDisposableTracker?.disposed(disposable)
+    }
+}
+
+class DisposableTracker {
+    /**
+     * Mapping of tracked disposables to their liveness state. True means live, false means
+     * disposed.
+     */
+    private var disposables: MutableMap<Disposable, Boolean> = mutableMapOf()
+
+    /** Keep count as optimization for check in [checkLeakCountZero]. */
+    private var liveCount = 0
+
+    fun track(disposable: Disposable) {
+        val live = disposables.put(disposable, true)
+
+        if (live == true) {
+            error("${getName(disposable)} was already tracked.")
+        } else if (live == false) {
+            disposables[disposable] = false
+            error("${getName(disposable)} was already tracked and then disposed.")
+        }
+
+        liveCount++
+    }
+
+    fun disposed(disposable: Disposable) {
+        val live = disposables.put(disposable, false)
+
+        if (live == null) {
+            disposables.remove(disposable)
+            error("${getName(disposable)} was never tracked.")
+        } else if (!live) {
+            error("${getName(disposable)} was already disposed.")
+        }
+
+        liveCount--
+    }
+
+    fun checkLeakCountZero() {
+        check(liveCount == 0) {
+            buildString {
+                append(liveCount)
+                append(" Disposables were leaked: ")
+
+                val leakedDisposables = disposables.entries
+                    .asSequence()
+                    .filter { (_, live) -> live }
+                    .take(DISPOSABLE_PRINT_COUNT)
+                    .map { (disposable, _) -> disposable }
+                    .toList()
+
+                check(liveCount >= leakedDisposables.size)
+
+                leakedDisposables.joinTo(this, transform = ::getName)
+
+                if (liveCount > DISPOSABLE_PRINT_COUNT) {
+                    append(",...")
+                } else {
+                    append(".")
+                }
+            }
+        }
+    }
+}
+
+private fun getName(disposable: Disposable): String =
+    disposable::class.simpleName ?: "(anonymous class)"

core/src/commonMain/kotlin/world/phantasmal/core/disposable/TrackedDisposable.kt

@@ -1,10 +1,8 @@
 package world.phantasmal.core.disposable
 
 /**
- * A global count is kept of all undisposed instances of this class.
- * This count can be used to find memory leaks.
- *
- * Tracking is not thread-safe.
+ * Subclasses of this class are automatically tracked. Subclasses are required to call
+ * super.[dispose].
  */
 abstract class TrackedDisposable : Disposable {
     var disposed = false
@@ -13,76 +11,15 @@ abstract class TrackedDisposable : Disposable {
     init {
         // Suppress this warning, because track simply adds this disposable to a set at this point.
         @Suppress("LeakingThis")
-        track(this)
+        DisposableTracking.track(this)
     }
 
     override fun dispose() {
-        if (!disposed) {
-            disposed = true
-            untrack(this)
-        }
-    }
-
-    companion object {
-        private const val DISPOSABLE_PRINT_COUNT = 10
-
-        var disposables: MutableSet<Disposable> = mutableSetOf()
-        var trackPrecise = false
-        var disposableCount: Int = 0
-            private set
-
-        inline fun checkNoLeaks(trackPrecise: Boolean = false, block: () -> Unit) {
-            val initialCount = disposableCount
-            val initialTrackPrecise = this.trackPrecise
-            val initialDisposables = disposables
-
-            this.trackPrecise = trackPrecise
-            disposables = mutableSetOf()
-
-            try {
-                block()
-                checkLeakCountZero(disposableCount - initialCount)
-            } finally {
-                this.trackPrecise = initialTrackPrecise
-                disposables = initialDisposables
-            }
+        require(!disposed) {
+            "${this::class.simpleName ?: "(Anonymous class)"} already disposed."
         }
 
-        fun track(disposable: Disposable) {
-            disposableCount++
-
-            if (trackPrecise) {
-                disposables.add(disposable)
-            }
-        }
-
-        fun untrack(disposable: Disposable) {
-            disposableCount--
-
-            if (trackPrecise) {
-                disposables.remove(disposable)
-            }
-        }
-
-        fun checkLeakCountZero(leakCount: Int) {
-            check(leakCount == 0) {
-                buildString {
-                    append("$leakCount TrackedDisposables were leaked")
-
-                    if (trackPrecise) {
-                        append(": ")
-                        disposables.take(DISPOSABLE_PRINT_COUNT).joinTo(this) {
-                            it::class.simpleName ?: "Anonymous"
-                        }
-
-                        if (disposables.size > DISPOSABLE_PRINT_COUNT) {
-                            append(",..")
-                        }
-                    }
-
-                    append(".")
-                }
-            }
-        }
+        disposed = true
+        DisposableTracking.disposed(this)
     }
 }

core/src/commonMain/kotlin/world/phantasmal/core/unsafe/UnsafeMap.kt

@@ -17,7 +17,8 @@ package world.phantasmal.core.unsafe
  *
  * 3. The keys used do not require equals or hashCode to be called in JS.
  *    E.g. Int, String, objects which you consider equal if and only if they are the exact same
- *    instance.
+ *    instance. Note that some objects that compile to primitives on JVM, such as Long, compile to
+ *    an object in JS and thus will not behave the way you expect.
  */
 expect class UnsafeMap<K, V>() {
     fun get(key: K): V?

core/src/commonMain/kotlin/world/phantasmal/core/unsafe/UnsafeSet.kt

@@ -17,7 +17,8 @@ package world.phantasmal.core.unsafe
  *
  * 3. The values used do not require equals or hashCode to be called in JS.
  *    E.g. Int, String, objects which you consider equal if and only if they are the exact same
- *    instance.
+ *    instance. Note that some objects that compile to primitives on JVM, such as Long, compile to
+ *    an object in JS and thus will not behave the way you expect.
  */
 expect class UnsafeSet<T> {
     constructor()

core/src/commonTest/kotlin/world/phantasmal/core/disposable/DisposerTests.kt

@@ -5,7 +5,7 @@ import kotlin.test.*
 class DisposerTests {
     @Test
     fun calling_add_or_addAll_increases_size_correctly() {
-        TrackedDisposable.checkNoLeaks {
+        DisposableTracking.checkNoLeaks {
             val disposer = Disposer()
             assertEquals(disposer.size, 0)
 
@@ -29,7 +29,7 @@ class DisposerTests {
 
     @Test
     fun disposes_all_its_disposables_when_disposed() {
-        TrackedDisposable.checkNoLeaks {
+        DisposableTracking.checkNoLeaks {
             val disposer = Disposer()
             var disposablesDisposed = 0
 
@@ -57,7 +57,7 @@ class DisposerTests {
 
     @Test
     fun disposeAll_disposes_all_disposables() {
-        TrackedDisposable.checkNoLeaks {
+        DisposableTracking.checkNoLeaks {
             val disposer = Disposer()
 
             var disposablesDisposed = 0
@@ -80,7 +80,7 @@ class DisposerTests {
 
     @Test
     fun size_and_is_empty_should_correctly_reflect_the_contained_disposables() {
-        TrackedDisposable.checkNoLeaks {
+        DisposableTracking.checkNoLeaks {
             val disposer = Disposer()
 
             assertEquals(disposer.size, 0)
@@ -102,7 +102,7 @@ class DisposerTests {
 
     @Test
     fun adding_disposables_after_being_disposed_throws() {
-        TrackedDisposable.checkNoLeaks {
+        DisposableTracking.checkNoLeaks {
             val disposer = Disposer()
             disposer.dispose()
 

core/src/commonTest/kotlin/world/phantasmal/core/disposable/TrackedDisposableTests.kt

@@ -1,35 +1,34 @@
 package world.phantasmal.core.disposable
 
 import kotlin.test.Test
-import kotlin.test.assertEquals
+import kotlin.test.assertFails
 import kotlin.test.assertFalse
 import kotlin.test.assertTrue
 
 class TrackedDisposableTests {
     @Test
-    fun count_goes_up_when_created_and_down_when_disposed() {
-        val initialCount = TrackedDisposable.disposableCount
+    fun is_correctly_tracked() {
+        assertFails {
+            checkNoDisposableLeaks {
+                object : TrackedDisposable() {}
+            }
+        }
 
-        val disposable = object : TrackedDisposable() {}
-
-        assertEquals(initialCount + 1, TrackedDisposable.disposableCount)
-
-        disposable.dispose()
-
-        assertEquals(initialCount, TrackedDisposable.disposableCount)
+        checkNoDisposableLeaks {
+            val disposable = object : TrackedDisposable() {}
+            disposable.dispose()
+        }
     }
 
     @Test
-    fun double_dispose_does_not_increase_count() {
-        val initialCount = TrackedDisposable.disposableCount
-
+    fun double_dispose_throws() {
         val disposable = object : TrackedDisposable() {}
 
-        for (i in 1..5) {
+        disposable.dispose()
+
+        assertFails {
             disposable.dispose()
         }
-
-        assertEquals(initialCount, TrackedDisposable.disposableCount)
     }
 
     @Test

core/src/jsMain/kotlin/world/phantasmal/core/Assertions.kt

@@ -1,5 +1,13 @@
 package world.phantasmal.core
 
+import kotlin.contracts.InvocationKind.AT_MOST_ONCE
+import kotlin.contracts.contract
+
 actual inline fun assert(value: () -> Boolean, lazyMessage: () -> Any) {
+    contract {
+        callsInPlace(value, AT_MOST_ONCE)
+        callsInPlace(lazyMessage, AT_MOST_ONCE)
+    }
+
     // TODO: Figure out a sensible way to do dev assertions in JS.
 }

core/src/jvmMain/kotlin/world/phantasmal/core/Assertions.kt

@@ -2,9 +2,17 @@
 
 package world.phantasmal.core
 
+import kotlin.contracts.InvocationKind.AT_MOST_ONCE
+import kotlin.contracts.contract
+
 val ASSERTIONS_ENABLED: Boolean = {}.javaClass.desiredAssertionStatus()
 
 actual inline fun assert(value: () -> Boolean, lazyMessage: () -> Any) {
+    contract {
+        callsInPlace(value, AT_MOST_ONCE)
+        callsInPlace(lazyMessage, AT_MOST_ONCE)
+    }
+
     if (ASSERTIONS_ENABLED && !value()) {
         throw AssertionError(lazyMessage())
     }

observable/src/commonMain/kotlin/world/phantasmal/observable/AbstractDependency.kt

@@ -1,6 +1,9 @@
 package world.phantasmal.observable
 
-abstract class AbstractDependency : Dependency {
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
+abstract class AbstractDependency<T> : Dependency<T> {
     protected val dependents: MutableList<Dependent> = mutableListOf()
 
     override fun addDependent(dependent: Dependent) {
@@ -10,4 +13,21 @@ abstract class AbstractDependency : Dependency {
     override fun removeDependent(dependent: Dependent) {
         dependents.remove(dependent)
     }
+
+    protected fun emitDependencyInvalidated() {
+        for (dependent in dependents) {
+            dependent.dependencyInvalidated(this)
+        }
+    }
+
+    protected inline fun applyChange(block: () -> Unit) {
+        contract {
+            callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+        }
+
+        ChangeManager.changeDependency {
+            emitDependencyInvalidated()
+            block()
+        }
+    }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/CallbackChangeObserver.kt

@@ -7,11 +7,12 @@ import world.phantasmal.core.unsafe.unsafeCast
  * Calls [callback] when [dependency] changes.
  */
 class CallbackChangeObserver<T, E : ChangeEvent<T>>(
-    private val dependency: Dependency,
-    // We don't use Observer<T> because of type system limitations. It would break e.g.
+    private val dependency: Observable<T>,
+    // We don't use ChangeObserver<T> because of type system limitations. It would break e.g.
     // AbstractListCell.observeListChange.
     private val callback: (E) -> Unit,
-) : TrackedDisposable(), Dependent {
+) : TrackedDisposable(), Dependent, LeafDependent {
+
     init {
         dependency.addDependent(this)
     }
@@ -21,13 +22,12 @@ class CallbackChangeObserver<T, E : ChangeEvent<T>>(
         super.dispose()
     }
 
-    override fun dependencyMightChange() {
-        // Do nothing.
+    override fun dependencyInvalidated(dependency: Dependency<*>) {
+        ChangeManager.invalidated(this)
     }
 
-    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-        if (event != null) {
-            callback(unsafeCast(event))
-        }
+    override fun pull() {
+        // See comment above callback property to understand why this is safe.
+        dependency.changeEvent?.let(unsafeCast<(ChangeEvent<T>) -> Unit>(callback))
     }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/CallbackObserver.kt

@@ -3,14 +3,12 @@ package world.phantasmal.observable
 import world.phantasmal.core.disposable.TrackedDisposable
 
 /**
- * Calls [callback] when one or more dependency in [dependencies] changes.
+ * Calls [callback] when one or more observable in [dependencies] changes.
  */
 class CallbackObserver(
-    private vararg val dependencies: Dependency,
+    private vararg val dependencies: Observable<*>,
     private val callback: () -> Unit,
-) : TrackedDisposable(), Dependent {
-    private var changingDependencies = 0
-    private var dependenciesActuallyChanged = false
+) : TrackedDisposable(), Dependent, LeafDependent {
 
     init {
         for (dependency in dependencies) {
@@ -26,20 +24,21 @@ class CallbackObserver(
         super.dispose()
     }
 
-    override fun dependencyMightChange() {
-        changingDependencies++
+    override fun dependencyInvalidated(dependency: Dependency<*>) {
+        ChangeManager.invalidated(this)
     }
 
-    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-        if (event != null) {
-            dependenciesActuallyChanged = true
+    override fun pull() {
+        var changed = false
+
+        // We loop through all dependencies to ensure they're valid again.
+        for (dependency in dependencies) {
+            if (dependency.changeEvent != null) {
+                changed = true
+            }
         }
 
-        changingDependencies--
-
-        if (changingDependencies == 0 && dependenciesActuallyChanged) {
-            dependenciesActuallyChanged = false
-
+        if (changed) {
             callback()
         }
     }

observable/src/commonMain/kotlin/world/phantasmal/observable/ChangeManager.kt

@@ -1,57 +1,56 @@
 package world.phantasmal.observable
 
+import kotlin.contracts.InvocationKind.EXACTLY_ONCE
+import kotlin.contracts.contract
+
+// TODO: Throw exception by default when triggering early recomputation during change set. Allow to
+//       to turn this check off, because partial early recomputation might be useful in rare cases.
+//       Dependencies will need to partially apply ListChangeEvents etc. and remember which part of
+//       the event they've already applied (i.e. an index into the changes list).
+// TODO: Think about nested change sets. Initially don't allow nesting?
 object ChangeManager {
-    private var currentChangeSet: ChangeSet? = null
+    private val invalidatedLeaves = HashSet<LeafDependent>()
+
+    /** Whether a dependency's value is changing at the moment. */
+    private var dependencyChanging = false
 
     fun inChangeSet(block: () -> Unit) {
-        // TODO: Figure out change set bug and enable change sets again.
-//        val existingChangeSet = currentChangeSet
-//        val changeSet = existingChangeSet ?: ChangeSet().also {
-//            currentChangeSet = it
-//        }
-//
-//        try {
-            block()
-//        } finally {
-//            if (existingChangeSet == null) {
-//                // Set to null so changed calls are turned into emitDependencyChanged calls
-//                // immediately instead of being deferred.
-//                currentChangeSet = null
-//                changeSet.complete()
-//            }
-//        }
+        // TODO: Implement inChangeSet correctly.
+        block()
     }
 
-    fun changed(dependency: Dependency) {
-        val changeSet = currentChangeSet
+    fun invalidated(dependent: LeafDependent) {
+        invalidatedLeaves.add(dependent)
+    }
 
-        if (changeSet == null) {
-            dependency.emitDependencyChanged()
-        } else {
-            changeSet.changed(dependency)
+    inline fun changeDependency(block: () -> Unit) {
+        contract {
+            callsInPlace(block, EXACTLY_ONCE)
+        }
+
+        dependencyStartedChanging()
+
+        try {
+            block()
+        } finally {
+            dependencyFinishedChanging()
         }
     }
-}
 
-private class ChangeSet {
-    private var completing = false
-    private val changedDependencies: MutableList<Dependency> = mutableListOf()
+    fun dependencyStartedChanging() {
+        check(!dependencyChanging) { "An observable is already changing." }
 
-    fun changed(dependency: Dependency) {
-        check(!completing)
-
-        changedDependencies.add(dependency)
+        dependencyChanging = true
     }
 
-    fun complete() {
+    fun dependencyFinishedChanging() {
         try {
-            completing = true
-
-            for (dependency in changedDependencies) {
-                dependency.emitDependencyChanged()
+            for (dependent in invalidatedLeaves) {
+                dependent.pull()
             }
         } finally {
-            completing = false
+            dependencyChanging = false
+            invalidatedLeaves.clear()
         }
     }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/Dependency.kt

@@ -1,6 +1,9 @@
 package world.phantasmal.observable
 
-interface Dependency {
+interface Dependency<out T> {
+    // TODO: Docs.
+    val changeEvent: ChangeEvent<T>?
+
     /**
      * This method is not meant to be called from typical application code. Usually you'll want to
      * use [Observable.observeChange].
@@ -11,9 +14,4 @@ interface Dependency {
      * This method is not meant to be called from typical application code.
      */
     fun removeDependent(dependent: Dependent)
-
-    /**
-     * This method is not meant to be called from typical application code.
-     */
-    fun emitDependencyChanged()
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/Dependent.kt

@@ -2,6 +2,7 @@ package world.phantasmal.observable
 
 interface Dependent {
     /**
+     * TODO: Fix documentation.
      * This method is not meant to be called from typical application code.
      *
      * Called whenever a dependency of this dependent might change. Sometimes a dependency doesn't
@@ -9,16 +10,14 @@ interface Dependent {
      * after calling this method.
      *
      * E.g. C depends on B and B depends on A. A is about to change, so it calls
-     * [dependencyMightChange] on B. At this point B doesn't know whether it will actually change
+     * [dependencyInvalidated] on B. At this point B doesn't know whether it will actually change
      * since the new value of A doesn't necessarily result in a new value for B (e.g. B = A % 2 and
-     * A changes from 0 to 2). So B then calls [dependencyMightChange] on C.
+     * A changes from 0 to 2). So B then calls [dependencyInvalidated] on C.
      */
-    fun dependencyMightChange()
-
-    /**
-     * This method is not meant to be called from typical application code.
-     *
-     * Always call [dependencyMightChange] before calling this method.
-     */
-    fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?)
+    fun dependencyInvalidated(dependency: Dependency<*>)
+}
+
+interface LeafDependent {
+    // TODO: Sensible name for `pull`.
+    fun pull()
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/Observable.kt

@@ -2,7 +2,7 @@ package world.phantasmal.observable
 
 import world.phantasmal.core.disposable.Disposable
 
-interface Observable<out T> : Dependency {
+interface Observable<out T> : Dependency<T> {
     /**
      * [observer] will be called whenever this observable changes.
      */

observable/src/commonMain/kotlin/world/phantasmal/observable/SimpleEmitter.kt

@@ -2,31 +2,18 @@ package world.phantasmal.observable
 
 import world.phantasmal.core.disposable.Disposable
 
-class SimpleEmitter<T> : AbstractDependency(), Emitter<T> {
-    private var event: ChangeEvent<T>? = null
+// TODO: Should multiple events be emitted somehow during a change set? At the moment no application
+//       code seems to care.
+class SimpleEmitter<T> : AbstractDependency<T>(), Emitter<T> {
+    override var changeEvent: ChangeEvent<T>? = null
+        private set
 
     override fun emit(event: ChangeEvent<T>) {
-        for (dependent in dependents) {
-            dependent.dependencyMightChange()
+        applyChange {
+            this.changeEvent = event
         }
-
-        this.event = event
-
-        ChangeManager.changed(this)
     }
 
     override fun observeChange(observer: ChangeObserver<T>): Disposable =
         CallbackChangeObserver(this, observer)
-
-    override fun emitDependencyChanged() {
-        if (event != null) {
-            try {
-                for (dependent in dependents) {
-                    dependent.dependencyChanged(this, event)
-                }
-            } finally {
-                event = null
-            }
-        }
-    }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/AbstractCell.kt

@@ -3,34 +3,11 @@ package world.phantasmal.observable.cell
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.observable.AbstractDependency
 import world.phantasmal.observable.CallbackChangeObserver
-import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.ChangeObserver
 
-abstract class AbstractCell<T> : AbstractDependency(), Cell<T> {
-    private var mightChangeEmitted = false
-
+abstract class AbstractCell<T> : AbstractDependency<T>(), Cell<T> {
     override fun observeChange(observer: ChangeObserver<T>): Disposable =
         CallbackChangeObserver(this, observer)
 
-    protected fun emitMightChange() {
-        if (!mightChangeEmitted) {
-            mightChangeEmitted = true
-
-            for (dependent in dependents) {
-                dependent.dependencyMightChange()
-            }
-        }
-    }
-
-    protected fun emitDependencyChangedEvent(event: ChangeEvent<*>?) {
-        if (mightChangeEmitted) {
-            mightChangeEmitted = false
-
-            for (dependent in dependents) {
-                dependent.dependencyChanged(this, event)
-            }
-        }
-    }
-
-    override fun toString(): String = "${this::class.simpleName}[$value]"
+    override fun toString(): String = cellToString(this)
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/AbstractDependentCell.kt

@@ -1,45 +1,31 @@
 package world.phantasmal.observable.cell
 
+import world.phantasmal.core.unsafe.unsafeCast
 import world.phantasmal.observable.ChangeEvent
-import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
 
 abstract class AbstractDependentCell<T> : AbstractCell<T>(), Dependent {
-    private var changingDependencies = 0
-    private var dependenciesActuallyChanged = false
 
-    override fun dependencyMightChange() {
-        changingDependencies++
-        emitMightChange()
-    }
-
-    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-        if (event != null) {
-            dependenciesActuallyChanged = true
+    private var _value: T? = null
+    final override val value: T
+        get() {
+            computeValueAndEvent()
+            // We cast instead of asserting _value is non-null because T might actually be a
+            // nullable type.
+            return unsafeCast(_value)
         }
 
-        changingDependencies--
-
-        if (changingDependencies == 0) {
-            if (dependenciesActuallyChanged) {
-                dependenciesActuallyChanged = false
-
-                dependenciesFinishedChanging()
-            } else {
-                emitDependencyChangedEvent(null)
-            }
+    final override var changeEvent: ChangeEvent<T>? = null
+        get() {
+            computeValueAndEvent()
+            return field
         }
-    }
+        private set
 
-    override fun emitDependencyChanged() {
-        // Nothing to do because dependent cells emit dependencyChanged immediately. They don't
-        // defer this operation because they only change when there is no change set or the current
-        // change set is being completed.
-    }
+    protected abstract fun computeValueAndEvent()
 
-    /**
-     * Called after a wave of dependencyMightChange notifications followed by an equal amount of
-     * dependencyChanged notifications of which at least one signified an actual change.
-     */
-    protected abstract fun dependenciesFinishedChanging()
+    protected fun setValueAndEvent(value: T, changeEvent: ChangeEvent<T>?) {
+        _value = value
+        this.changeEvent = changeEvent
+    }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/CellUtils.kt

@@ -42,9 +42,10 @@ fun <T> mutableCell(getter: () -> T, setter: (T) -> Unit): MutableCell<T> =
 fun <T> Cell<T>.observeNow(
     observer: (T) -> Unit,
 ): Disposable {
+    val disposable = observeChange { observer(it.value) }
+    // Call observer after observeChange to avoid double recomputation in most observables.
     observer(value)
-
-    return observeChange { observer(it.value) }
+    return disposable
 }
 
 fun <T1, T2> observeNow(
@@ -52,9 +53,10 @@ fun <T1, T2> observeNow(
     c2: Cell<T2>,
     observer: (T1, T2) -> Unit,
 ): Disposable {
+    val disposable = CallbackObserver(c1, c2) { observer(c1.value, c2.value) }
+    // Call observer after observeChange to avoid double recomputation in most observables.
     observer(c1.value, c2.value)
-
-    return CallbackObserver(c1, c2) { observer(c1.value, c2.value) }
+    return disposable
 }
 
 fun <T1, T2, T3> observeNow(
@@ -63,9 +65,10 @@ fun <T1, T2, T3> observeNow(
     c3: Cell<T3>,
     observer: (T1, T2, T3) -> Unit,
 ): Disposable {
+    val disposable = CallbackObserver(c1, c2, c3) { observer(c1.value, c2.value, c3.value) }
+    // Call observer after observeChange to avoid double recomputation in most observables.
     observer(c1.value, c2.value, c3.value)
-
-    return CallbackObserver(c1, c2, c3) { observer(c1.value, c2.value, c3.value) }
+    return disposable
 }
 
 fun <T1, T2, T3, T4> observeNow(
@@ -75,9 +78,11 @@ fun <T1, T2, T3, T4> observeNow(
     c4: Cell<T4>,
     observer: (T1, T2, T3, T4) -> Unit,
 ): Disposable {
+    val disposable =
+        CallbackObserver(c1, c2, c3, c4) { observer(c1.value, c2.value, c3.value, c4.value) }
+    // Call observer after observeChange to avoid double recomputation in most observables.
     observer(c1.value, c2.value, c3.value, c4.value)
-
-    return CallbackObserver(c1, c2, c3, c4) { observer(c1.value, c2.value, c3.value, c4.value) }
+    return disposable
 }
 
 fun <T1, T2, T3, T4, T5> observeNow(
@@ -88,11 +93,12 @@ fun <T1, T2, T3, T4, T5> observeNow(
     c5: Cell<T5>,
     observer: (T1, T2, T3, T4, T5) -> Unit,
 ): Disposable {
-    observer(c1.value, c2.value, c3.value, c4.value, c5.value)
-
-    return CallbackObserver(c1, c2, c3, c4, c5) {
+    val disposable = CallbackObserver(c1, c2, c3, c4, c5) {
         observer(c1.value, c2.value, c3.value, c4.value, c5.value)
     }
+    // Call observer after observeChange to avoid double recomputation in most observables.
+    observer(c1.value, c2.value, c3.value, c4.value, c5.value)
+    return disposable
 }
 
 /**
@@ -234,3 +240,9 @@ fun Cell<String>.isBlank(): Cell<Boolean> =
 
 fun Cell<String>.isNotBlank(): Cell<Boolean> =
     map { it.isNotBlank() }
+
+fun cellToString(cell: Cell<*>): String {
+    val className = cell::class.simpleName
+    val value = cell.value
+    return "$className{$value}"
+}

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/DelegatingCell.kt

@@ -1,27 +1,24 @@
 package world.phantasmal.observable.cell
 
 import world.phantasmal.observable.ChangeEvent
-import world.phantasmal.observable.ChangeManager
 
 class DelegatingCell<T>(
     private val getter: () -> T,
     private val setter: (T) -> Unit,
 ) : AbstractCell<T>(), MutableCell<T> {
-    override var value: T
-        get() = getter()
+    override var value: T = getter()
         set(value) {
-            val oldValue = getter()
+            setter(value)
+            val newValue = getter()
 
-            if (value != oldValue) {
-                emitMightChange()
-
-                setter(value)
-
-                ChangeManager.changed(this)
+            if (newValue != field) {
+                applyChange {
+                    field = newValue
+                    changeEvent = ChangeEvent(newValue)
+                }
             }
         }
 
-    override fun emitDependencyChanged() {
-        emitDependencyChangedEvent(ChangeEvent(value))
-    }
+    override var changeEvent: ChangeEvent<T>? = null
+        private set
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/DependentCell.kt

@@ -1,38 +1,34 @@
 package world.phantasmal.observable.cell
 
-import world.phantasmal.core.unsafe.unsafeCast
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
+import world.phantasmal.observable.Observable
 
 /**
  * Cell of which the value depends on 0 or more dependencies.
  */
 class DependentCell<T>(
-    private vararg val dependencies: Dependency,
+    private vararg val dependencies: Observable<*>,
     private val compute: () -> T,
 ) : AbstractDependentCell<T>() {
 
-    private var _value: T? = null
-    override val value: T
-        get() {
-            // Recompute value every time when we have no dependents. At this point we're not yet a
-            // dependent of our own dependencies, and thus we won't automatically recompute our
-            // value when they change.
-            if (dependents.isEmpty()) {
-                _value = compute()
-            }
+    private var valid = false
 
-            return unsafeCast(_value)
+    override fun computeValueAndEvent() {
+        // Recompute value every time when we have no dependents. At that point we're not yet a
+        // dependent of our own dependencies, and thus we won't automatically recompute our value
+        // when they change.
+        if (!valid) {
+            val newValue = compute()
+            setValueAndEvent(newValue, ChangeEvent(newValue))
+            valid = dependents.isNotEmpty()
         }
+    }
 
     override fun addDependent(dependent: Dependent) {
         if (dependents.isEmpty()) {
-            // Start actually depending on or dependencies when we get our first dependent.
-            // Make sure value is up-to-date here, because from now on `compute` will only be called
-            // when our dependencies change.
-            _value = compute()
-
+            // Start actually depending on our dependencies when we get our first dependent.
             for (dependency in dependencies) {
                 dependency.addDependent(this)
             }
@@ -45,6 +41,10 @@ class DependentCell<T>(
         super.removeDependent(dependent)
 
         if (dependents.isEmpty()) {
+            // As long as we don't have any dependents we're permanently "invalid", i.e. we always
+            // recompute our value.
+            valid = false
+
             // Stop actually depending on our dependencies when we no longer have any dependents.
             for (dependency in dependencies) {
                 dependency.removeDependent(this)
@@ -52,9 +52,8 @@ class DependentCell<T>(
         }
     }
 
-    override fun dependenciesFinishedChanging() {
-        val newValue = compute()
-        _value = newValue
-        emitDependencyChangedEvent(ChangeEvent(newValue))
+    override fun dependencyInvalidated(dependency: Dependency<*>) {
+        valid = false
+        emitDependencyInvalidated()
     }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/FlatteningDependentCell.kt

@@ -1,56 +1,80 @@
 package world.phantasmal.observable.cell
 
 import world.phantasmal.core.unsafe.unsafeAssertNotNull
-import world.phantasmal.core.unsafe.unsafeCast
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
+import world.phantasmal.observable.Observable
 
 /**
  * Similar to [DependentCell], except that this cell's [compute] returns a cell.
  */
+// TODO: Shares 99% of its code with FlatteningDependentListCell, should use common super class.
 class FlatteningDependentCell<T>(
-    private vararg val dependencies: Dependency,
+    private vararg val dependencies: Observable<*>,
     private val compute: () -> Cell<T>,
 ) : AbstractDependentCell<T>() {
 
     private var computedCell: Cell<T>? = null
     private var computedInDeps = false
-    private var shouldRecompute = false
+    private var shouldRecomputeCell = true
+    private var valid = false
 
-    private var _value: T? = null
-    override val value: T
-        get() {
-            if (dependents.isEmpty()) {
-                _value = compute().value
+    override fun computeValueAndEvent() {
+        if (!valid) {
+            val hasDependents = dependents.isNotEmpty()
+
+            val computedCell: Cell<T>
+
+            if (shouldRecomputeCell) {
+                this.computedCell?.removeDependent(this)
+                computedCell = compute()
+
+                if (hasDependents) {
+                    // Only hold onto and depend on the computed cell if we have dependents
+                    // ourselves.
+                    computedCell.addDependent(this)
+                    this.computedCell = computedCell
+                    computedInDeps = dependencies.any { it === computedCell }
+                    shouldRecomputeCell = false
+                } else {
+                    // Set field to null to allow the cell to be garbage collected.
+                    this.computedCell = null
+                }
+            } else {
+                computedCell = unsafeAssertNotNull(this.computedCell)
             }
 
-            return unsafeCast(_value)
+            val newValue = computedCell.value
+            setValueAndEvent(newValue, ChangeEvent(newValue))
+            // We stay invalid if we have no dependents to ensure our value is always recomputed.
+            valid = hasDependents
         }
+    }
 
     override fun addDependent(dependent: Dependent) {
-        if (dependents.isEmpty()) {
+        super.addDependent(dependent)
+
+        if (dependents.size == 1) {
             for (dependency in dependencies) {
                 dependency.addDependent(this)
             }
 
-            computedCell = compute().also { computedCell ->
-                computedCell.addDependent(this)
-                computedInDeps = dependencies.any { it === computedCell }
-                _value = computedCell.value
-            }
+            // Called to ensure that we depend on the computed cell. This could be optimized by
+            // avoiding the value and changeEvent calculation.
+            computeValueAndEvent()
         }
-
-        super.addDependent(dependent)
     }
 
     override fun removeDependent(dependent: Dependent) {
         super.removeDependent(dependent)
 
         if (dependents.isEmpty()) {
+            valid = false
             computedCell?.removeDependent(this)
+            // Set field to null to allow the cell to be garbage collected.
             computedCell = null
-            computedInDeps = false
+            shouldRecomputeCell = true
 
             for (dependency in dependencies) {
                 dependency.removeDependent(this)
@@ -58,28 +82,19 @@ class FlatteningDependentCell<T>(
         }
     }
 
-    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-        if ((dependency !== computedCell || computedInDeps) && event != null) {
-            shouldRecompute = true
+    override fun dependencyInvalidated(dependency: Dependency<*>) {
+        valid = false
+
+        // We should recompute the computed cell when any dependency except the computed cell is
+        // invalidated. When the computed cell is in our dependency array (i.e. the computed cell
+        // itself takes part in determining what the computed cell is) we should also recompute.
+        if (dependency !== computedCell || computedInDeps) {
+            // We're not allowed to change the dependency graph at this point, so we just set this
+            // field to true and remove ourselves as dependency from the computed cell right before
+            // we recompute it.
+            shouldRecomputeCell = true
         }
 
-        super.dependencyChanged(dependency, event)
-    }
-
-    override fun dependenciesFinishedChanging() {
-        if (shouldRecompute) {
-            computedCell?.removeDependent(this)
-
-            computedCell = compute().also { computedCell ->
-                computedCell.addDependent(this)
-                computedInDeps = dependencies.any { it === computedCell }
-            }
-
-            shouldRecompute = false
-        }
-
-        val newValue = unsafeAssertNotNull(computedCell).value
-        _value = newValue
-        emitDependencyChangedEvent(ChangeEvent(newValue))
+        emitDependencyInvalidated()
     }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/ImmutableCell.kt

@@ -2,11 +2,14 @@ package world.phantasmal.observable.cell
 
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.nopDisposable
+import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.ChangeObserver
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
 
-class ImmutableCell<T>(override val value: T) : Dependency, Cell<T> {
+class ImmutableCell<T>(override val value: T) : Dependency<T>, Cell<T> {
+    override val changeEvent: ChangeEvent<T>? get() = null
+
     override fun addDependent(dependent: Dependent) {
         // We don't remember our dependents because we never need to notify them of changes.
     }
@@ -17,7 +20,5 @@ class ImmutableCell<T>(override val value: T) : Dependency, Cell<T> {
 
     override fun observeChange(observer: ChangeObserver<T>): Disposable = nopDisposable()
 
-    override fun emitDependencyChanged() {
-        error("ImmutableCell can't change.")
-    }
+    override fun toString(): String = cellToString(this)
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/SimpleCell.kt

@@ -1,21 +1,18 @@
 package world.phantasmal.observable.cell
 
 import world.phantasmal.observable.ChangeEvent
-import world.phantasmal.observable.ChangeManager
 
 class SimpleCell<T>(value: T) : AbstractCell<T>(), MutableCell<T> {
     override var value: T = value
         set(value) {
             if (value != field) {
-                emitMightChange()
-
-                field = value
-
-                ChangeManager.changed(this)
+                applyChange {
+                    field = value
+                    changeEvent = ChangeEvent(value)
+                }
             }
         }
 
-    override fun emitDependencyChanged() {
-        emitDependencyChangedEvent(ChangeEvent(value))
-    }
+    override var changeEvent: ChangeEvent<T>? = null
+        private set
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/AbstractDependentListCell.kt

@@ -1,83 +0,0 @@
-package world.phantasmal.observable.cell.list
-
-import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.unsafe.unsafeAssertNotNull
-import world.phantasmal.observable.CallbackChangeObserver
-import world.phantasmal.observable.ChangeObserver
-import world.phantasmal.observable.Dependent
-import world.phantasmal.observable.cell.AbstractDependentCell
-import world.phantasmal.observable.cell.Cell
-import world.phantasmal.observable.cell.DependentCell
-
-abstract class AbstractDependentListCell<E> :
-    AbstractDependentCell<List<E>>(),
-    ListCell<E>,
-    Dependent {
-
-    protected abstract val elements: List<E>
-
-    override val value: List<E>
-        get() {
-            if (dependents.isEmpty()) {
-                computeElements()
-            }
-
-            return elements
-        }
-
-    private var _size: Cell<Int>? = null
-    final override val size: Cell<Int>
-        get() {
-            if (_size == null) {
-                _size = DependentCell(this) { value.size }
-            }
-
-            return unsafeAssertNotNull(_size)
-        }
-
-    private var _empty: Cell<Boolean>? = null
-    final override val empty: Cell<Boolean>
-        get() {
-            if (_empty == null) {
-                _empty = DependentCell(this) { value.isEmpty() }
-            }
-
-            return unsafeAssertNotNull(_empty)
-        }
-
-    private var _notEmpty: Cell<Boolean>? = null
-    final override val notEmpty: Cell<Boolean>
-        get() {
-            if (_notEmpty == null) {
-                _notEmpty = DependentCell(this) { value.isNotEmpty() }
-            }
-
-            return unsafeAssertNotNull(_notEmpty)
-        }
-
-    final override fun observeChange(observer: ChangeObserver<List<E>>): Disposable =
-        observeListChange(observer)
-
-    override fun observeListChange(observer: ListChangeObserver<E>): Disposable =
-        CallbackChangeObserver(this, observer)
-
-    final override fun dependenciesFinishedChanging() {
-        val oldElements = value
-
-        computeElements()
-
-        emitDependencyChangedEvent(
-            ListChangeEvent(
-                elements,
-                listOf(ListChange(
-                    index = 0,
-                    prevSize = oldElements.size,
-                    removed = oldElements,
-                    inserted = elements,
-                )),
-            )
-        )
-    }
-
-    protected abstract fun computeElements()
-}

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/AbstractElementsWrappingListCell.kt

@@ -0,0 +1,34 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.core.unsafe.unsafeAssertNotNull
+
+abstract class AbstractElementsWrappingListCell<E> : AbstractListCell<E>() {
+    /**
+     * When [value] is accessed and this property is null, a new wrapper is created that points to
+     * [elements]. Before changes to [elements] are made, if there's a wrapper, the current
+     * wrapper's backing list is set to a copy of [elements] and this property is set to null. This
+     * way, accessing [value] acts like accessing a snapshot without making an actual copy
+     * everytime. This is necessary because the contract is that a cell's new value is always != to
+     * its old value whenever a change event was emitted (TODO: is this still the contract?).
+     */
+    // TODO: Optimize this by using a weak reference to avoid copying when nothing references the
+    //       wrapper.
+    // TODO: Just remove this because it's a huge headache? Does it matter that events are
+    //       immutable?
+    private var _elementsWrapper: DelegatingList<E>? = null
+    protected val elementsWrapper: DelegatingList<E>
+        get() {
+            if (_elementsWrapper == null) {
+                _elementsWrapper = DelegatingList(elements)
+            }
+
+            return unsafeAssertNotNull(_elementsWrapper)
+        }
+
+    protected abstract val elements: List<E>
+
+    protected fun copyAndResetWrapper() {
+        _elementsWrapper?.backingList = elements.toList()
+        _elementsWrapper = null
+    }
+}

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/AbstractFilteredListCell.kt

@@ -1,42 +1,156 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.core.unsafe.unsafeCast
-import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
 
 abstract class AbstractFilteredListCell<E>(
     protected val list: ListCell<E>,
-) : AbstractListCell<E>(), Dependent {
-
-    /** Keeps track of number of changing dependencies during a change wave. */
-    private var changingDependencies = 0
+) : AbstractElementsWrappingListCell<E>(), Dependent {
 
     /** Set during a change wave when [list] changes. */
-    private var listChangeEvent: ListChangeEvent<E>? = null
+    private var listInvalidated = false
 
     /** Set during a change wave when [predicateDependency] changes. */
-    private var predicateChanged = false
+    private var predicateInvalidated = false
 
-    override val elements = mutableListOf<E>()
+    private var valid = false
 
-    protected abstract val predicateDependency: Dependency
+    final override val elements = mutableListOf<E>()
 
-    override val value: List<E>
+    protected abstract val predicateDependency: Dependency<*>
+
+    final override val value: List<E>
         get() {
-            if (dependents.isEmpty()) {
-                recompute()
-            }
-
+            computeValueAndEvent()
             return elementsWrapper
         }
 
-    override fun addDependent(dependent: Dependent) {
-        val wasEmpty = dependents.isEmpty()
+    final override var changeEvent: ListChangeEvent<E>? = null
+        get() {
+            computeValueAndEvent()
+            return field
+        }
+        private set
 
+    private fun computeValueAndEvent() {
+        if (!valid) {
+            val hasDependents = dependents.isNotEmpty()
+
+            if (predicateInvalidated || !hasDependents) {
+                // Simply assume the entire list changes and recompute.
+                val removed = elementsWrapper
+
+                ignoreOtherChanges()
+                recompute()
+
+                changeEvent = ListChangeEvent(
+                    elementsWrapper,
+                    listOf(ListChange(0, removed.size, removed, elementsWrapper)),
+                )
+            } else {
+                // TODO: Conditionally copyAndResetWrapper?
+                copyAndResetWrapper()
+                val filteredChanges = mutableListOf<ListChange<E>>()
+
+                if (listInvalidated) {
+                    list.changeEvent?.let { listChangeEvent ->
+                        for (change in listChangeEvent.changes) {
+                            val prevSize = elements.size
+                            // Map the incoming change index to an index into our own elements list.
+                            // TODO: Avoid this loop by storing the index where an element "would"
+                            //       be if it passed the predicate.
+                            var eventIndex = prevSize
+
+                            for (index in change.index..maxDepIndex()) {
+                                val i = mapIndex(index)
+
+                                if (i != -1) {
+                                    eventIndex = i
+                                    break
+                                }
+                            }
+
+                            // Process removals.
+                            val removed = mutableListOf<E>()
+
+                            for (element in change.removed) {
+                                val index = removeIndexMapping(change.index)
+
+                                if (index != -1) {
+                                    elements.removeAt(eventIndex)
+                                    removed.add(element)
+                                }
+                            }
+
+                            // Process insertions.
+                            val inserted = mutableListOf<E>()
+                            var insertionIndex = eventIndex
+
+                            for ((i, element) in change.inserted.withIndex()) {
+                                if (applyPredicate(element)) {
+                                    insertIndexMapping(change.index + i, insertionIndex, element)
+                                    elements.add(insertionIndex, element)
+                                    inserted.add(element)
+                                    insertionIndex++
+                                } else {
+                                    insertIndexMapping(change.index + i, -1, element)
+                                }
+                            }
+
+                            // Shift mapped indices by a certain amount. This amount can be
+                            // positive, negative or zero.
+                            val diff = inserted.size - removed.size
+
+                            if (diff != 0) {
+                                // Indices before the change index stay the same. Newly inserted
+                                // indices are already correct. So we only need to shift everything
+                                // after the new indices.
+                                val startIndex = change.index + change.inserted.size
+
+                                for (index in startIndex..maxDepIndex()) {
+                                    shiftIndexMapping(index, diff)
+                                }
+                            }
+
+                            // Add a list change if something actually changed.
+                            if (removed.isNotEmpty() || inserted.isNotEmpty()) {
+                                filteredChanges.add(
+                                    ListChange(
+                                        eventIndex,
+                                        prevSize,
+                                        removed,
+                                        inserted,
+                                    )
+                                )
+                            }
+                        }
+                    }
+                }
+
+                processOtherChanges(filteredChanges)
+
+                changeEvent =
+                    if (filteredChanges.isEmpty()) {
+                        null
+                    } else {
+                        ListChangeEvent(elementsWrapper, filteredChanges)
+                    }
+            }
+
+            // Reset for next change wave.
+            listInvalidated = false
+            predicateInvalidated = false
+            // We stay invalid if we have no dependents to ensure our value is always recomputed.
+            valid = hasDependents
+        }
+
+        resetChangeWaveData()
+    }
+
+    override fun addDependent(dependent: Dependent) {
         super.addDependent(dependent)
 
-        if (wasEmpty) {
+        if (dependents.size == 1) {
             list.addDependent(this)
             predicateDependency.addDependent(this)
             recompute()
@@ -47,150 +161,33 @@ abstract class AbstractFilteredListCell<E>(
         super.removeDependent(dependent)
 
         if (dependents.isEmpty()) {
+            valid = false
             predicateDependency.removeDependent(this)
             list.removeDependent(this)
         }
     }
 
-    override fun dependencyMightChange() {
-        changingDependencies++
-        emitMightChange()
-    }
+    override fun dependencyInvalidated(dependency: Dependency<*>) {
+        valid = false
 
-    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
         if (dependency === list) {
-            listChangeEvent = unsafeCast(event)
+            listInvalidated = true
         } else if (dependency === predicateDependency) {
-            predicateChanged = event != null
-        } else if (event != null) {
-            otherDependencyChanged(dependency)
+            predicateInvalidated = true
+        } else {
+            otherDependencyInvalidated(dependency)
         }
 
-        changingDependencies--
-
-        if (changingDependencies == 0) {
-            if (predicateChanged) {
-                // Simply assume the entire list changes and recompute.
-                val removed = elementsWrapper
-
-                ignoreOtherChanges()
-                recompute()
-
-                emitDependencyChangedEvent(
-                    ListChangeEvent(
-                        elementsWrapper,
-                        listOf(ListChange(0, removed.size, removed, elementsWrapper)),
-                    )
-                )
-            } else {
-                // TODO: Conditionally copyAndResetWrapper?
-                copyAndResetWrapper()
-                val filteredChanges = mutableListOf<ListChange<E>>()
-
-                val listChangeEvent = this.listChangeEvent
-
-                if (listChangeEvent != null) {
-                    for (change in listChangeEvent.changes) {
-                        val prevSize = elements.size
-                        // Map the incoming change index to an index into our own elements list.
-                        // TODO: Avoid this loop by storing the index where an element "would" be
-                        //       if it passed the predicate.
-                        var eventIndex = prevSize
-
-                        for (index in change.index..maxDepIndex()) {
-                            val i = mapIndex(index)
-
-                            if (i != -1) {
-                                eventIndex = i
-                                break
-                            }
-                        }
-
-                        // Process removals.
-                        val removed = mutableListOf<E>()
-
-                        for (element in change.removed) {
-                            val index = removeIndexMapping(change.index)
-
-                            if (index != -1) {
-                                elements.removeAt(eventIndex)
-                                removed.add(element)
-                            }
-                        }
-
-                        // Process insertions.
-                        val inserted = mutableListOf<E>()
-                        var insertionIndex = eventIndex
-
-                        for ((i, element) in change.inserted.withIndex()) {
-                            if (applyPredicate(element)) {
-                                insertIndexMapping(change.index + i, insertionIndex, element)
-                                elements.add(insertionIndex, element)
-                                inserted.add(element)
-                                insertionIndex++
-                            } else {
-                                insertIndexMapping(change.index + i, -1, element)
-                            }
-                        }
-
-                        // Shift mapped indices by a certain amount. This amount can be positive,
-                        // negative or zero.
-                        val diff = inserted.size - removed.size
-
-                        if (diff != 0) {
-                            // Indices before the change index stay the same. Newly inserted indices
-                            // are already correct. So we only need to shift everything after the
-                            // new indices.
-                            for (index in (change.index + change.inserted.size)..maxDepIndex()) {
-                                shiftIndexMapping(index, diff)
-                            }
-                        }
-
-                        // Add a list change if something actually changed.
-                        if (removed.isNotEmpty() || inserted.isNotEmpty()) {
-                            filteredChanges.add(
-                                ListChange(
-                                    eventIndex,
-                                    prevSize,
-                                    removed,
-                                    inserted,
-                                )
-                            )
-                        }
-                    }
-                }
-
-                processOtherChanges(filteredChanges)
-
-                if (filteredChanges.isEmpty()) {
-                    emitDependencyChangedEvent(null)
-                } else {
-                    emitDependencyChangedEvent(
-                        ListChangeEvent(elementsWrapper, filteredChanges)
-                    )
-                }
-            }
-
-            // Reset for next change wave.
-            listChangeEvent = null
-            predicateChanged = false
-            resetChangeWaveData()
-        }
+        emitDependencyInvalidated()
     }
 
     /** Called when a dependency that's neither [list] nor [predicateDependency] has changed. */
-    protected abstract fun otherDependencyChanged(dependency: Dependency)
+    protected abstract fun otherDependencyInvalidated(dependency: Dependency<*>)
 
     protected abstract fun ignoreOtherChanges()
 
     protected abstract fun processOtherChanges(filteredChanges: MutableList<ListChange<E>>)
 
-    override fun emitDependencyChanged() {
-        // Nothing to do because AbstractFilteredListCell emits dependencyChanged immediately. We
-        // don't defer this operation because AbstractFilteredListCell only changes when there is no
-        // change set or the current change set is being completed.
-    }
-
     protected abstract fun applyPredicate(element: E): Boolean
 
     protected abstract fun maxDepIndex(): Int

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/AbstractListCell.kt

@@ -7,37 +7,38 @@ import world.phantasmal.observable.ChangeObserver
 import world.phantasmal.observable.cell.AbstractCell
 import world.phantasmal.observable.cell.Cell
 import world.phantasmal.observable.cell.DependentCell
-import world.phantasmal.observable.cell.not
 
 abstract class AbstractListCell<E> : AbstractCell<List<E>>(), ListCell<E> {
-    /**
-     * When [value] is accessed and this property is null, a new wrapper is created that points to
-     * [elements]. Before changes to [elements] are made, if there's a wrapper, the current
-     * wrapper's backing list is set to a copy of [elements] and this property is set to null. This
-     * way, accessing [value] acts like accessing a snapshot without making an actual copy
-     * everytime. This is necessary because the contract is that a cell's new value is always != to
-     * its old value whenever a change event was emitted.
-     */
-    // TODO: Optimize this by using a weak reference to avoid copying when nothing references the
-    //       wrapper.
-    private var _elementsWrapper: DelegatingList<E>? = null
-    protected val elementsWrapper: DelegatingList<E>
+
+    private var _size: Cell<Int>? = null
+    final override val size: Cell<Int>
         get() {
-            if (_elementsWrapper == null) {
-                _elementsWrapper = DelegatingList(elements)
+            if (_size == null) {
+                _size = DependentCell(this) { value.size }
             }
 
-            return unsafeAssertNotNull(_elementsWrapper)
+            return unsafeAssertNotNull(_size)
         }
 
-    protected abstract val elements: List<E>
+    private var _empty: Cell<Boolean>? = null
+    final override val empty: Cell<Boolean>
+        get() {
+            if (_empty == null) {
+                _empty = DependentCell(this) { value.isEmpty() }
+            }
 
-    @Suppress("LeakingThis")
-    final override val size: Cell<Int> = DependentCell(this) { value.size }
+            return unsafeAssertNotNull(_empty)
+        }
 
-    final override val empty: Cell<Boolean> = DependentCell(size) { size.value == 0 }
+    private var _notEmpty: Cell<Boolean>? = null
+    final override val notEmpty: Cell<Boolean>
+        get() {
+            if (_notEmpty == null) {
+                _notEmpty = DependentCell(this) { value.isNotEmpty() }
+            }
 
-    final override val notEmpty: Cell<Boolean> = !empty
+            return unsafeAssertNotNull(_notEmpty)
+        }
 
     final override fun observeChange(observer: ChangeObserver<List<E>>): Disposable =
         observeListChange(observer)
@@ -45,8 +46,5 @@ abstract class AbstractListCell<E> : AbstractCell<List<E>>(), ListCell<E> {
     override fun observeListChange(observer: ListChangeObserver<E>): Disposable =
         CallbackChangeObserver(this, observer)
 
-    protected fun copyAndResetWrapper() {
-        _elementsWrapper?.backingList = elements.toList()
-        _elementsWrapper = null
-    }
+    override fun toString(): String = listCellToString(this)
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/DependentListCell.kt

@@ -1,23 +1,53 @@
 package world.phantasmal.observable.cell.list
 
+import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
-import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.Observable
 
 /**
- * ListCell of which the value depends on 0 or more other cells.
+ * ListCell of which the value depends on 0 or more other observables.
  */
 class DependentListCell<E>(
-    private vararg val dependencies: Cell<*>,
+    private vararg val dependencies: Observable<*>,
     private val computeElements: () -> List<E>,
-) : AbstractDependentListCell<E>() {
+) : AbstractListCell<E>(), Dependent {
 
-    override var elements: List<E> = emptyList()
+    private var valid = false
+
+    private var _value: List<E> = emptyList()
+    override val value: List<E>
+        get() {
+            computeValueAndEvent()
+            return _value
+        }
+
+    override var changeEvent: ListChangeEvent<E>? = null
+        get() {
+            computeValueAndEvent()
+            return field
+        }
         private set
 
+    private fun computeValueAndEvent() {
+        if (!valid) {
+            val oldElements = _value
+            val newElements = computeElements()
+            _value = newElements
+            changeEvent = ListChangeEvent(
+                newElements,
+                listOf(ListChange(
+                    index = 0,
+                    prevSize = oldElements.size,
+                    removed = oldElements,
+                    inserted = newElements,
+                )),
+            )
+            valid = dependents.isNotEmpty()
+        }
+    }
+
     override fun addDependent(dependent: Dependent) {
         if (dependents.isEmpty()) {
-            computeElements()
-
             for (dependency in dependencies) {
                 dependency.addDependent(this)
             }
@@ -30,13 +60,16 @@ class DependentListCell<E>(
         super.removeDependent(dependent)
 
         if (dependents.isEmpty()) {
+            valid = false
+
             for (dependency in dependencies) {
                 dependency.removeDependent(this)
             }
         }
     }
 
-    override fun computeElements() {
-        elements = computeElements.invoke()
+    override fun dependencyInvalidated(dependency: Dependency<*>) {
+        valid = false
+        emitDependencyInvalidated()
     }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/FilteredListCell.kt

@@ -1,6 +1,7 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.assert
+import world.phantasmal.core.assertUnreachable
 import world.phantasmal.core.unsafe.unsafeCast
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Dependency
@@ -20,7 +21,7 @@ class FilteredListCell<E>(
 
     private val changedPredicateResults = mutableListOf<Mapping>()
 
-    override val predicateDependency: Dependency
+    override val predicateDependency: Dependency<*>
         get() = predicate
 
     override fun removeDependent(dependent: Dependent) {
@@ -33,8 +34,11 @@ class FilteredListCell<E>(
         }
     }
 
-    override fun otherDependencyChanged(dependency: Dependency) {
-        assert { dependency is FilteredListCell<*>.Mapping }
+    override fun otherDependencyInvalidated(dependency: Dependency<*>) {
+        assert(
+            { dependency is FilteredListCell<*>.Mapping },
+            { "Expected $dependency to be a mapping." },
+        )
 
         changedPredicateResults.add(unsafeCast(dependency))
     }
@@ -105,7 +109,8 @@ class FilteredListCell<E>(
         }
 
         // Can still contain changed mappings at this point if e.g. an element was removed after its
-        // predicate result changed.
+        // predicate result changed or a predicate result emitted multiple invalidation
+        // notifications.
         changedPredicateResults.clear()
     }
 
@@ -182,15 +187,17 @@ class FilteredListCell<E>(
          * pass the predicate.
          */
         var index: Int,
-    ) : Dependent, Dependency {
-        override fun dependencyMightChange() {
-            this@FilteredListCell.dependencyMightChange()
-        }
+    ) : Dependent, Dependency<Boolean> {
+        override val changeEvent: ChangeEvent<Boolean>?
+            get() {
+                assertUnreachable { "Change event is never computed." }
+                return null
+            }
 
-        override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+        override fun dependencyInvalidated(dependency: Dependency<*>) {
             assert { dependency === predicateResult }
 
-            this@FilteredListCell.dependencyChanged(this, event)
+            this@FilteredListCell.dependencyInvalidated(this)
         }
 
         override fun addDependent(dependent: Dependent) {
@@ -204,9 +211,5 @@ class FilteredListCell<E>(
 
             predicateResult.removeDependent(this)
         }
-
-        override fun emitDependencyChanged() {
-            // Nothing to do.
-        }
     }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/FlatteningDependentListCell.kt

@@ -1,48 +1,103 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.unsafe.unsafeAssertNotNull
-import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
+import world.phantasmal.observable.Observable
 
 /**
  * Similar to [DependentListCell], except that this cell's [computeElements] returns a [ListCell].
  */
+// TODO: Shares 99% of its code with FlatteningDependentCell, should use common super class.
 class FlatteningDependentListCell<E>(
-    private vararg val dependencies: Dependency,
+    private vararg val dependencies: Observable<*>,
     private val computeElements: () -> ListCell<E>,
-) : AbstractDependentListCell<E>() {
+) : AbstractListCell<E>(), Dependent {
 
     private var computedCell: ListCell<E>? = null
     private var computedInDeps = false
-    private var shouldRecompute = false
+    private var shouldRecomputeCell = true
+    private var valid = false
 
-    override var elements: List<E> = emptyList()
+    private var _value:List<E> = emptyList()
+    override val value: List<E>
+        get() {
+            computeValueAndEvent()
+            return _value
+        }
+
+    override var changeEvent: ListChangeEvent<E>? = null
+        get() {
+            computeValueAndEvent()
+            return field
+        }
         private set
 
+    private fun computeValueAndEvent() {
+        if (!valid) {
+            val oldElements = _value
+            val hasDependents = dependents.isNotEmpty()
+
+            val computedCell: ListCell<E>
+
+            if (shouldRecomputeCell) {
+                this.computedCell?.removeDependent(this)
+                computedCell = computeElements()
+
+                if (hasDependents) {
+                    // Only hold onto and depend on the computed cell if we have dependents
+                    // ourselves.
+                    computedCell.addDependent(this)
+                    this.computedCell = computedCell
+                    computedInDeps = dependencies.any { it === computedCell }
+                    shouldRecomputeCell = false
+                } else {
+                    // Set field to null to allow the cell to be garbage collected.
+                    this.computedCell = null
+                }
+            } else {
+                computedCell = unsafeAssertNotNull(this.computedCell)
+            }
+
+            val newElements = computedCell.value
+            _value = newElements
+            changeEvent = ListChangeEvent(
+                newElements,
+                listOf(ListChange(
+                    index = 0,
+                    prevSize = oldElements.size,
+                    removed = oldElements,
+                    inserted = newElements,
+                )),
+            )
+            // We stay invalid if we have no dependents to ensure our value is always recomputed.
+            valid = hasDependents
+        }
+    }
+
     override fun addDependent(dependent: Dependent) {
-        if (dependents.isEmpty()) {
+        super.addDependent(dependent)
+
+        if (dependents.size == 1) {
             for (dependency in dependencies) {
                 dependency.addDependent(this)
             }
 
-            computedCell = computeElements.invoke().also { computedCell ->
-                computedCell.addDependent(this)
-                computedInDeps = dependencies.any { it === computedCell }
-                elements = computedCell.value
-            }
+            // Called to ensure that we depend on the computed cell. This could be optimized by
+            // avoiding the value and changeEvent calculation.
+            computeValueAndEvent()
         }
-
-        super.addDependent(dependent)
     }
 
     override fun removeDependent(dependent: Dependent) {
         super.removeDependent(dependent)
 
         if (dependents.isEmpty()) {
+            valid = false
             computedCell?.removeDependent(this)
+            // Set field to null to allow the cell to be garbage collected.
             computedCell = null
-            computedInDeps = false
+            shouldRecomputeCell = true
 
             for (dependency in dependencies) {
                 dependency.removeDependent(this)
@@ -50,26 +105,19 @@ class FlatteningDependentListCell<E>(
         }
     }
 
-    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-        if ((dependency !== computedCell || computedInDeps) && event != null) {
-            shouldRecompute = true
+    override fun dependencyInvalidated(dependency: Dependency<*>) {
+        valid = false
+
+        // We should recompute the computed cell when any dependency except the computed cell is
+        // invalidated. When the computed cell is in our dependency array (i.e. the computed cell
+        // itself takes part in determining what the computed cell is) we should also recompute.
+        if (dependency !== computedCell || computedInDeps) {
+            // We're not allowed to change the dependency graph at this point, so we just set this
+            // field to true and remove ourselves as dependency from the computed cell right before
+            // we recompute it.
+            shouldRecomputeCell = true
         }
 
-        super.dependencyChanged(dependency, event)
-    }
-
-    override fun computeElements() {
-        if (shouldRecompute || dependents.isEmpty()) {
-            computedCell?.removeDependent(this)
-
-            computedCell = computeElements.invoke().also { computedCell ->
-                computedCell.addDependent(this)
-                computedInDeps = dependencies.any { it === computedCell }
-            }
-
-            shouldRecompute = false
-        }
-
-        elements = unsafeAssertNotNull(computedCell).value
+        emitDependencyInvalidated()
     }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/ImmutableListCell.kt

@@ -10,13 +10,15 @@ import world.phantasmal.observable.cell.cell
 import world.phantasmal.observable.cell.falseCell
 import world.phantasmal.observable.cell.trueCell
 
-class ImmutableListCell<E>(private val elements: List<E>) : Dependency, ListCell<E> {
+class ImmutableListCell<E>(private val elements: List<E>) : Dependency<List<E>>, ListCell<E> {
     override val size: Cell<Int> = cell(elements.size)
     override val empty: Cell<Boolean> = if (elements.isEmpty()) trueCell() else falseCell()
     override val notEmpty: Cell<Boolean> = if (elements.isNotEmpty()) trueCell() else falseCell()
 
     override val value: List<E> = elements
 
+    override val changeEvent: ListChangeEvent<E>? get() = null
+
     override fun addDependent(dependent: Dependent) {
         // We don't remember our dependents because we never need to notify them of changes.
     }
@@ -25,14 +27,11 @@ class ImmutableListCell<E>(private val elements: List<E>) : Dependency, ListCell
         // Nothing to remove because we don't remember our dependents.
     }
 
-    override fun get(index: Int): E =
-        elements[index]
+    override fun get(index: Int): E = elements[index]
 
     override fun observeChange(observer: ChangeObserver<List<E>>): Disposable = nopDisposable()
 
     override fun observeListChange(observer: ListChangeObserver<E>): Disposable = nopDisposable()
 
-    override fun emitDependencyChanged() {
-        error("ImmutableListCell can't change.")
-    }
+    override fun toString(): String = listCellToString(this)
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/ListCell.kt

@@ -6,6 +6,8 @@ import world.phantasmal.observable.cell.Cell
 interface ListCell<out E> : Cell<List<E>> {
     override val value: List<E>
 
+    override val changeEvent: ListChangeEvent<E>?
+
     val size: Cell<Int>
 
     val empty: Cell<Boolean>

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/ListCellUtils.kt

@@ -68,6 +68,14 @@ fun <T1, T2, R> mapToList(
 ): ListCell<R> =
     DependentListCell(c1, c2) { transform(c1.value, c2.value) }
 
+fun <T1, T2, T3, R> mapToList(
+    c1: Cell<T1>,
+    c2: Cell<T2>,
+    c3: Cell<T3>,
+    transform: (T1, T2, T3) -> List<R>,
+): ListCell<R> =
+    DependentListCell(c1, c2, c3) { transform(c1.value, c2.value, c3.value) }
+
 fun <T, R> Cell<T>.flatMapToList(
     transform: (T) -> ListCell<R>,
 ): ListCell<R> =
@@ -79,3 +87,12 @@ fun <T1, T2, R> flatMapToList(
     transform: (T1, T2) -> ListCell<R>,
 ): ListCell<R> =
     FlatteningDependentListCell(c1, c2) { transform(c1.value, c2.value) }
+
+fun listCellToString(cell: ListCell<*>): String = buildString {
+    append(this::class.simpleName)
+    append('[')
+    cell.value.joinTo(this, limit = 20) {
+        if (it === this) "(this cell)" else it.toString()
+    }
+    append(']')
+}

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/ListElementsDependentCell.kt

@@ -1,7 +1,6 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.splice
-import world.phantasmal.core.unsafe.unsafeCast
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
@@ -9,28 +8,70 @@ import world.phantasmal.observable.Observable
 import world.phantasmal.observable.cell.AbstractCell
 
 /**
- * Depends on a [ListCell] and zero or more [Observable]s per element in the list.
+ * Depends on a [ListCell] and zero or more observables per element in the list.
  */
 class ListElementsDependentCell<E>(
     private val list: ListCell<E>,
     private val extractObservables: (element: E) -> Array<out Observable<*>>,
 ) : AbstractCell<List<E>>(), Dependent {
     /** An array of dependencies per [list] element, extracted by [extractObservables]. */
-    private val elementDependencies = mutableListOf<Array<out Dependency>>()
+    private val elementDependencies = mutableListOf<Array<out Dependency<*>>>()
 
-    /** Keeps track of how many of our dependencies are about to (maybe) change. */
-    private var changingDependencies = 0
-
-    /**
-     * Set to true once one of our dependencies has actually changed. Reset to false whenever
-     * [changingDependencies] hits 0 again.
-     */
-    private var dependenciesActuallyChanged = false
-
-    private var listChangeEvent: ListChangeEvent<E>? = null
+    private var valid = false
+    private var listInvalidated = false
 
     override val value: List<E>
-        get() = list.value
+        get() {
+            updateElementDependenciesAndEvent()
+            return list.value
+        }
+
+    override var changeEvent: ChangeEvent<List<E>>? = null
+        get() {
+            updateElementDependenciesAndEvent()
+            return field
+        }
+        private set
+
+    private fun updateElementDependenciesAndEvent() {
+        if (!valid) {
+            if (listInvalidated) {
+                // At this point we can remove this dependent from the removed elements' dependencies
+                // and add it to the newly inserted elements' dependencies.
+                list.changeEvent?.let { listChangeEvent ->
+                    for (change in listChangeEvent.changes) {
+                        for (i in change.index until (change.index + change.removed.size)) {
+                            for (elementDependency in elementDependencies[i]) {
+                                elementDependency.removeDependent(this)
+                            }
+                        }
+
+                        val inserted = change.inserted.map(extractObservables)
+
+                        elementDependencies.splice(
+                            startIndex = change.index,
+                            amount = change.removed.size,
+                            elements = inserted,
+                        )
+
+                        for (elementDependencies in inserted) {
+                            for (elementDependency in elementDependencies) {
+                                elementDependency.addDependent(this)
+                            }
+                        }
+                    }
+                }
+
+                // Reset for the next change wave.
+                listInvalidated = false
+            }
+
+            changeEvent = ChangeEvent(list.value)
+            // We stay invalid if we have no dependents to ensure our change event is always
+            // recomputed.
+            valid = dependents.isNotEmpty()
+        }
+    }
 
     override fun addDependent(dependent: Dependent) {
         if (dependents.isEmpty()) {
@@ -55,6 +96,9 @@ class ListElementsDependentCell<E>(
         super.removeDependent(dependent)
 
         if (dependents.isEmpty()) {
+            valid = false
+            listInvalidated = false
+
             // At this point we have no more dependents, so we can stop depending on our own
             // dependencies.
             for (dependencies in elementDependencies) {
@@ -68,73 +112,13 @@ class ListElementsDependentCell<E>(
         }
     }
 
-    override fun dependencyMightChange() {
-        changingDependencies++
-        emitMightChange()
-    }
+    override fun dependencyInvalidated(dependency: Dependency<*>) {
+        valid = false
 
-    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-        if (event != null) {
-            dependenciesActuallyChanged = true
-
-            // Simply store all list changes when the changing dependency is our list dependency. We
-            // don't update our dependencies yet to avoid receiving dependencyChanged notifications
-            // from newly inserted dependencies for which we haven't received any
-            // dependencyMightChange notifications and to avoid *NOT* receiving dependencyChanged
-            // notifications from removed dependencies for which we *HAVE* received
-            // dependencyMightChange notifications.
-            if (dependency === list) {
-                listChangeEvent = unsafeCast(event)
-            }
+        if (dependency === list) {
+            listInvalidated = true
         }
 
-        changingDependencies--
-
-        if (changingDependencies == 0) {
-            // All of our dependencies have finished changing.
-
-            // At this point we can remove this dependent from the removed elements' dependencies
-            // and add it to the newly inserted elements' dependencies.
-            listChangeEvent?.let { listChangeEvent ->
-                for (change in listChangeEvent.changes) {
-                    for (i in change.index until (change.index + change.removed.size)) {
-                        for (elementDependency in elementDependencies[i]) {
-                            elementDependency.removeDependent(this)
-                        }
-                    }
-
-                    val inserted = change.inserted.map(extractObservables)
-
-                    elementDependencies.splice(
-                        startIndex = change.index,
-                        amount = change.removed.size,
-                        elements = inserted,
-                    )
-
-                    for (elementDependencies in inserted) {
-                        for (elementDependency in elementDependencies) {
-                            elementDependency.addDependent(this)
-                        }
-                    }
-                }
-            }
-
-            // Reset for the next change wave.
-            listChangeEvent = null
-
-            if (dependenciesActuallyChanged) {
-                dependenciesActuallyChanged = false
-
-                emitDependencyChangedEvent(ChangeEvent(list.value))
-            } else {
-                emitDependencyChangedEvent(null)
-            }
-        }
-    }
-
-    override fun emitDependencyChanged() {
-        // Nothing to do because ListElementsDependentCell emits dependencyChanged immediately. We
-        // don't defer this operation because ListElementsDependentCell only changes when there is
-        // no transaction or the current transaction is being committed.
+        emitDependencyInvalidated()
     }
 }

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/SimpleFilteredListCell.kt

@@ -1,5 +1,6 @@
 package world.phantasmal.observable.cell.list
 
+import world.phantasmal.core.assertUnreachable
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.cell.Cell
 
@@ -17,12 +18,11 @@ class SimpleFilteredListCell<E>(
      */
     private val indexMap = mutableListOf<Int>()
 
-    override val predicateDependency: Dependency
+    override val predicateDependency: Dependency<*>
         get() = predicate
 
-    override fun otherDependencyChanged(dependency: Dependency) {
-        // Unreachable code path.
-        error("Unexpected dependency.")
+    override fun otherDependencyInvalidated(dependency: Dependency<*>) {
+        assertUnreachable { "Unexpected dependency $dependency." }
     }
 
     override fun ignoreOtherChanges() {

observable/src/commonMain/kotlin/world/phantasmal/observable/cell/list/SimpleListCell.kt

@@ -1,16 +1,14 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.replaceAll
-import world.phantasmal.observable.ChangeManager
 
 /**
  * @param elements The backing list for this [ListCell].
  */
+// TODO: Support change sets by sometimes appending to changeEvent instead of always overwriting it.
 class SimpleListCell<E>(
     override val elements: MutableList<E>,
-) : AbstractListCell<E>(), MutableListCell<E> {
-
-    private var changes = mutableListOf<ListChange<E>>()
+) : AbstractElementsWrappingListCell<E>(), MutableListCell<E> {
 
     override var value: List<E>
         get() = elementsWrapper
@@ -18,50 +16,55 @@ class SimpleListCell<E>(
             replaceAll(value)
         }
 
+    override var changeEvent: ListChangeEvent<E>? = null
+        private set
+
     override operator fun get(index: Int): E =
         elements[index]
 
     override operator fun set(index: Int, element: E): E {
         checkIndex(index, elements.lastIndex)
-        emitMightChange()
 
-        copyAndResetWrapper()
-        val removed = elements.set(index, element)
+        applyChange {
+            copyAndResetWrapper()
+            val removed = elements.set(index, element)
 
-        finalizeChange(
-            index,
-            prevSize = elements.size,
-            removed = listOf(removed),
-            inserted = listOf(element),
-        )
+            finalizeChange(
+                index,
+                prevSize = elements.size,
+                removed = listOf(removed),
+                inserted = listOf(element),
+            )
 
-        return removed
+            return removed
+        }
     }
 
     override fun add(element: E) {
-        emitMightChange()
+        applyChange {
+            val index = elements.size
+            copyAndResetWrapper()
+            elements.add(element)
 
-        val index = elements.size
-        copyAndResetWrapper()
-        elements.add(element)
-
-        finalizeChange(
-            index,
-            prevSize = index,
-            removed = emptyList(),
-            inserted = listOf(element),
-        )
+            finalizeChange(
+                index,
+                prevSize = index,
+                removed = emptyList(),
+                inserted = listOf(element),
+            )
+        }
     }
 
     override fun add(index: Int, element: E) {
         val prevSize = elements.size
         checkIndex(index, prevSize)
-        emitMightChange()
 
-        copyAndResetWrapper()
-        elements.add(index, element)
+        applyChange {
+            copyAndResetWrapper()
+            elements.add(index, element)
 
-        finalizeChange(index, prevSize, removed = emptyList(), inserted = listOf(element))
+            finalizeChange(index, prevSize, removed = emptyList(), inserted = listOf(element))
+        }
     }
 
     override fun remove(element: E): Boolean {
@@ -77,99 +80,95 @@ class SimpleListCell<E>(
 
     override fun removeAt(index: Int): E {
         checkIndex(index, elements.lastIndex)
-        emitMightChange()
 
-        val prevSize = elements.size
+        applyChange {
+            val prevSize = elements.size
 
-        copyAndResetWrapper()
-        val removed = elements.removeAt(index)
+            copyAndResetWrapper()
+            val removed = elements.removeAt(index)
 
-        finalizeChange(index, prevSize, removed = listOf(removed), inserted = emptyList())
-        return removed
+            finalizeChange(index, prevSize, removed = listOf(removed), inserted = emptyList())
+            return removed
+        }
     }
 
     override fun replaceAll(elements: Iterable<E>) {
-        emitMightChange()
+        applyChange {
+            val prevSize = this.elements.size
+            val removed = elementsWrapper
 
-        val prevSize = this.elements.size
-        val removed = elementsWrapper
+            copyAndResetWrapper()
+            this.elements.replaceAll(elements)
 
-        copyAndResetWrapper()
-        this.elements.replaceAll(elements)
-
-        finalizeChange(index = 0, prevSize, removed, inserted = elementsWrapper)
+            finalizeChange(index = 0, prevSize, removed, inserted = elementsWrapper)
+        }
     }
 
     override fun replaceAll(elements: Sequence<E>) {
-        emitMightChange()
+        applyChange {
+            val prevSize = this.elements.size
+            val removed = elementsWrapper
 
-        val prevSize = this.elements.size
-        val removed = elementsWrapper
+            copyAndResetWrapper()
+            this.elements.replaceAll(elements)
 
-        copyAndResetWrapper()
-        this.elements.replaceAll(elements)
-
-        finalizeChange(index = 0, prevSize, removed, inserted = elementsWrapper)
+            finalizeChange(index = 0, prevSize, removed, inserted = elementsWrapper)
+        }
     }
 
     override fun splice(fromIndex: Int, removeCount: Int, newElement: E) {
         val prevSize = elements.size
         val removed = ArrayList<E>(removeCount)
 
+        // Do this loop outside applyChange because it will throw when any index is out of bounds.
         for (i in fromIndex until (fromIndex + removeCount)) {
             removed.add(elements[i])
         }
 
-        emitMightChange()
+        applyChange {
+            copyAndResetWrapper()
+            repeat(removeCount) { elements.removeAt(fromIndex) }
+            elements.add(fromIndex, newElement)
 
-        copyAndResetWrapper()
-        repeat(removeCount) { elements.removeAt(fromIndex) }
-        elements.add(fromIndex, newElement)
-
-        finalizeChange(fromIndex, prevSize, removed, inserted = listOf(newElement))
+            finalizeChange(fromIndex, prevSize, removed, inserted = listOf(newElement))
+        }
     }
 
     override fun clear() {
-        emitMightChange()
+        applyChange {
+            val prevSize = elements.size
+            val removed = elementsWrapper
 
-        val prevSize = elements.size
-        val removed = elementsWrapper
+            copyAndResetWrapper()
+            elements.clear()
 
-        copyAndResetWrapper()
-        elements.clear()
-
-        finalizeChange(index = 0, prevSize, removed, inserted = emptyList())
+            finalizeChange(index = 0, prevSize, removed, inserted = emptyList())
+        }
     }
 
     override fun sortWith(comparator: Comparator<E>) {
-        emitMightChange()
+        applyChange {
+            val removed = elementsWrapper
+            copyAndResetWrapper()
+            var throwable: Throwable? = null
 
-        val removed = elementsWrapper
-        copyAndResetWrapper()
-        var throwable: Throwable? = null
+            try {
+                elements.sortWith(comparator)
+            } catch (e: Throwable) {
+                throwable = e
+            }
 
-        try {
-            elements.sortWith(comparator)
-        } catch (e: Throwable) {
-            throwable = e
+            finalizeChange(
+                index = 0,
+                prevSize = elements.size,
+                removed,
+                inserted = elementsWrapper,
+            )
+
+            if (throwable != null) {
+                throw throwable
+            }
         }
-
-        finalizeChange(
-            index = 0,
-            prevSize = elements.size,
-            removed,
-            inserted = elementsWrapper,
-        )
-
-        if (throwable != null) {
-            throw throwable
-        }
-    }
-
-    override fun emitDependencyChanged() {
-        val currentChanges = changes
-        changes = mutableListOf()
-        emitDependencyChangedEvent(ListChangeEvent(elementsWrapper, currentChanges))
     }
 
     private fun checkIndex(index: Int, maxIndex: Int) {
@@ -186,7 +185,9 @@ class SimpleListCell<E>(
         removed: List<E>,
         inserted: List<E>,
     ) {
-        changes.add(ListChange(index, prevSize, removed, inserted))
-        ChangeManager.changed(this)
+        changeEvent = ListChangeEvent(
+            elementsWrapper,
+            listOf(ListChange(index, prevSize, removed, inserted)),
+        )
     }
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/DependencyTests.kt

@@ -7,44 +7,31 @@ interface DependencyTests : ObservableTestSuite {
     fun createProvider(): Provider
 
     @Test
-    fun correctly_emits_changes_to_its_dependents() = test {
+    fun correctly_emits_invalidation_notifications_to_its_dependents() = test {
         val p = createProvider()
-        var dependencyMightChangeCalled = false
-        var dependencyChangedCalled = false
+        var dependencyInvalidatedCalled: Boolean
 
         p.dependency.addDependent(object : Dependent {
-            override fun dependencyMightChange() {
-                assertFalse(dependencyMightChangeCalled)
-                assertFalse(dependencyChangedCalled)
-                dependencyMightChangeCalled = true
-            }
-
-            override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-                assertTrue(dependencyMightChangeCalled)
-                assertFalse(dependencyChangedCalled)
+            override fun dependencyInvalidated(dependency: Dependency<*>) {
                 assertEquals(p.dependency, dependency)
-                assertNotNull(event)
-                dependencyChangedCalled = true
+                dependencyInvalidatedCalled = true
             }
         })
 
         repeat(5) { index ->
-            dependencyMightChangeCalled = false
-            dependencyChangedCalled = false
+            dependencyInvalidatedCalled = false
 
             p.emit()
 
-            assertTrue(dependencyMightChangeCalled, "repetition $index")
-            assertTrue(dependencyChangedCalled, "repetition $index")
+            assertTrue(dependencyInvalidatedCalled, "repetition $index")
         }
     }
 
     interface Provider {
-        val dependency: Dependency
+        val dependency: Dependency<*>
 
         /**
-         * Makes [dependency] emit [Dependent.dependencyMightChange] followed by
-         * [Dependent.dependencyChanged] with a non-null event.
+         * Makes [dependency] call [Dependent.dependencyInvalidated] on its dependents.
          */
         fun emit()
     }

observable/src/commonTest/kotlin/world/phantasmal/observable/ObservableTests.kt

@@ -57,6 +57,6 @@ interface ObservableTests : DependencyTests {
     interface Provider : DependencyTests.Provider {
         val observable: Observable<*>
 
-        override val dependency: Dependency get() = observable
+        override val dependency: Dependency<*> get() = observable
     }
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/CellWithDependenciesTests.kt

@@ -3,7 +3,9 @@ package world.phantasmal.observable.cell
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
-import kotlin.test.*
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
 
 interface CellWithDependenciesTests : CellTests {
     fun createWithDependencies(
@@ -13,39 +15,26 @@ interface CellWithDependenciesTests : CellTests {
     ): Cell<Any>
 
     @Test
-    fun emits_precisely_once_when_all_of_its_dependencies_emit() = test {
+    fun emits_at_least_once_when_all_of_its_dependencies_emit() = test {
         val root = SimpleCell(5)
         val branch1 = DependentCell(root) { root.value * 2 }
         val branch2 = DependentCell(root) { root.value * 3 }
         val branch3 = DependentCell(root) { root.value * 4 }
         val leaf = createWithDependencies(branch1, branch2, branch3)
-        var dependencyMightChangeCalled = false
-        var dependencyChangedCalled = false
+        var dependencyInvalidatedCalled: Boolean
 
         leaf.addDependent(object : Dependent {
-            override fun dependencyMightChange() {
-                assertFalse(dependencyMightChangeCalled)
-                assertFalse(dependencyChangedCalled)
-                dependencyMightChangeCalled = true
-            }
-
-            override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-                assertTrue(dependencyMightChangeCalled)
-                assertFalse(dependencyChangedCalled)
-                assertEquals(leaf, dependency)
-                assertNotNull(event)
-                dependencyChangedCalled = true
+            override fun dependencyInvalidated(dependency: Dependency<*>) {
+                dependencyInvalidatedCalled = true
             }
         })
 
         repeat(5) { index ->
-            dependencyMightChangeCalled = false
-            dependencyChangedCalled = false
+            dependencyInvalidatedCalled = false
 
             root.value += 1
 
-            assertTrue(dependencyMightChangeCalled, "repetition $index")
-            assertTrue(dependencyChangedCalled, "repetition $index")
+            assertTrue(dependencyInvalidatedCalled, "repetition $index")
         }
     }
 
@@ -91,10 +80,6 @@ interface CellWithDependenciesTests : CellTests {
         val publicDependents: List<Dependent> = dependents
 
         override val value: Int = 5
-
-        override fun emitDependencyChanged() {
-            // Not going to change.
-            throw NotImplementedError()
-        }
+        override val changeEvent: ChangeEvent<Int> = ChangeEvent(value)
     }
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/FlatteningDependentCellDirectAndTransitiveDependencyEmitTests.kt

@@ -0,0 +1,20 @@
+package world.phantasmal.observable.cell
+
+/**
+ * In these tests both the direct dependency and the transitive dependency of the
+ * [FlatteningDependentCell] change.
+ */
+class FlatteningDependentCellDirectAndTransitiveDependencyEmitTests : CellTests {
+    override fun createProvider() = Provider()
+
+    class Provider : CellTests.Provider {
+        // This cell is both the direct and transitive dependency.
+        private val dependencyCell = SimpleCell('a')
+
+        override val observable = FlatteningDependentCell(dependencyCell) { dependencyCell }
+
+        override fun emit() {
+            dependencyCell.value += 1
+        }
+    }
+}

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/FlatteningDependentCellWithSimpleListCellTests.kt

@@ -1,17 +0,0 @@
-package world.phantasmal.observable.cell
-
-import world.phantasmal.observable.cell.list.SimpleListCell
-
-class FlatteningDependentCellWithSimpleListCellTests : CellTests {
-    override fun createProvider() = Provider()
-
-    class Provider : CellTests.Provider {
-        private val dependencyCell = SimpleListCell(mutableListOf("a", "b", "c"))
-
-        override val observable = FlatteningDependentCell(dependencyCell) { dependencyCell }
-
-        override fun emit() {
-            dependencyCell.add("x")
-        }
-    }
-}

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/ImmutableCellTests.kt

@@ -1,16 +1,19 @@
 package world.phantasmal.observable.cell
 
-import world.phantasmal.core.disposable.TrackedDisposable
+import world.phantasmal.core.disposable.DisposableTracking
 import world.phantasmal.observable.test.ObservableTestSuite
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
 class ImmutableCellTests : ObservableTestSuite {
+    /**
+     * As an optimization we simply ignore any observers and return a singleton Nop disposable.
+     */
     @Test
     fun observing_it_never_creates_leaks() = test {
         val cell = ImmutableCell("test value")
 
-        TrackedDisposable.checkNoLeaks {
+        DisposableTracking.checkNoLeaks {
             // We never call dispose on the returned disposable.
             cell.observeChange {}
         }

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/MutableCellTests.kt

@@ -70,7 +70,7 @@ interface MutableCellTests<T : Any> : CellTests {
     // TODO: Figure out change set bug and enable change sets again.
     /**
      * Modifying a mutable cell multiple times in one change set results in a single call to
-     * [Dependent.dependencyMightChange] and [Dependent.dependencyChanged].
+     * [Dependent.dependencyInvalidated] and [Dependent.dependencyChanged].
      */
 //    @Test
 //    fun multiple_changes_to_one_cell_in_change_set() = test {

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/RegularCellTests.kt

@@ -1,6 +1,9 @@
 package world.phantasmal.observable.cell
 
-import kotlin.test.*
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
 
 /**
  * Test suite for all [Cell] implementations that aren't ListCells. There is a subclass of this
@@ -9,6 +12,7 @@ import kotlin.test.*
 interface RegularCellTests : CellTests {
     fun <T> createWithValue(value: T): Cell<T>
 
+    // TODO: Move this test to CellTests.
     @Test
     fun convenience_methods() = test {
         listOf(Any(), null).forEach { any ->
@@ -25,6 +29,7 @@ interface RegularCellTests : CellTests {
         }
     }
 
+    // TODO: Move this test to CellTests.
     @Test
     fun generic_extensions() = test {
         listOf(Any(), null).forEach { any ->
@@ -54,6 +59,9 @@ interface RegularCellTests : CellTests {
             assertEquals(a != b, (aCell ne bCell).value)
         }
 
+        testEqNe(null, null)
+        testEqNe(null, Unit)
+        testEqNe(Unit, Unit)
         testEqNe(10, 10)
         testEqNe(5, 99)
         testEqNe("a", "a")

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/list/FilteredListCellListDependencyEmitsTests.kt

@@ -1,12 +1,8 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.observable.cell.Cell
-import world.phantasmal.observable.cell.DependentCell
-import world.phantasmal.observable.cell.ImmutableCell
+import world.phantasmal.observable.cell.*
 
-// TODO: A test suite that tests FilteredListCell while its predicate dependency is changing.
-// TODO: A test suite that tests FilteredListCell while the predicate results are changing.
-class FilteredListCellListDependencyEmitsTests : AbstractFilteredListCellTests {
+class FilteredListCellListDependencyEmitsTests : ListCellTests, CellWithDependenciesTests {
     override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
         private val dependencyCell =
             SimpleListCell(if (empty) mutableListOf(5) else mutableListOf(5, 10))
@@ -14,7 +10,7 @@ class FilteredListCellListDependencyEmitsTests : AbstractFilteredListCellTests {
         override val observable =
             FilteredListCell(
                 list = dependencyCell,
-                predicate = ImmutableCell { ImmutableCell(it % 2 == 0) },
+                predicate = cell { cell(it % 2 == 0) },
             )
 
         override fun addElement() {
@@ -28,14 +24,12 @@ class FilteredListCellListDependencyEmitsTests : AbstractFilteredListCellTests {
         dependency3: Cell<Int>,
     ) =
         FilteredListCell(
-            list = DependentListCell(dependency1, computeElements = {
-                listOf(dependency1.value)
-            }),
-            predicate = DependentCell(dependency2, compute = {
-                fun predicate(element: Int) =
-                    DependentCell(dependency3, compute = { element < dependency2.value })
+            list = dependency1.mapToList { listOf(it) },
+            predicate = dependency2.map { value2 ->
+                fun predicate(element: Int): Cell<Boolean> =
+                    dependency3.map { value3 -> (element % 2) == ((value2 + value3) % 2) }
 
                 ::predicate
-            }),
+            },
         )
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/list/FilteredListCellTests.kt

@@ -0,0 +1,13 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.cell
+import world.phantasmal.observable.cell.map
+
+// TODO: A test suite that tests FilteredListCell while its predicate dependency is changing.
+// TODO: A test suite that tests FilteredListCell while the predicate results are changing.
+// TODO: A test suite that tests FilteredListCell while all 3 types of dependencies are changing.
+class FilteredListCellTests : SuperFilteredListCellTests {
+    override fun <E> createFilteredListCell(list: ListCell<E>, predicate: Cell<(E) -> Boolean>) =
+        FilteredListCell(list, predicate.map { p -> { cell(p(it)) } })
+}

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/list/ImmutableListCellTests.kt

@@ -1,17 +1,20 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.core.disposable.TrackedDisposable
+import world.phantasmal.core.disposable.DisposableTracking
 import world.phantasmal.observable.cell.observeNow
 import world.phantasmal.observable.test.ObservableTestSuite
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
 class ImmutableListCellTests : ObservableTestSuite {
+    /**
+     * As an optimization we simply ignore any observers and return a singleton Nop disposable.
+     */
     @Test
     fun observing_it_never_creates_leaks() = test {
         val listCell = ImmutableListCell(listOf(1, 2, 3))
 
-        TrackedDisposable.checkNoLeaks {
+        DisposableTracking.checkNoLeaks {
             // We never call dispose on the returned disposables.
             listCell.observeChange {}
             listCell.observeListChange {}

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/list/SimpleFilteredListCellListDependencyEmitsTests.kt

@@ -1,21 +1,23 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.observable.cell.Cell
-import world.phantasmal.observable.cell.DependentCell
-import world.phantasmal.observable.cell.ImmutableCell
+import world.phantasmal.observable.cell.CellWithDependenciesTests
+import world.phantasmal.observable.cell.cell
 
 /**
  * In these tests the list dependency of the [SimpleListCell] changes and the predicate
  * dependency does not.
  */
-class SimpleFilteredListCellListDependencyEmitsTests : AbstractFilteredListCellTests {
+class SimpleFilteredListCellListDependencyEmitsTests :
+    ListCellTests, CellWithDependenciesTests {
+
     override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
         private val dependencyCell =
             SimpleListCell(if (empty) mutableListOf(5) else mutableListOf(5, 10))
 
         override val observable = SimpleFilteredListCell(
             list = dependencyCell,
-            predicate = ImmutableCell { it % 2 == 0 },
+            predicate = cell { it % 2 == 0 },
         )
 
         override fun addElement() {
@@ -29,11 +31,9 @@ class SimpleFilteredListCellListDependencyEmitsTests : AbstractFilteredListCellT
         dependency3: Cell<Int>,
     ) =
         SimpleFilteredListCell(
-            list = DependentListCell(dependency1, dependency2) {
-                listOf(dependency1.value, dependency2.value)
-            },
-            predicate = DependentCell(dependency3) {
-                { it < dependency3.value }
+            list = mapToList(dependency1, dependency2, dependency3) { value1, value2, value3 ->
+                listOf(value1, value2, value3)
             },
+            predicate = cell { it % 2 == 0 },
         )
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/list/SimpleFilteredListCellPredicateDependencyEmitsTests.kt

@@ -1,14 +1,17 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.observable.cell.Cell
-import world.phantasmal.observable.cell.DependentCell
+import world.phantasmal.observable.cell.CellWithDependenciesTests
 import world.phantasmal.observable.cell.SimpleCell
+import world.phantasmal.observable.cell.map
 
 /**
  * In these tests the predicate dependency of the [SimpleListCell] changes and the list dependency
  * does not.
  */
-class SimpleFilteredListCellPredicateDependencyEmitsTests : AbstractFilteredListCellTests {
+class SimpleFilteredListCellPredicateDependencyEmitsTests :
+    ListCellTests, CellWithDependenciesTests {
+
     override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
         private var maxValue = if (empty) 0 else 1
         private val predicateCell = SimpleCell<(Int) -> Boolean> { it <= maxValue }
@@ -31,11 +34,9 @@ class SimpleFilteredListCellPredicateDependencyEmitsTests : AbstractFilteredList
         dependency3: Cell<Int>,
     ) =
         SimpleFilteredListCell(
-            list = DependentListCell(dependency1, dependency2) {
-                listOf(dependency1.value, dependency2.value)
-            },
-            predicate = DependentCell(dependency3) {
-                { it < dependency3.value }
+            list = listCell(1, 2, 3, 4, 5, 6, 7, 8, 9),
+            predicate = map(dependency1, dependency2, dependency3) { value1, value2, value3 ->
+                { (it % 2) == ((value1 + value2 + value3) % 2) }
             },
         )
 }

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/list/SimpleFilteredListCellTests.kt

@@ -0,0 +1,10 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.cell.Cell
+
+// TODO: A test suite that tests SimpleFilteredListCell while both types of dependencies are
+//       changing.
+class SimpleFilteredListCellTests : SuperFilteredListCellTests {
+    override fun <E> createFilteredListCell(list: ListCell<E>, predicate: Cell<(E) -> Boolean>) =
+        SimpleFilteredListCell(list, predicate)
+}

observable/src/commonTest/kotlin/world/phantasmal/observable/cell/list/SuperFilteredListCellTests.kt

@@ -1,16 +1,21 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.observable.ChangeManager
-import world.phantasmal.observable.cell.CellWithDependenciesTests
+import world.phantasmal.observable.cell.Cell
 import world.phantasmal.observable.cell.ImmutableCell
 import world.phantasmal.observable.cell.SimpleCell
+import world.phantasmal.observable.test.ObservableTestSuite
 import kotlin.test.*
 
-interface AbstractFilteredListCellTests : ListCellTests, CellWithDependenciesTests {
+/**
+ * Tests that apply to all filtered list implementations.
+ */
+interface SuperFilteredListCellTests : ObservableTestSuite {
+    fun <E> createFilteredListCell(list: ListCell<E>, predicate: Cell<(E) -> Boolean>): ListCell<E>
+
     @Test
     fun contains_only_values_that_match_the_predicate() = test {
         val dep = SimpleListCell(mutableListOf("a", "b"))
-        val list = SimpleFilteredListCell(dep, predicate = ImmutableCell { 'a' in it })
+        val list = createFilteredListCell(dep, predicate = ImmutableCell { 'a' in it })
 
         assertEquals(1, list.value.size)
         assertEquals("a", list.value[0])
@@ -34,7 +39,7 @@ interface AbstractFilteredListCellTests : ListCellTests, CellWithDependenciesTes
     @Test
     fun only_emits_when_necessary() = test {
         val dep = SimpleListCell<Int>(mutableListOf())
-        val list = SimpleFilteredListCell(dep, predicate = ImmutableCell { it % 2 == 0 })
+        val list = createFilteredListCell(dep, predicate = ImmutableCell { it % 2 == 0 })
         var changes = 0
         var listChanges = 0
 
@@ -63,7 +68,7 @@ interface AbstractFilteredListCellTests : ListCellTests, CellWithDependenciesTes
     @Test
     fun emits_correct_change_events() = test {
         val dep = SimpleListCell<Int>(mutableListOf())
-        val list = SimpleFilteredListCell(dep, predicate = ImmutableCell { it % 2 == 0 })
+        val list = createFilteredListCell(dep, predicate = ImmutableCell { it % 2 == 0 })
         var event: ListChangeEvent<Int>? = null
 
         disposer.add(list.observeListChange {
@@ -107,7 +112,7 @@ interface AbstractFilteredListCellTests : ListCellTests, CellWithDependenciesTes
     @Test
     fun value_changes_and_emits_when_predicate_changes() = test {
         val predicate: SimpleCell<(Int) -> Boolean> = SimpleCell { it % 2 == 0 }
-        val list = SimpleFilteredListCell(ImmutableListCell(listOf(1, 2, 3, 4, 5)), predicate)
+        val list = createFilteredListCell(ImmutableListCell(listOf(1, 2, 3, 4, 5)), predicate)
         var event: ListChangeEvent<Int>? = null
 
         disposer.add(list.observeListChange {
@@ -157,38 +162,31 @@ interface AbstractFilteredListCellTests : ListCellTests, CellWithDependenciesTes
     @Test
     fun emits_correctly_when_multiple_changes_happen_at_once() = test {
         val dependency = object : AbstractListCell<Int>() {
-            private val changes: MutableList<Pair<Int, Int>> = mutableListOf()
-            override val elements: MutableList<Int> = mutableListOf()
-            override val value = elements
-
-            override fun emitDependencyChanged() {
-                emitDependencyChangedEvent(ListChangeEvent(
-                    elementsWrapper,
-                    changes.map { (index, newElement) ->
-                        ListChange(
-                            index = index,
-                            prevSize = index,
-                            removed = emptyList(),
-                            inserted = listOf(newElement),
-                        )
-                    }
-                ))
-                changes.clear()
-            }
+            private val elements: MutableList<Int> = mutableListOf()
+            override val value: List<Int> get() = elements
+            override var changeEvent: ListChangeEvent<Int>? = null
+                private set
 
             fun makeChanges(newElements: List<Int>) {
-                emitMightChange()
+                applyChange {
+                    val changes: MutableList<ListChange<Int>> = mutableListOf()
 
-                for (newElement in newElements) {
-                    changes.add(Pair(elements.size, newElement))
-                    elements.add(newElement)
+                    for (newElement in newElements) {
+                        changes.add(ListChange(
+                            index = elements.size,
+                            prevSize = elements.size,
+                            removed = emptyList(),
+                            inserted = listOf(newElement),
+                        ))
+                        elements.add(newElement)
+                    }
+
+                    changeEvent = ListChangeEvent(elements.toList(), changes)
                 }
-
-                ChangeManager.changed(this)
             }
         }
 
-        val list = SimpleFilteredListCell(dependency, ImmutableCell { true })
+        val list = createFilteredListCell(dependency, ImmutableCell { true })
         var event: ListChangeEvent<Int>? = null
 
         disposer.add(list.observeListChange {
@@ -235,7 +233,7 @@ interface AbstractFilteredListCellTests : ListCellTests, CellWithDependenciesTes
         val y = "y"
         val z = "z"
         val dependency = SimpleListCell(mutableListOf(x, y, z, x, y, z))
-        val list = SimpleFilteredListCell(dependency, SimpleCell { it != y })
+        val list = createFilteredListCell(dependency, SimpleCell { it != y })
         var event: ListChangeEvent<String>? = null
 
         disposer.add(list.observeListChange {

observable/src/commonTest/kotlin/world/phantasmal/observable/test/Utils.kt

@@ -5,6 +5,5 @@ import kotlin.test.assertEquals
 
 fun <E> assertListCellEquals(expected: List<E>, actual: ListCell<E>) {
     assertEquals(expected.size, actual.size.value)
-    assertEquals(expected.size, actual.value.size)
     assertEquals(expected, actual.value)
 }

test-utils/src/commonMain/kotlin/world/phantasmal/testUtils/AbstractTestSuite.kt

@@ -1,16 +1,16 @@
 package world.phantasmal.testUtils
 
 import world.phantasmal.core.disposable.Disposer
-import world.phantasmal.core.disposable.TrackedDisposable
+import world.phantasmal.core.disposable.DisposableTracking
 
 interface AbstractTestSuite<Ctx : TestContext> {
     fun test(slow: Boolean = false, testBlock: Ctx.() -> Unit) {
         if (slow && !canExecuteSlowTests()) return
 
-        TrackedDisposable.checkNoLeaks(trackPrecise = true) {
+        DisposableTracking.checkNoLeaks {
             val disposer = Disposer()
 
-            testBlock(createContext(disposer))
+            createContext(disposer).testBlock()
 
             disposer.dispose()
         }
@@ -20,10 +20,10 @@ interface AbstractTestSuite<Ctx : TestContext> {
         world.phantasmal.testUtils.testAsync lambda@{
             if (slow && !canExecuteSlowTests()) return@lambda
 
-            TrackedDisposable.checkNoLeaks(trackPrecise = true) {
+            DisposableTracking.checkNoLeaks {
                 val disposer = Disposer()
 
-                testBlock(createContext(disposer))
+                createContext(disposer).testBlock()
 
                 disposer.dispose()
             }

web/src/main/kotlin/world/phantasmal/web/Main.kt

@@ -15,7 +15,6 @@ import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.Disposer
 import world.phantasmal.core.disposable.TrackedDisposable
 import world.phantasmal.core.disposable.disposable
-import world.phantasmal.observable.cell.mutableCell
 import world.phantasmal.web.application.Application
 import world.phantasmal.web.core.loading.AssetLoader
 import world.phantasmal.web.core.persistence.LocalStorageKeyValueStore
@@ -91,11 +90,21 @@ private fun createThreeRenderer(canvas: HTMLCanvasElement): DisposableThreeRende
 
 private class HistoryApplicationUrl : TrackedDisposable(), ApplicationUrl {
     private val path: String get() = window.location.pathname
+    private val popCallbacks = mutableListOf<(String) -> Unit>()
 
-    override val url = mutableCell(window.location.hash.substring(1))
+    override var pathAndParams = window.location.hash.substring(1)
+        private set
 
     private val popStateListener = window.disposableListener<PopStateEvent>("popstate", {
-        url.value = window.location.hash.substring(1)
+        val newPathAndParams = window.location.hash.substring(1)
+
+        if (newPathAndParams != pathAndParams) {
+            pathAndParams = newPathAndParams
+
+            for (callback in popCallbacks) {
+                callback(newPathAndParams)
+            }
+        }
     })
 
     override fun dispose() {
@@ -103,18 +112,19 @@ private class HistoryApplicationUrl : TrackedDisposable(), ApplicationUrl {
         super.dispose()
     }
 
-    override fun pushUrl(url: String) {
-        window.history.pushState(null, TITLE, "$path#$url")
-        // Do after pushState to avoid triggering observers that call pushUrl or replaceUrl before
-        // the current change has happened.
-        this.url.value = url
+    override fun pushPathAndParams(pathAndParams: String) {
+        this.pathAndParams = pathAndParams
+        window.history.pushState(null, TITLE, "$path#$pathAndParams")
     }
 
-    override fun replaceUrl(url: String) {
-        window.history.replaceState(null, TITLE, "$path#$url")
-        // Do after replaceState to avoid triggering observers that call pushUrl or replaceUrl
-        // before the current change has happened.
-        this.url.value = url
+    override fun replacePathAndParams(pathAndParams: String) {
+        this.pathAndParams = pathAndParams
+        window.history.replaceState(null, TITLE, "$path#$pathAndParams")
+    }
+
+    override fun onPopPathAndParams(callback: (String) -> Unit): Disposable {
+        popCallbacks.add(callback)
+        return disposable { popCallbacks.remove(callback) }
     }
 
     companion object {

web/src/main/kotlin/world/phantasmal/web/core/commands/Command.kt

@@ -1,6 +1,6 @@
-package world.phantasmal.web.core.actions
+package world.phantasmal.web.core.commands
 
-interface Action {
+interface Command {
     val description: String
     fun execute()
     fun undo()

web/src/main/kotlin/world/phantasmal/web/core/controllers/PathAwareTabContainerController.kt

@@ -1,5 +1,8 @@
 package world.phantasmal.web.core.controllers
 
+import kotlinx.browser.window
+import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.map
 import world.phantasmal.web.core.PwToolType
 import world.phantasmal.web.core.stores.UiStore
 import world.phantasmal.webui.controllers.Tab
@@ -12,33 +15,40 @@ interface PathAwareTab : Tab {
 open class PathAwareTabContainerController<T : PathAwareTab>(
     private val uiStore: UiStore,
     private val tool: PwToolType,
-    tabs: List<T>,
-) : TabContainerController<T>(tabs) {
-    init {
-        observeNow(uiStore.path) { path ->
-            if (uiStore.currentTool.value == tool) {
-                tabs.find { path.startsWith(it.path) }?.let {
-                    setActiveTab(it, replaceUrl = true)
-                }
+    final override val tabs: List<T>,
+) : TabContainerController<T>() {
+    final override val activeTab: Cell<T?> =
+        map(uiStore.currentTool, uiStore.path) { currentTool, path ->
+            if (currentTool == tool) {
+                tabs.find { path.startsWith(it.path) } ?: tabs.firstOrNull()
+            } else {
+                null
             }
         }
+
+    init {
+        setPathPrefix(activeTab.value, replace = true)
     }
 
-    override fun setActiveTab(tab: T?, replaceUrl: Boolean) {
-        if (tab != null && uiStore.currentTool.value == tool) {
-            uiStore.setPathPrefix(tab.path, replaceUrl)
-        }
-
-        super.setActiveTab(tab)
+    final override fun setActiveTab(tab: T?) {
+        setPathPrefix(tab, replace = false)
     }
 
-    override fun visibleChanged(visible: Boolean) {
+    final override fun visibleChanged(visible: Boolean) {
         super.visibleChanged(visible)
 
-        if (visible && uiStore.currentTool.value == tool) {
-            activeTab.value?.let {
-                uiStore.setPathPrefix(it.path, replace = true)
-            }
+        if (visible) {
+            // TODO: Remove this hack.
+            window.setTimeout({
+                if (disposed) return@setTimeout
+                setPathPrefix(activeTab.value, replace = true)
+            }, 0)
+        }
+    }
+
+    private fun setPathPrefix(tab: T?, replace: Boolean) {
+        if (tab != null && uiStore.currentTool.value == tool) {
+            uiStore.setPathPrefix(tab.path, replace)
         }
     }
 }

web/src/main/kotlin/world/phantasmal/web/core/loading/AssetLoader.kt

@@ -5,12 +5,14 @@ import io.ktor.client.request.*
 import io.ktor.client.statement.*
 import io.ktor.http.*
 import kotlinx.browser.window
+import kotlinx.coroutines.delay
 import org.khronos.webgl.ArrayBuffer
+import world.phantasmal.web.shared.dto.QuestDto
 
 class AssetLoader(
     val httpClient: HttpClient,
     val origin: String = window.location.origin,
-    val basePath: String = window.location.pathname.removeSuffix("/") + "/assets",
+    val basePath: String = defaultBasePath(),
 ) {
     suspend inline fun <reified T> load(path: String): T =
         httpClient.get("$origin$basePath$path")
@@ -23,4 +25,19 @@ class AssetLoader(
         check(channel.availableForRead == 0) { "Couldn't read all data." }
         return arrayBuffer
     }
+
+    companion object {
+        fun defaultBasePath(): String {
+            val pathname = window.location.pathname
+
+            val appPath =
+                if (pathname.endsWith(".html")) {
+                    pathname.substring(0, pathname.lastIndexOf('/'))
+                } else {
+                    pathname.removeSuffix("/")
+                }
+
+            return "$appPath/assets"
+        }
+    }
 }

web/src/main/kotlin/world/phantasmal/web/core/stores/UiStore.kt

@@ -4,150 +4,96 @@ import kotlinx.browser.window
 import kotlinx.coroutines.launch
 import org.w3c.dom.events.KeyboardEvent
 import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.Disposer
+import world.phantasmal.core.disposable.TrackedDisposable
 import world.phantasmal.core.disposable.disposable
 import world.phantasmal.observable.cell.Cell
-import world.phantasmal.observable.cell.MutableCell
 import world.phantasmal.observable.cell.eq
 import world.phantasmal.observable.cell.mutableCell
 import world.phantasmal.web.core.PwToolType
 import world.phantasmal.web.core.models.Server
+import world.phantasmal.webui.DisposableContainer
 import world.phantasmal.webui.dom.disposableListener
 import world.phantasmal.webui.stores.Store
 
+/**
+ * Represents the current path and parameters without hash (#). E.g. `/viewer/models?model=HUmar`.
+ * In production this string is actually appended to the base URL after the hash. The above path and
+ * parameters would become `https://www.phantasmal.world/#/viewer/models?model=HUmar`.
+ */
 interface ApplicationUrl {
-    val url: Cell<String>
+    val pathAndParams: String
 
-    fun pushUrl(url: String)
+    fun pushPathAndParams(pathAndParams: String)
 
-    fun replaceUrl(url: String)
+    fun replacePathAndParams(pathAndParams: String)
+
+    fun onPopPathAndParams(callback: (String) -> Unit): Disposable
 }
 
-class UiStore(private val applicationUrl: ApplicationUrl) : Store() {
-    private val _currentTool: MutableCell<PwToolType>
+interface Param : Disposable {
+    val value: String?
 
-    private val _path = mutableCell("")
+    fun set(value: String?)
+}
+
+class UiStore(applicationUrl: ApplicationUrl) : Store() {
     private val _server = mutableCell(Server.Ephinea)
 
-    /**
-     * Maps full paths to maps of parameters and their values. In other words we keep track of
-     * parameter values per [applicationUrl].
-     */
-    private val parameters: MutableMap<String, MutableMap<String, MutableCell<String?>>> =
-        mutableMapOf()
+    // TODO: Remove this dependency and add it to each component that actually needs it.
+    private val navigationStore = addDisposable(NavigationStore(applicationUrl))
+
     private val globalKeyDownHandlers: MutableMap<String, suspend (e: KeyboardEvent) -> Unit> =
         mutableMapOf()
 
-    /**
-     * Enabled alpha features. Alpha features can be turned on by adding a features parameter with
-     * the comma-separated feature names as value.
-     * E.g. `/viewer?features=f1,f2,f3`
-     */
-    private val features: MutableSet<String> = mutableSetOf()
-
     private val tools: List<PwToolType> = PwToolType.values().toList()
 
-    /**
-     * The default tool that is loaded.
-     */
-    val defaultTool: PwToolType = PwToolType.Viewer
-
     /**
      * The tool that is currently visible.
      */
-    val currentTool: Cell<PwToolType>
+    val currentTool: Cell<PwToolType> get() = navigationStore.currentTool
 
     /**
      * Map of tools to a boolean cell that says whether they are the current tool or not.
+     * At all times, exactly one of these booleans is true.
      */
-    val toolToActive: Map<PwToolType, Cell<Boolean>>
+    val toolToActive: Map<PwToolType, Cell<Boolean>> =
+        tools.associateWith { tool -> currentTool eq tool }
 
     /**
      * Application URL without the tool path prefix.
+     * E.g. when the full path is `/viewer/models`, [path] will be `/models`.
      */
-    val path: Cell<String> = _path
+    val path: Cell<String> get() = navigationStore.path
 
     /**
      * The private server we're currently showing data and tools for.
      */
-    val server: Cell<Server> = _server
+    val server: Cell<Server> get() = _server
 
     init {
-        _currentTool = mutableCell(defaultTool)
-        currentTool = _currentTool
-
-        toolToActive = tools.associateWith { tool -> currentTool eq tool }
-
         addDisposables(
             window.disposableListener("keydown", ::dispatchGlobalKeyDown),
         )
-
-        observeNow(applicationUrl.url) { setDataFromUrl(it) }
     }
 
     fun setCurrentTool(tool: PwToolType) {
-        if (tool != currentTool.value) {
-            updateApplicationUrl(tool, path = "", replace = false)
-            setCurrentTool(tool, path = "")
-        }
+        navigationStore.setCurrentTool(tool)
     }
 
     /**
      * Updates [path] to [prefix] if the current path doesn't start with [prefix].
      */
     fun setPathPrefix(prefix: String, replace: Boolean) {
-        if (!path.value.startsWith(prefix)) {
-            updateApplicationUrl(currentTool.value, prefix, replace)
-            _path.value = prefix
-        }
+        navigationStore.setPathPrefix(prefix, replace)
     }
 
     fun registerParameter(
         tool: PwToolType,
         path: String,
         parameter: String,
-        setInitialValue: (String?) -> Unit,
-        value: Cell<String?>,
         onChange: (String?) -> Unit,
-    ): Disposable {
-        require(parameter !== FEATURES_PARAM) {
-            "$FEATURES_PARAM can't be set because it is a global parameter."
-        }
-
-        val pathParams = parameters.getOrPut("/${tool.slug}$path", ::mutableMapOf)
-        val param = pathParams.getOrPut(parameter) { mutableCell(null) }
-
-        setInitialValue(param.value)
-
-        value.value.let { v ->
-            if (v != param.value) {
-                setParameter(tool, path, param, v, replaceUrl = true)
-            }
-        }
-
-        return Disposer(
-            value.observeChange {
-                if (it.value != param.value) {
-                    setParameter(tool, path, param, it.value, replaceUrl = false)
-                }
-            },
-            param.observeChange { onChange(it.value) },
-        )
-    }
-
-    private fun setParameter(
-        tool: PwToolType,
-        path: String,
-        parameter: MutableCell<String?>,
-        value: String?,
-        replaceUrl: Boolean,
-    ) {
-        parameter.value = value
-
-        if (this.currentTool.value == tool && this.path.value == path) {
-            updateApplicationUrl(tool, path, replaceUrl)
-        }
-    }
+    ): Param =
+        navigationStore.registerParameter(tool, path, parameter, onChange)
 
     fun onGlobalKeyDown(
         tool: PwToolType,
@@ -164,75 +110,6 @@ class UiStore(private val applicationUrl: ApplicationUrl) : Store() {
         return disposable { globalKeyDownHandlers.remove(key) }
     }
 
-    /**
-     * Sets [currentTool], [path], [parameters] and [features].
-     */
-    private fun setDataFromUrl(url: String) {
-        val urlSplit = url.split("?")
-        val fullPath = urlSplit[0]
-        val paramsStr = urlSplit.getOrNull(1)
-        val secondSlashIdx = fullPath.indexOf("/", 1)
-        val toolStr =
-            if (secondSlashIdx == -1) fullPath.substring(1)
-            else fullPath.substring(1, secondSlashIdx)
-
-        val tool = SLUG_TO_PW_TOOL[toolStr]
-        val path = if (secondSlashIdx == -1) "" else fullPath.substring(secondSlashIdx)
-
-        if (paramsStr != null) {
-            val params = parameters.getOrPut(fullPath, ::mutableMapOf)
-
-            for (p in paramsStr.split("&")) {
-                val (param, value) = p.split("=", limit = 2)
-
-                if (param == FEATURES_PARAM) {
-                    for (feature in value.split(",")) {
-                        features.add(feature)
-                    }
-                } else {
-                    params.getOrPut(param) { mutableCell(value) }.value = value
-                }
-            }
-        }
-
-        val actualTool = tool ?: defaultTool
-        this.setCurrentTool(actualTool, path)
-
-        if (tool == null) {
-            updateApplicationUrl(actualTool, path, replace = true)
-        }
-    }
-
-    private fun setCurrentTool(tool: PwToolType, path: String) {
-        _path.value = path
-        _currentTool.value = tool
-    }
-
-    private fun updateApplicationUrl(tool: PwToolType, path: String, replace: Boolean) {
-        val fullPath = "/${tool.slug}${path}"
-        val params = mutableMapOf<String, String>()
-
-        parameters[fullPath]?.forEach { (k, v) ->
-            v.value?.let { params[k] = it }
-        }
-
-        if (features.isNotEmpty()) {
-            params[FEATURES_PARAM] = features.joinToString(",")
-        }
-
-        val paramStr =
-            if (params.isEmpty()) ""
-            else "?" + params.map { (k, v) -> "$k=$v" }.joinToString("&")
-
-        val url = "${fullPath}${paramStr}"
-
-        if (replace) {
-            applicationUrl.replaceUrl(url)
-        } else {
-            applicationUrl.pushUrl(url)
-        }
-    }
-
     private fun dispatchGlobalKeyDown(e: KeyboardEvent) {
         val bindingParts = mutableListOf<String>()
 
@@ -255,9 +132,226 @@ class UiStore(private val applicationUrl: ApplicationUrl) : Store() {
         return "$tool -> $binding"
     }
 
+    companion object {
+        /**
+         * The default tool that's loaded if the initial URL has no specific path.
+         */
+        val DEFAULT_TOOL: PwToolType = PwToolType.Viewer
+    }
+}
+
+/**
+ * Deconstructs the URL into [currentTool], [path], [parameters] and [features], propagates changes
+ * to the URL (e.g. when the user navigates backward or forward through the browser history) to the
+ * rest of the application and allows the application to update the URL.
+ */
+class NavigationStore(private val applicationUrl: ApplicationUrl) : DisposableContainer() {
+
+    private val _currentTool = mutableCell(UiStore.DEFAULT_TOOL)
+
+    /**
+     * The tool that is currently visible.
+     */
+    val currentTool: Cell<PwToolType> = _currentTool
+
+    private val _path = mutableCell("")
+
+    /**
+     * Application URL without the tool path prefix.
+     * E.g. when the full path is `/viewer/models`, [path] will be `/models`.
+     */
+    val path: Cell<String> = _path
+
+    /**
+     * Maps full paths to maps of parameters and their values. In other words we keep track of
+     * parameter values per full path.
+     */
+    private val parameters: MutableMap<String, MutableMap<String, ParamValue>> =
+        mutableMapOf()
+
+    /**
+     * Enabled alpha features. Alpha features can be turned on by adding a features parameter with
+     * the comma-separated feature names as value.
+     * E.g. `/viewer?features=f1,f2,f3` to enable features f1, f2 and f3.
+     */
+    private val features: MutableSet<String> = mutableSetOf()
+
+    init {
+        deconstructPathAndParams(applicationUrl.pathAndParams)
+        addDisposable(applicationUrl.onPopPathAndParams(::deconstructPathAndParams))
+    }
+
+    fun setCurrentTool(tool: PwToolType) {
+        if (tool != currentTool.value) {
+            updateApplicationUrl(tool, path = "", replace = false)
+            setCurrentTool(tool, path = "")
+        }
+    }
+
+    private fun setCurrentTool(tool: PwToolType, path: String) {
+        _path.value = path
+        _currentTool.value = tool
+    }
+
+    /**
+     * Updates [path] to [prefix] if the current path doesn't start with [prefix].
+     */
+    fun setPathPrefix(prefix: String, replace: Boolean) {
+        if (!path.value.startsWith(prefix)) {
+            updateApplicationUrl(currentTool.value, prefix, replace)
+            _path.value = prefix
+        }
+    }
+
+    fun registerParameter(
+        tool: PwToolType,
+        path: String,
+        parameter: String,
+        onChange: (String?) -> Unit,
+    ): Param {
+        require(parameter !== FEATURES_PARAM) {
+            "$FEATURES_PARAM can't be set because it is a global parameter."
+        }
+
+        val pathParams = parameters.getOrPut("/${tool.slug}$path", ::mutableMapOf)
+        val paramCtx =
+            pathParams.getOrPut(parameter) { ParamValue(value = null, onChange = null) }
+
+        require(paramCtx.onChange == null) {
+            "Parameter $parameter is already registered."
+        }
+
+        return ParamImpl(paramCtx, tool, path, onChange)
+    }
+
+    /**
+     * Sets [currentTool], [path], [parameters] and [features].
+     */
+    private fun deconstructPathAndParams(url: String) {
+        val urlSplit = url.split("?")
+        val fullPath = urlSplit[0]
+        val paramsStr = urlSplit.getOrNull(1)
+        val secondSlashIdx = fullPath.indexOf("/", 1)
+        val toolStr =
+            if (secondSlashIdx == -1) fullPath.substring(1)
+            else fullPath.substring(1, secondSlashIdx)
+
+        val tool = SLUG_TO_PW_TOOL[toolStr]
+        val path = if (secondSlashIdx == -1) "" else fullPath.substring(secondSlashIdx)
+
+        if (paramsStr != null) {
+            val params = parameters.getOrPut(fullPath, ::mutableMapOf)
+
+            for (paramNameAndValue in paramsStr.split("&")) {
+                val paramNameAndValueSplit = paramNameAndValue.split("=", limit = 2)
+                val paramName = paramNameAndValueSplit[0]
+                val value = paramNameAndValueSplit.getOrNull(1)
+
+                if (paramName == FEATURES_PARAM) {
+                    if (value != null) {
+                        for (feature in value.split(",")) {
+                            features.add(feature)
+                        }
+                    }
+                } else {
+                    val param = params[paramName]
+
+                    if (param == null) {
+                        params[paramName] = ParamValue(value, onChange = null)
+                    } else {
+                        param.updateAndCallOnChange(value)
+                    }
+                }
+            }
+        }
+
+        val actualTool = tool ?: UiStore.DEFAULT_TOOL
+        this.setCurrentTool(actualTool, path)
+
+        if (tool == null) {
+            updateApplicationUrl(actualTool, path, replace = true)
+        }
+    }
+
+    // TODO: Use buildString.
+    private fun updateApplicationUrl(tool: PwToolType, path: String, replace: Boolean) {
+        val fullPath = "/${tool.slug}${path}"
+        val params = mutableMapOf<String, String>()
+
+        parameters[fullPath]?.forEach { (k, v) ->
+            v.value?.let { params[k] = it }
+        }
+
+        if (features.isNotEmpty()) {
+            params[FEATURES_PARAM] = features.joinToString(",")
+        }
+
+        val paramStr =
+            if (params.isEmpty()) ""
+            else "?" + params.map { (k, v) -> "$k=$v" }.joinToString("&")
+
+        val url = "${fullPath}${paramStr}"
+
+        if (replace) {
+            applicationUrl.replacePathAndParams(url)
+        } else {
+            applicationUrl.pushPathAndParams(url)
+        }
+    }
+
     companion object {
         private const val FEATURES_PARAM = "features"
         private val SLUG_TO_PW_TOOL: Map<String, PwToolType> =
             PwToolType.values().associateBy { it.slug }
     }
+
+    private class ParamValue(
+        value: String?,
+        var onChange: ((String?) -> Unit)?,
+    ) {
+        var value: String? = value
+            private set
+
+        fun update(value: String?): Boolean {
+            val changed = value != this.value
+            this.value = value
+            return changed
+        }
+
+        fun updateAndCallOnChange(value: String?) {
+            if (update(value)) {
+                onChange?.invoke(value)
+            }
+        }
+    }
+
+    private inner class ParamImpl(
+        private val paramValue: ParamValue,
+        private val tool: PwToolType,
+        private val paramPath: String,
+        onChange: (String?) -> Unit,
+    ) : TrackedDisposable(), Param {
+
+        override val value: String? get() = paramValue.value
+
+        init {
+            require(paramValue.onChange == null)
+            paramValue.onChange = onChange
+        }
+
+        override fun set(value: String?) {
+            // Always update parameter value.
+            if (paramValue.update(value)) {
+                // Only update URL if current part of the tool is visible.
+                if (currentTool.value == tool && path.value == paramPath) {
+                    updateApplicationUrl(tool, paramPath, replace = true)
+                }
+            }
+        }
+
+        override fun dispose() {
+            paramValue.onChange = null
+            super.dispose()
+        }
+    }
 }

web/src/main/kotlin/world/phantasmal/web/core/undo/Undo.kt

@@ -1,21 +1,21 @@
 package world.phantasmal.web.core.undo
 
 import world.phantasmal.observable.cell.Cell
-import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.commands.Command
 
 interface Undo {
     val canUndo: Cell<Boolean>
     val canRedo: Cell<Boolean>
 
     /**
-     * The first action that will be undone when calling undo().
+     * The first command that will be undone when calling undo().
      */
-    val firstUndo: Cell<Action?>
+    val firstUndo: Cell<Command?>
 
     /**
-     * The first action that will be redone when calling redo().
+     * The first command that will be redone when calling redo().
      */
-    val firstRedo: Cell<Action?>
+    val firstRedo: Cell<Command?>
 
     /**
      * True if this undo is at the point in time where the last save happened. See [savePoint].

web/src/main/kotlin/world/phantasmal/web/core/undo/UndoManager.kt

@@ -3,7 +3,7 @@ package world.phantasmal.web.core.undo
 import world.phantasmal.observable.cell.*
 import world.phantasmal.observable.cell.list.fold
 import world.phantasmal.observable.cell.list.mutableListCell
-import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.commands.Command
 
 class UndoManager {
     private val undos = mutableListCell<Undo>(NopUndo)
@@ -13,8 +13,8 @@ class UndoManager {
 
     val canUndo: Cell<Boolean> = current.flatMap { it.canUndo }
     val canRedo: Cell<Boolean> = current.flatMap { it.canRedo }
-    val firstUndo: Cell<Action?> = current.flatMap { it.firstUndo }
-    val firstRedo: Cell<Action?> = current.flatMap { it.firstRedo }
+    val firstUndo: Cell<Command?> = current.flatMap { it.firstUndo }
+    val firstRedo: Cell<Command?> = current.flatMap { it.firstRedo }
 
     /**
      * True if all undos are at the most recent save point. I.e., true if there are no changes to

web/src/main/kotlin/world/phantasmal/web/core/undo/UndoStack.kt

@@ -2,17 +2,17 @@ package world.phantasmal.web.core.undo
 
 import world.phantasmal.observable.cell.*
 import world.phantasmal.observable.cell.list.mutableListCell
-import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.commands.Command
 
 /**
  * Full-fledged linear undo/redo implementation.
  */
 class UndoStack(manager: UndoManager) : Undo {
-    private val stack = mutableListCell<Action>()
+    private val stack = mutableListCell<Command>()
 
     /**
-     * The index where new actions are inserted. If not equal to the [stack]'s size, points to the
-     * action that will be redone when calling [redo].
+     * The index where new commands are inserted. If not equal to the [stack]'s size, points to the
+     * command that will be redone when calling [redo].
      */
     private val index = mutableCell(0)
     private val savePointIndex = mutableCell(0)
@@ -22,9 +22,9 @@ class UndoStack(manager: UndoManager) : Undo {
 
     override val canRedo: Cell<Boolean> = map(stack, index) { stack, index -> index < stack.size }
 
-    override val firstUndo: Cell<Action?> = index.map { stack.value.getOrNull(it - 1) }
+    override val firstUndo: Cell<Command?> = index.map { stack.value.getOrNull(it - 1) }
 
-    override val firstRedo: Cell<Action?> = index.map { stack.value.getOrNull(it) }
+    override val firstRedo: Cell<Command?> = index.map { stack.value.getOrNull(it) }
 
     override val atSavePoint: Cell<Boolean> = index eq savePointIndex
 
@@ -32,13 +32,13 @@ class UndoStack(manager: UndoManager) : Undo {
         manager.addUndo(this)
     }
 
-    fun push(action: Action): Action {
+    fun push(command: Command): Command {
         if (!undoingOrRedoing) {
-            stack.splice(index.value, stack.value.size - index.value, action)
+            stack.splice(index.value, stack.value.size - index.value, command)
             index.value++
         }
 
-        return action
+        return command
     }
 
     override fun undo(): Boolean {

web/src/main/kotlin/world/phantasmal/web/huntOptimizer/stores/HuntMethodStore.kt

@@ -48,7 +48,7 @@ class HuntMethodStore(
         val server = uiStore.server.value
 
         withContext(Dispatchers.Default) {
-            val quests = assetLoader.load<List<QuestDto>>("/quests.${server.slug}.json")
+            val quests: List<QuestDto> = assetLoader.load("/quests.${server.slug}.json")
 
             val methods = quests
                 .asSequence()
@@ -84,7 +84,7 @@ class HuntMethodStore(
                     }
 
                     val duration = when {
-                        quest.name.matches(Regex("""^\d-\d.*""")) ->
+                        quest.name.matches(GOVERNMENT_QUEST_NAME_REGEX) ->
                             DEFAULT_GOVERNMENT_TEST_DURATION
 
                         totalEnemyCount > 400 ->
@@ -117,6 +117,7 @@ class HuntMethodStore(
     }
 
     companion object {
+        private val GOVERNMENT_QUEST_NAME_REGEX = Regex("""^\d-\d.*""")
         private val DEFAULT_DURATION = Duration.minutes(30)
         private val DEFAULT_GOVERNMENT_TEST_DURATION = Duration.minutes(45)
         private val DEFAULT_LARGE_ENEMY_COUNT_DURATION = Duration.minutes(45)

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/CreateEntityAction.kt

@@ -1,25 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.questEditor.models.QuestEntityModel
-import world.phantasmal.web.questEditor.models.QuestModel
-
-class CreateEntityAction(
-    private val setSelectedEntity: (QuestEntityModel<*, *>) -> Unit,
-    private val quest: QuestModel,
-    private val entity: QuestEntityModel<*, *>,
-) : Action {
-    override val description: String = "Add ${entity.type.name}"
-
-    override fun execute() {
-        change {
-            quest.addEntity(entity)
-            setSelectedEntity(entity)
-        }
-    }
-
-    override fun undo() {
-        quest.removeEntity(entity)
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/CreateEventAction.kt

@@ -1,29 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.questEditor.models.QuestEventModel
-import world.phantasmal.web.questEditor.models.QuestModel
-
-class CreateEventAction(
-    private val setSelectedEvent: (QuestEventModel?) -> Unit,
-    private val quest: QuestModel,
-    private val index: Int,
-    private val event: QuestEventModel,
-) : Action {
-    override val description: String = "Add event"
-
-    override fun execute() {
-        change {
-            quest.addEvent(index, event)
-            setSelectedEvent(event)
-        }
-    }
-
-    override fun undo() {
-        change {
-            setSelectedEvent(null)
-            quest.removeEvent(event)
-        }
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/CreateEventActionAction.kt

@@ -1,32 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.questEditor.models.QuestEventActionModel
-import world.phantasmal.web.questEditor.models.QuestEventModel
-
-/**
- * Creates a quest event action.
- */
-class CreateEventActionAction(
-    private val setSelectedEvent: (QuestEventModel) -> Unit,
-    private val event: QuestEventModel,
-    private val action: QuestEventActionModel,
-) : Action {
-    override val description: String =
-        "Add ${action.shortName} action to event ${event.id.value}"
-
-    override fun execute() {
-        change {
-            event.addAction(action)
-            setSelectedEvent(event)
-        }
-    }
-
-    override fun undo() {
-        change {
-            event.removeAction(action)
-            setSelectedEvent(event)
-        }
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/DeleteEntityAction.kt

@@ -1,25 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.questEditor.models.QuestEntityModel
-import world.phantasmal.web.questEditor.models.QuestModel
-
-class DeleteEntityAction(
-    private val setSelectedEntity: (QuestEntityModel<*, *>) -> Unit,
-    private val quest: QuestModel,
-    private val entity: QuestEntityModel<*, *>,
-) : Action {
-    override val description: String = "Delete ${entity.type.name}"
-
-    override fun execute() {
-        quest.removeEntity(entity)
-    }
-
-    override fun undo() {
-        change {
-            quest.addEntity(entity)
-            setSelectedEntity(entity)
-        }
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/DeleteEventAction.kt

@@ -1,29 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.questEditor.models.QuestEventModel
-import world.phantasmal.web.questEditor.models.QuestModel
-
-class DeleteEventAction(
-    private val setSelectedEvent: (QuestEventModel?) -> Unit,
-    private val quest: QuestModel,
-    private val index: Int,
-    private val event: QuestEventModel,
-) : Action {
-    override val description: String = "Delete event ${event.id.value}"
-
-    override fun execute() {
-        change {
-            setSelectedEvent(null)
-            quest.removeEvent(event)
-        }
-    }
-
-    override fun undo() {
-        change {
-            quest.addEvent(index, event)
-            setSelectedEvent(event)
-        }
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/EditEntityPropAction.kt

@@ -1,30 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.questEditor.models.QuestEntityModel
-import world.phantasmal.web.questEditor.models.QuestEntityPropModel
-
-class EditEntityPropAction(
-    private val setSelectedEntity: (QuestEntityModel<*, *>) -> Unit,
-    private val entity: QuestEntityModel<*, *>,
-    private val prop: QuestEntityPropModel,
-    private val newValue: Any,
-    private val oldValue: Any,
-) : Action {
-    override val description: String = "Edit ${entity.type.simpleName} ${prop.name}"
-
-    override fun execute() {
-        change {
-            setSelectedEntity(entity)
-            prop.setValue(newValue)
-        }
-    }
-
-    override fun undo() {
-        change {
-            setSelectedEntity(entity)
-            prop.setValue(oldValue)
-        }
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/EditEventPropertyAction.kt

@@ -1,28 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.questEditor.models.QuestEventModel
-
-class EditEventPropertyAction<T>(
-    override val description: String,
-    private val setSelectedEvent: (QuestEventModel) -> Unit,
-    private val event: QuestEventModel,
-    private val setter: (T) -> Unit,
-    private val newValue: T,
-    private val oldValue: T,
-) : Action {
-    override fun execute() {
-        change {
-            setSelectedEvent(event)
-            setter(newValue)
-        }
-    }
-
-    override fun undo() {
-        change {
-            setSelectedEvent(event)
-            setter(oldValue)
-        }
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/EditPropertyAction.kt

@@ -1,18 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.web.core.actions.Action
-
-class EditPropertyAction<T>(
-    override val description: String,
-    private val setter: (T) -> Unit,
-    private val newValue: T,
-    private val oldValue: T,
-) : Action {
-    override fun execute() {
-        setter(newValue)
-    }
-
-    override fun undo() {
-        setter(oldValue)
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/RotateEntityAction.kt

@@ -1,40 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.externals.three.Euler
-import world.phantasmal.web.questEditor.models.QuestEntityModel
-
-class RotateEntityAction(
-    private val setSelectedEntity: (QuestEntityModel<*, *>) -> Unit,
-    private val entity: QuestEntityModel<*, *>,
-    private val newRotation: Euler,
-    private val oldRotation: Euler,
-    private val world: Boolean,
-) : Action {
-    override val description: String = "Rotate ${entity.type.simpleName}"
-
-    override fun execute() {
-        change {
-            setSelectedEntity(entity)
-
-            if (world) {
-                entity.setWorldRotation(newRotation)
-            } else {
-                entity.setRotation(newRotation)
-            }
-        }
-    }
-
-    override fun undo() {
-        change {
-            setSelectedEntity(entity)
-
-            if (world) {
-                entity.setWorldRotation(oldRotation)
-            } else {
-                entity.setRotation(oldRotation)
-            }
-        }
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/actions/TranslateEntityAction.kt

@@ -1,38 +0,0 @@
-package world.phantasmal.web.questEditor.actions
-
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
-import world.phantasmal.web.externals.three.Vector3
-import world.phantasmal.web.questEditor.models.QuestEntityModel
-
-class TranslateEntityAction(
-    private val setSelectedEntity: (QuestEntityModel<*, *>) -> Unit,
-    private val setEntitySection: (Int) -> Unit,
-    private val entity: QuestEntityModel<*, *>,
-    private val newSection: Int?,
-    private val oldSection: Int?,
-    private val newPosition: Vector3,
-    private val oldPosition: Vector3,
-) : Action {
-    override val description: String = "Move ${entity.type.simpleName}"
-
-    override fun execute() {
-        change {
-            setSelectedEntity(entity)
-
-            newSection?.let(setEntitySection)
-
-            entity.setPosition(newPosition)
-        }
-    }
-
-    override fun undo() {
-        change {
-            setSelectedEntity(entity)
-
-            oldSection?.let(setEntitySection)
-
-            entity.setPosition(oldPosition)
-        }
-    }
-}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/CreateEntityCommand.kt

@@ -0,0 +1,22 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEntityModel
+import world.phantasmal.web.questEditor.models.QuestModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class CreateEntityCommand(
+    private val questEditorStore: QuestEditorStore,
+    private val quest: QuestModel,
+    private val entity: QuestEntityModel<*, *>,
+) : Command {
+    override val description: String = "Add ${entity.type.name}"
+
+    override fun execute() {
+        questEditorStore.addEntity(quest, entity)
+    }
+
+    override fun undo() {
+        questEditorStore.removeEntity(quest, entity)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/CreateEventActionCommand.kt

@@ -0,0 +1,26 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEventActionModel
+import world.phantasmal.web.questEditor.models.QuestEventModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+/**
+ * Creates a quest event action.
+ */
+class CreateEventActionCommand(
+    private val questEditorStore: QuestEditorStore,
+    private val event: QuestEventModel,
+    private val action: QuestEventActionModel,
+) : Command {
+    override val description: String =
+        "Add ${action.shortName} action to event ${event.id.value}"
+
+    override fun execute() {
+        questEditorStore.addEventAction(event, action)
+    }
+
+    override fun undo() {
+        questEditorStore.removeEventAction(event, action)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/CreateEventCommand.kt

@@ -0,0 +1,23 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEventModel
+import world.phantasmal.web.questEditor.models.QuestModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class CreateEventCommand(
+    private val questEditorStore: QuestEditorStore,
+    private val quest: QuestModel,
+    private val index: Int,
+    private val event: QuestEventModel,
+) : Command {
+    override val description: String = "Add event"
+
+    override fun execute() {
+        questEditorStore.addEvent(quest, index, event)
+    }
+
+    override fun undo() {
+        questEditorStore.removeEvent(quest, event)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/DeleteEntityCommand.kt

@@ -0,0 +1,22 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEntityModel
+import world.phantasmal.web.questEditor.models.QuestModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class DeleteEntityCommand(
+    private val questEditorStore: QuestEditorStore,
+    private val quest: QuestModel,
+    private val entity: QuestEntityModel<*, *>,
+) : Command {
+    override val description: String = "Delete ${entity.type.name}"
+
+    override fun execute() {
+        questEditorStore.removeEntity(quest, entity)
+    }
+
+    override fun undo() {
+        questEditorStore.addEntity(quest, entity)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/DeleteEventActionCommand.kt

@@ -1,33 +1,27 @@
-package world.phantasmal.web.questEditor.actions
+package world.phantasmal.web.questEditor.commands
 
-import world.phantasmal.observable.change
-import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.commands.Command
 import world.phantasmal.web.questEditor.models.QuestEventActionModel
 import world.phantasmal.web.questEditor.models.QuestEventModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
 
 /**
  * Deletes a quest event action.
  */
-class DeleteEventActionAction(
-    private val setSelectedEvent: (QuestEventModel) -> Unit,
+class DeleteEventActionCommand(
+    private val questEditorStore: QuestEditorStore,
     private val event: QuestEventModel,
     private val index: Int,
     private val action: QuestEventActionModel,
-) : Action {
+) : Command {
     override val description: String =
         "Remove ${action.shortName} action from event ${event.id.value}"
 
     override fun execute() {
-        change {
-            setSelectedEvent(event)
-            event.removeAction(action)
-        }
+        questEditorStore.removeEventAction(event, action)
     }
 
     override fun undo() {
-        change {
-            setSelectedEvent(event)
-            event.addAction(index, action)
-        }
+        questEditorStore.addEventAction(event, index, action)
     }
 }

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/DeleteEventCommand.kt

@@ -0,0 +1,23 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEventModel
+import world.phantasmal.web.questEditor.models.QuestModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class DeleteEventCommand(
+    private val questEditorStore: QuestEditorStore,
+    private val quest: QuestModel,
+    private val index: Int,
+    private val event: QuestEventModel,
+) : Command {
+    override val description: String = "Delete event ${event.id.value}"
+
+    override fun execute() {
+        questEditorStore.removeEvent(quest, event)
+    }
+
+    override fun undo() {
+        questEditorStore.addEvent(quest, index, event)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/EditEntityPropCommand.kt

@@ -0,0 +1,27 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEntityModel
+import world.phantasmal.web.questEditor.models.QuestEntityPropModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+/**
+ * Edits a dynamic entity property.
+ */
+class EditEntityPropCommand(
+    private val questEditorStore: QuestEditorStore,
+    private val entity: QuestEntityModel<*, *>,
+    private val prop: QuestEntityPropModel,
+    private val newValue: Any,
+    private val oldValue: Any,
+) : Command {
+    override val description: String = "Edit ${entity.type.simpleName} ${prop.name}"
+
+    override fun execute() {
+        questEditorStore.setEntityProp(entity, prop, newValue)
+    }
+
+    override fun undo() {
+        questEditorStore.setEntityProp(entity, prop, oldValue)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/EditEntityPropertyCommand.kt

@@ -0,0 +1,25 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEntityModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+/**
+ * Edits a simple entity property.
+ */
+class EditEntityPropertyCommand<Entity : QuestEntityModel<*, *>, T>(
+    private val questEditorStore: QuestEditorStore,
+    override val description: String,
+    private val entity: Entity,
+    private val setter: (Entity, T) -> Unit,
+    private val newValue: T,
+    private val oldValue: T,
+) : Command {
+    override fun execute() {
+        questEditorStore.setEntityProperty(entity, setter, newValue)
+    }
+
+    override fun undo() {
+        questEditorStore.setEntityProperty(entity, setter, oldValue)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/EditEntitySectionCommand.kt

@@ -1,16 +1,18 @@
-package world.phantasmal.web.questEditor.actions
+package world.phantasmal.web.questEditor.commands
 
-import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.commands.Command
 import world.phantasmal.web.questEditor.models.QuestEntityModel
 import world.phantasmal.web.questEditor.models.SectionModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
 
-class EditEntitySectionAction(
+class EditEntitySectionCommand(
+    private val questEditorStore: QuestEditorStore,
     private val entity: QuestEntityModel<*, *>,
     private val newSectionId: Int,
     private val newSection: SectionModel?,
     private val oldSectionId: Int,
     private val oldSection: SectionModel?,
-) : Action {
+) : Command {
     override val description: String = "Edit ${entity.type.simpleName} section"
 
     init {
@@ -20,17 +22,17 @@ class EditEntitySectionAction(
 
     override fun execute() {
         if (newSection != null) {
-            entity.setSection(newSection)
+            questEditorStore.setEntitySection(entity, newSection)
         } else {
-            entity.setSectionId(newSectionId)
+            questEditorStore.setEntitySectionId(entity, newSectionId)
         }
     }
 
     override fun undo() {
         if (oldSection != null) {
-            entity.setSection(oldSection)
+            questEditorStore.setEntitySection(entity, oldSection)
         } else {
-            entity.setSectionId(oldSectionId)
+            questEditorStore.setEntitySectionId(entity, oldSectionId)
         }
     }
 }

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/EditEventActionPropertyCommand.kt

@@ -0,0 +1,24 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEventActionModel
+import world.phantasmal.web.questEditor.models.QuestEventModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class EditEventActionPropertyCommand<EventAction : QuestEventActionModel, T>(
+    private val questEditorStore: QuestEditorStore,
+    override val description: String,
+    private val event: QuestEventModel,
+    private val action: EventAction,
+    private val setter: (EventAction, T) -> Unit,
+    private val newValue: T,
+    private val oldValue: T,
+) : Command {
+    override fun execute() {
+        questEditorStore.setEventActionProperty(event, action, setter, newValue)
+    }
+
+    override fun undo() {
+        questEditorStore.setEventActionProperty(event, action, setter, oldValue)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/EditEventPropertyCommand.kt

@@ -0,0 +1,22 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestEventModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class EditEventPropertyCommand<T>(
+    private val questEditorStore: QuestEditorStore,
+    override val description: String,
+    private val event: QuestEventModel,
+    private val setter: (QuestEventModel, T) -> Unit,
+    private val newValue: T,
+    private val oldValue: T,
+) : Command {
+    override fun execute() {
+        questEditorStore.setEventProperty(event, setter, newValue)
+    }
+
+    override fun undo() {
+        questEditorStore.setEventProperty(event, setter, oldValue)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/EditQuestPropertyCommand.kt

@@ -0,0 +1,22 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.questEditor.models.QuestModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class EditQuestPropertyCommand<T>(
+    private val questEditorStore: QuestEditorStore,
+    override val description: String,
+    private val quest: QuestModel,
+    private val setter: (QuestModel, T) -> Unit,
+    private val newValue: T,
+    private val oldValue: T,
+) : Command {
+    override fun execute() {
+        questEditorStore.setQuestProperty(quest, setter, newValue)
+    }
+
+    override fun undo() {
+        questEditorStore.setQuestProperty(quest, setter, oldValue)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/RotateEntityCommand.kt

@@ -0,0 +1,32 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.externals.three.Euler
+import world.phantasmal.web.questEditor.models.QuestEntityModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class RotateEntityCommand(
+    private val questEditorStore: QuestEditorStore,
+    private val entity: QuestEntityModel<*, *>,
+    private val newRotation: Euler,
+    private val oldRotation: Euler,
+    private val world: Boolean,
+) : Command {
+    override val description: String = "Rotate ${entity.type.simpleName}"
+
+    override fun execute() {
+        if (world) {
+            questEditorStore.setEntityWorldRotation(entity, newRotation)
+        } else {
+            questEditorStore.setEntityRotation(entity, newRotation)
+        }
+    }
+
+    override fun undo() {
+        if (world) {
+            questEditorStore.setEntityWorldRotation(entity, oldRotation)
+        } else {
+            questEditorStore.setEntityRotation(entity, oldRotation)
+        }
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/commands/TranslateEntityCommand.kt

@@ -0,0 +1,25 @@
+package world.phantasmal.web.questEditor.commands
+
+import world.phantasmal.web.core.commands.Command
+import world.phantasmal.web.externals.three.Vector3
+import world.phantasmal.web.questEditor.models.QuestEntityModel
+import world.phantasmal.web.questEditor.stores.QuestEditorStore
+
+class TranslateEntityCommand(
+    private val questEditorStore: QuestEditorStore,
+    private val entity: QuestEntityModel<*, *>,
+    private val newSection: Int?,
+    private val oldSection: Int?,
+    private val newPosition: Vector3,
+    private val oldPosition: Vector3,
+) : Command {
+    override val description: String = "Move ${entity.type.simpleName}"
+
+    override fun execute() {
+        questEditorStore.setEntityPosition(entity, newSection, newPosition)
+    }
+
+    override fun undo() {
+        questEditorStore.setEntityPosition(entity, oldSection, oldPosition)
+    }
+}

web/src/main/kotlin/world/phantasmal/web/questEditor/controllers/EntityInfoController.kt

@@ -11,7 +11,7 @@ import world.phantasmal.psolib.fileFormats.quest.EntityPropType
 import world.phantasmal.web.core.euler
 import world.phantasmal.web.externals.three.Euler
 import world.phantasmal.web.externals.three.Vector3
-import world.phantasmal.web.questEditor.actions.*
+import world.phantasmal.web.questEditor.commands.*
 import world.phantasmal.web.questEditor.models.QuestEntityModel
 import world.phantasmal.web.questEditor.models.QuestEntityPropModel
 import world.phantasmal.web.questEditor.models.QuestNpcModel
@@ -28,8 +28,8 @@ sealed class EntityInfoPropModel(
     protected fun setPropValue(prop: QuestEntityPropModel, value: Any) {
         store.selectedEntity.value?.let { entity ->
             store.executeAction(
-                EditEntityPropAction(
-                    setSelectedEntity = store::setSelectedEntity,
+                EditEntityPropCommand(
+                    store,
                     entity,
                     prop,
                     value,
@@ -142,7 +142,8 @@ class EntityInfoController(
                     sectionId,
                 )
                 questEditorStore.executeAction(
-                    EditEntitySectionAction(
+                    EditEntitySectionCommand(
+                        questEditorStore,
                         entity,
                         sectionId,
                         section,
@@ -157,9 +158,11 @@ class EntityInfoController(
     fun setWaveId(waveId: Int) {
         (questEditorStore.selectedEntity.value as? QuestNpcModel)?.let { npc ->
             questEditorStore.executeAction(
-                EditPropertyAction(
+                EditEntityPropertyCommand(
+                    questEditorStore,
                     "Edit ${npc.type.simpleName} wave",
-                    npc::setWaveId,
+                    npc,
+                    QuestNpcModel::setWaveId,
                     waveId,
                     npc.wave.value.id,
                 )
@@ -192,9 +195,8 @@ class EntityInfoController(
         if (!enabled.value) return
 
         questEditorStore.executeAction(
-            TranslateEntityAction(
-                setSelectedEntity = questEditorStore::setSelectedEntity,
-                setEntitySection = { /* Won't be called. */ },
+            TranslateEntityCommand(
+                questEditorStore,
                 entity,
                 newSection = null,
                 oldSection = null,
@@ -229,12 +231,12 @@ class EntityInfoController(
         if (!enabled.value) return
 
         questEditorStore.executeAction(
-            RotateEntityAction(
-                setSelectedEntity = questEditorStore::setSelectedEntity,
+            RotateEntityCommand(
+                questEditorStore,
                 entity,
                 euler(x, y, z),
                 entity.rotation.value,
-                false,
+                world = false,
             )
         )
     }

web/src/main/kotlin/world/phantasmal/web/questEditor/controllers/EventsController.kt

@@ -3,7 +3,7 @@ package world.phantasmal.web.questEditor.controllers
 import world.phantasmal.observable.cell.*
 import world.phantasmal.observable.cell.list.ListCell
 import world.phantasmal.observable.cell.list.listCell
-import world.phantasmal.web.questEditor.actions.*
+import world.phantasmal.web.questEditor.commands.*
 import world.phantasmal.web.questEditor.models.QuestEventActionModel
 import world.phantasmal.web.questEditor.models.QuestEventModel
 import world.phantasmal.web.questEditor.stores.QuestEditorStore
@@ -48,8 +48,8 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
                 else quest.events.value.indexOf(selectedEvent) + 1
 
             store.executeAction(
-                CreateEventAction(
-                    ::selectEvent,
+                CreateEventCommand(
+                    store,
                     quest,
                     index,
                     QuestEventModel(
@@ -78,7 +78,7 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
 
             if (index != -1) {
                 store.executeAction(
-                    DeleteEventAction(::selectEvent, quest, index, event)
+                    DeleteEventCommand(store, quest, index, event)
                 )
             }
         }
@@ -86,11 +86,11 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
 
     fun setId(event: QuestEventModel, id: Int) {
         store.executeAction(
-            EditEventPropertyAction(
+            EditEventPropertyCommand(
+                store,
                 "Edit ID of event ${event.id.value}",
-                ::selectEvent,
                 event,
-                event::setId,
+                QuestEventModel::setId,
                 id,
                 event.id.value,
             )
@@ -99,11 +99,11 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
 
     fun setSectionId(event: QuestEventModel, sectionId: Int) {
         store.executeAction(
-            EditEventPropertyAction(
+            EditEventPropertyCommand(
+                store,
                 "Edit section of event ${event.id.value}",
-                ::selectEvent,
                 event,
-                event::setSectionId,
+                QuestEventModel::setSectionId,
                 sectionId,
                 event.sectionId.value,
             )
@@ -112,11 +112,11 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
 
     fun setWaveId(event: QuestEventModel, waveId: Int) {
         store.executeAction(
-            EditEventPropertyAction(
+            EditEventPropertyCommand(
+                store,
                 "Edit wave of event ${event.id}",
-                ::selectEvent,
                 event,
-                event::setWaveId,
+                QuestEventModel::setWaveId,
                 waveId,
                 event.wave.value.id,
             )
@@ -125,11 +125,11 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
 
     fun setDelay(event: QuestEventModel, delay: Int) {
         store.executeAction(
-            EditEventPropertyAction(
+            EditEventPropertyCommand(
+                store,
                 "Edit delay of event ${event.id}",
-                ::selectEvent,
                 event,
-                event::setDelay,
+                QuestEventModel::setDelay,
                 delay,
                 event.delay.value,
             )
@@ -145,12 +145,12 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
             else -> error("""Unknown action type "$type".""")
         }
 
-        store.executeAction(CreateEventActionAction(::selectEvent, event, action))
+        store.executeAction(CreateEventActionCommand(store, event, action))
     }
 
     fun removeAction(event: QuestEventModel, action: QuestEventActionModel) {
         val index = event.actions.value.indexOf(action)
-        store.executeAction(DeleteEventActionAction(::selectEvent, event, index, action))
+        store.executeAction(DeleteEventActionCommand(store, event, index, action))
     }
 
     fun canGoToEvent(eventId: Cell<Int>): Cell<Boolean> = store.canGoToEvent(eventId)
@@ -165,11 +165,11 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
         sectionId: Int,
     ) {
         store.executeAction(
-            EditEventPropertyAction(
+            EditEventPropertyCommand(
+                store,
                 "Edit action section",
-                ::selectEvent,
                 event,
-                action::setSectionId,
+                QuestEventModel::setSectionId,
                 sectionId,
                 action.sectionId.value,
             )
@@ -182,11 +182,12 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
         appearFlag: Int,
     ) {
         store.executeAction(
-            EditEventPropertyAction(
+            EditEventActionPropertyCommand(
+                store,
                 "Edit action appear flag",
-                ::selectEvent,
                 event,
-                action::setAppearFlag,
+                action,
+                QuestEventActionModel.SpawnNpcs::setAppearFlag,
                 appearFlag,
                 action.appearFlag.value,
             )
@@ -199,11 +200,12 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
         doorId: Int,
     ) {
         store.executeAction(
-            EditEventPropertyAction(
+            EditEventActionPropertyCommand(
+                store,
                 "Edit action door",
-                ::selectEvent,
                 event,
-                action::setDoorId,
+                action,
+                QuestEventActionModel.Door::setDoorId,
                 doorId,
                 action.doorId.value,
             )
@@ -216,11 +218,12 @@ class EventsController(private val store: QuestEditorStore) : Controller() {
         eventId: Int,
     ) {
         store.executeAction(
-            EditEventPropertyAction(
+            EditEventActionPropertyCommand(
+                store,
                 "Edit action event",
-                ::selectEvent,
                 event,
-                action::setEventId,
+                action,
+                QuestEventActionModel.TriggerEvent::setEventId,
                 eventId,
                 action.eventId.value,
             )

web/src/main/kotlin/world/phantasmal/web/questEditor/controllers/QuestEditorToolbarController.kt

@@ -3,10 +3,10 @@ package world.phantasmal.web.questEditor.controllers
 import kotlinx.coroutines.await
 import mu.KotlinLogging
 import world.phantasmal.core.*
+import world.phantasmal.observable.cell.*
 import world.phantasmal.psolib.Endianness
 import world.phantasmal.psolib.Episode
 import world.phantasmal.psolib.fileFormats.quest.*
-import world.phantasmal.observable.cell.*
 import world.phantasmal.web.core.PwToolType
 import world.phantasmal.web.core.files.cursor
 import world.phantasmal.web.core.files.writeBuffer
@@ -74,16 +74,17 @@ class QuestEditorToolbarController(
 
     // Undo
 
-    val undoTooltip: Cell<String> = questEditorStore.firstUndo.map { action ->
-        (action?.let { "Undo \"${action.description}\"" } ?: "Nothing to undo") + " (Ctrl-Z)"
+    val undoTooltip: Cell<String> = questEditorStore.firstUndo.map { command ->
+        (command?.let { "Undo \"${command.description}\"" } ?: "Nothing to undo") + " (Ctrl-Z)"
     }
 
     val undoEnabled: Cell<Boolean> = questEditorStore.canUndo
 
     // Redo
 
-    val redoTooltip: Cell<String> = questEditorStore.firstRedo.map { action ->
-        (action?.let { "Redo \"${action.description}\"" } ?: "Nothing to redo") + " (Ctrl-Shift-Z)"
+    val redoTooltip: Cell<String> = questEditorStore.firstRedo.map { command ->
+        (command?.let { "Redo \"${command.description}\"" } ?: "Nothing to redo") +
+                " (Ctrl-Shift-Z)"
     }
 
     val redoEnabled: Cell<Boolean> = questEditorStore.canRedo

web/src/main/kotlin/world/phantasmal/web/questEditor/controllers/QuestInfoController.kt

@@ -1,7 +1,8 @@
 package world.phantasmal.web.questEditor.controllers
 
 import world.phantasmal.observable.cell.*
-import world.phantasmal.web.questEditor.actions.EditPropertyAction
+import world.phantasmal.web.questEditor.commands.EditQuestPropertyCommand
+import world.phantasmal.web.questEditor.models.QuestModel
 import world.phantasmal.web.questEditor.stores.QuestEditorStore
 import world.phantasmal.webui.controllers.Controller
 
@@ -25,7 +26,14 @@ class QuestInfoController(private val store: QuestEditorStore) : Controller() {
         if (!enabled.value) return
 
         store.currentQuest.value?.let { quest ->
-            store.executeAction(EditPropertyAction("Edit ID", quest::setId, id, quest.id.value))
+            store.executeAction(EditQuestPropertyCommand(
+                store,
+                "Edit ID",
+                quest,
+                QuestModel::setId,
+                id,
+                quest.id.value,
+            ))
         }
     }
 
@@ -34,7 +42,14 @@ class QuestInfoController(private val store: QuestEditorStore) : Controller() {
 
         store.currentQuest.value?.let { quest ->
             store.executeAction(
-                EditPropertyAction("Edit name", quest::setName, name, quest.name.value)
+                EditQuestPropertyCommand(
+                    store,
+                    "Edit name",
+                    quest,
+                    QuestModel::setName,
+                    name,
+                    quest.name.value,
+                )
             )
         }
     }
@@ -44,9 +59,11 @@ class QuestInfoController(private val store: QuestEditorStore) : Controller() {
 
         store.currentQuest.value?.let { quest ->
             store.executeAction(
-                EditPropertyAction(
+                EditQuestPropertyCommand(
+                    store,
                     "Edit short description",
-                    quest::setShortDescription,
+                    quest,
+                    QuestModel::setShortDescription,
                     shortDescription,
                     quest.shortDescription.value,
                 )
@@ -59,9 +76,11 @@ class QuestInfoController(private val store: QuestEditorStore) : Controller() {
 
         store.currentQuest.value?.let { quest ->
             store.executeAction(
-                EditPropertyAction(
+                EditQuestPropertyCommand(
+                    store,
                     "Edit long description",
-                    quest::setLongDescription,
+                    quest,
+                    QuestModel::setLongDescription,
                     longDescription,
                     quest.longDescription.value,
                 )

web/src/main/kotlin/world/phantasmal/web/questEditor/rendering/input/QuestInputManager.kt

@@ -84,7 +84,6 @@ class QuestInputManager(
         stateContext = StateContext(questEditorStore, renderContext, cameraInputManager)
         state = IdleState(stateContext, entityManipulationEnabled)
 
-        observeNow(questEditorStore.selectedEntity) { returnToIdleState() }
         observeNow(questEditorStore.questEditingEnabled) { entityManipulationEnabled = it }
 
         pointerTrap.className = "pw-quest-editor-input-manager-pointer-trap"
@@ -176,6 +175,7 @@ class QuestInputManager(
                 renderContext.canvas.disposableListener("pointermove", ::onPointerMove)
 
             window.setTimeout({
+                if (disposed) return@setTimeout
                 if (!pointerDragging) {
                     pointerTrap.hidden = true
                     contextMenuListener?.dispose()

web/src/main/kotlin/world/phantasmal/web/questEditor/rendering/input/state/CreationState.kt

@@ -79,7 +79,7 @@ class CreationState(
 
             is EntityDragLeaveEvt -> {
                 event.showDragElement()
-                quest.removeEntity(entity)
+                ctx.removeEntity(quest, entity)
                 IdleState(ctx, entityManipulationEnabled = true)
             }
 
@@ -115,7 +115,7 @@ class CreationState(
     }
 
     override fun cancel() {
-        quest.removeEntity(entity)
+        ctx.removeEntity(quest, entity)
     }
 
     companion object {

web/src/main/kotlin/world/phantasmal/web/questEditor/rendering/input/state/IdleState.kt

@@ -35,7 +35,7 @@ class IdleState(
                     val entity = ctx.selectedEntity.value
 
                     if (quest != null && entity != null && event.key == "Delete") {
-                        ctx.deleteEntity(quest, entity)
+                        ctx.finalizeEntityDelete(quest, entity)
                     }
                 }
             }

web/src/main/kotlin/world/phantasmal/web/questEditor/rendering/input/state/StateContext.kt

@@ -11,10 +11,10 @@ import world.phantasmal.web.core.plusAssign
 import world.phantasmal.web.core.rendering.OrbitalCameraInputManager
 import world.phantasmal.web.core.rendering.conversion.fingerPrint
 import world.phantasmal.web.externals.three.*
-import world.phantasmal.web.questEditor.actions.CreateEntityAction
-import world.phantasmal.web.questEditor.actions.DeleteEntityAction
-import world.phantasmal.web.questEditor.actions.RotateEntityAction
-import world.phantasmal.web.questEditor.actions.TranslateEntityAction
+import world.phantasmal.web.questEditor.commands.CreateEntityCommand
+import world.phantasmal.web.questEditor.commands.DeleteEntityCommand
+import world.phantasmal.web.questEditor.commands.RotateEntityCommand
+import world.phantasmal.web.questEditor.commands.TranslateEntityCommand
 import world.phantasmal.web.questEditor.loading.AreaUserData
 import world.phantasmal.web.questEditor.models.*
 import world.phantasmal.web.questEditor.rendering.QuestRenderContext
@@ -128,9 +128,8 @@ class StateContext(
         newPosition: Vector3,
         oldPosition: Vector3,
     ) {
-        questEditorStore.executeAction(TranslateEntityAction(
-            ::setSelectedEntity,
-            { questEditorStore.setEntitySection(entity, it) },
+        questEditorStore.executeAction(TranslateEntityCommand(
+            questEditorStore,
             entity,
             newSection?.id,
             oldSection?.id,
@@ -186,8 +185,8 @@ class StateContext(
         newRotation: Euler,
         oldRotation: Euler,
     ) {
-        questEditorStore.executeAction(RotateEntityAction(
-            ::setSelectedEntity,
+        questEditorStore.executeAction(RotateEntityCommand(
+            questEditorStore,
             entity,
             newRotation,
             oldRotation,
@@ -196,16 +195,20 @@ class StateContext(
     }
 
     fun finalizeEntityCreation(quest: QuestModel, entity: QuestEntityModel<*, *>) {
-        questEditorStore.pushAction(CreateEntityAction(
-            ::setSelectedEntity,
+        questEditorStore.pushAction(CreateEntityCommand(
+            questEditorStore,
             quest,
             entity,
         ))
     }
 
-    fun deleteEntity(quest: QuestModel, entity: QuestEntityModel<*, *>) {
-        questEditorStore.executeAction(DeleteEntityAction(
-            ::setSelectedEntity,
+    fun removeEntity(quest: QuestModel, entity: QuestEntityModel<*, *>) {
+        questEditorStore.removeEntity(quest, entity)
+    }
+
+    fun finalizeEntityDelete(quest: QuestModel, entity: QuestEntityModel<*, *>) {
+        questEditorStore.executeAction(DeleteEntityCommand(
+            questEditorStore,
             quest,
             entity,
         ))

web/src/main/kotlin/world/phantasmal/web/questEditor/stores/AsmStore.kt

@@ -5,12 +5,12 @@ import kotlinx.coroutines.launch
 import world.phantasmal.core.Severity
 import world.phantasmal.core.disposable.Disposer
 import world.phantasmal.core.disposable.disposable
-import world.phantasmal.psolib.asm.assemble
-import world.phantasmal.psolib.asm.disassemble
 import world.phantasmal.observable.Observable
 import world.phantasmal.observable.cell.Cell
 import world.phantasmal.observable.cell.list.ListCell
 import world.phantasmal.observable.cell.mutableCell
+import world.phantasmal.psolib.asm.assemble
+import world.phantasmal.psolib.asm.disassemble
 import world.phantasmal.web.core.undo.UndoManager
 import world.phantasmal.web.externals.monacoEditor.*
 import world.phantasmal.web.questEditor.asm.AsmAnalyser
@@ -111,43 +111,48 @@ class AsmStore(
     }
 
     private fun setTextModel(quest: QuestModel?, inlineStackArgs: Boolean) {
-        setBytecodeIrTimeout?.let { it ->
-            window.clearTimeout(it)
-            setBytecodeIrTimeout = null
-        }
-
-        modelDisposer.disposeAll()
-
-        quest ?: return
-
-        val asm = disassemble(quest.bytecodeIr, inlineStackArgs)
-        asmAnalyser.setAsm(asm, inlineStackArgs)
-
-        _textModel.value = createModel(asm.joinToString("\n"), ASM_LANG_ID).also { model ->
-            modelDisposer.add(disposable { model.dispose() })
-
-            model.onDidChangeContent { e ->
-                asmAnalyser.updateAsm(e.changes.map {
-                    AsmChange(
-                        AsmRange(
-                            it.range.startLineNumber,
-                            it.range.startColumn,
-                            it.range.endLineNumber,
-                            it.range.endColumn,
-                        ),
-                        it.text,
-                    )
-                })
-
-                setBytecodeIrTimeout?.let(window::clearTimeout)
-                setBytecodeIrTimeout = window.setTimeout(::setBytecodeIr, 1000)
-
-                // TODO: Update breakpoints.
+        // TODO: Remove this hack.
+        window.setTimeout({
+            setBytecodeIrTimeout?.let { it ->
+                window.clearTimeout(it)
+                setBytecodeIrTimeout = null
             }
-        }
+
+            modelDisposer.disposeAll()
+
+            quest ?: return@setTimeout
+
+            val asm = disassemble(quest.bytecodeIr, inlineStackArgs)
+            asmAnalyser.setAsm(asm, inlineStackArgs)
+
+            _textModel.value = createModel(asm.joinToString("\n"), ASM_LANG_ID).also { model ->
+                modelDisposer.add(disposable { model.dispose() })
+
+                model.onDidChangeContent { e ->
+                    asmAnalyser.updateAsm(e.changes.map {
+                        AsmChange(
+                            AsmRange(
+                                it.range.startLineNumber,
+                                it.range.startColumn,
+                                it.range.endLineNumber,
+                                it.range.endColumn,
+                            ),
+                            it.text,
+                        )
+                    })
+
+                    setBytecodeIrTimeout?.let(window::clearTimeout)
+                    setBytecodeIrTimeout = window.setTimeout(::setBytecodeIr, 1000)
+
+                    // TODO: Update breakpoints.
+                }
+            }
+        }, 0)
     }
 
     private fun setBytecodeIr() {
+        if (disposed) return
+
         setBytecodeIrTimeout = null
 
         val model = textModel.value ?: return

web/src/main/kotlin/world/phantasmal/web/questEditor/stores/QuestEditorStore.kt

@@ -2,17 +2,20 @@ package world.phantasmal.web.questEditor.stores
 
 import kotlinx.coroutines.launch
 import mu.KotlinLogging
-import world.phantasmal.psolib.Episode
 import world.phantasmal.observable.cell.*
 import world.phantasmal.observable.cell.list.ListCell
 import world.phantasmal.observable.cell.list.emptyListCell
 import world.phantasmal.observable.cell.list.filtered
 import world.phantasmal.observable.cell.list.flatMapToList
+import world.phantasmal.observable.change
+import world.phantasmal.psolib.Episode
 import world.phantasmal.web.core.PwToolType
-import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.commands.Command
 import world.phantasmal.web.core.stores.UiStore
 import world.phantasmal.web.core.undo.UndoManager
 import world.phantasmal.web.core.undo.UndoStack
+import world.phantasmal.web.externals.three.Euler
+import world.phantasmal.web.externals.three.Vector3
 import world.phantasmal.web.questEditor.QuestRunner
 import world.phantasmal.web.questEditor.loading.QuestLoader
 import world.phantasmal.web.questEditor.models.*
@@ -74,9 +77,9 @@ class QuestEditorStore(
     val questEditingEnabled: Cell<Boolean> = currentQuest.isNotNull() and !runner.running
 
     val canUndo: Cell<Boolean> = questEditingEnabled and undoManager.canUndo
-    val firstUndo: Cell<Action?> = undoManager.firstUndo
+    val firstUndo: Cell<Command?> = undoManager.firstUndo
     val canRedo: Cell<Boolean> = questEditingEnabled and undoManager.canRedo
-    val firstRedo: Cell<Action?> = undoManager.firstRedo
+    val firstRedo: Cell<Command?> = undoManager.firstRedo
 
     /**
      * True if there have been changes since the last save.
@@ -100,22 +103,6 @@ class QuestEditorStore(
             }
         }
 
-        observeNow(currentQuest.flatMap { it?.npcs ?: emptyListCell() }) { npcs ->
-            val selected = selectedEntity.value
-
-            if (selected is QuestNpcModel && selected !in npcs) {
-                _selectedEntity.value = null
-            }
-        }
-
-        observeNow(currentQuest.flatMap { it?.objects ?: emptyListCell() }) { objects ->
-            val selected = selectedEntity.value
-
-            if (selected is QuestObjectModel && selected !in objects) {
-                _selectedEntity.value = null
-            }
-        }
-
         if (initializeNewQuest) {
             scope.launch { setCurrentQuest(getDefaultQuest(Episode.I)) }
         }
@@ -166,6 +153,14 @@ class QuestEditorStore(
     suspend fun getDefaultQuest(episode: Episode): QuestModel =
         convertQuestToModel(questLoader.loadDefaultQuest(episode), areaStore::getVariant)
 
+    fun <T> setQuestProperty(
+        quest: QuestModel,
+        setter: (QuestModel, T) -> Unit,
+        value: T,
+    ) {
+        setter(quest, value)
+    }
+
     fun setCurrentArea(area: AreaModel?) {
         val event = selectedEvent.value
 
@@ -178,6 +173,20 @@ class QuestEditorStore(
         _currentArea.value = area
     }
 
+    fun addEvent(quest: QuestModel, index: Int, event: QuestEventModel) {
+        change {
+            quest.addEvent(index, event)
+            setSelectedEvent(event)
+        }
+    }
+
+    fun removeEvent(quest: QuestModel, event: QuestEventModel) {
+        change {
+            setSelectedEvent(null)
+            quest.removeEvent(event)
+        }
+    }
+
     fun setSelectedEvent(event: QuestEventModel?) {
         event?.let {
             val wave = event.wave.value
@@ -204,6 +213,50 @@ class QuestEditorStore(
         _selectedEvent.value = event
     }
 
+    fun <T> setEventProperty(
+        event: QuestEventModel,
+        setter: (QuestEventModel, T) -> Unit,
+        value: T,
+    ) {
+        change {
+            setSelectedEvent(event)
+            setter(event, value)
+        }
+    }
+
+    fun addEventAction(event: QuestEventModel, action: QuestEventActionModel) {
+        change {
+            setSelectedEvent(event)
+            event.addAction(action)
+        }
+    }
+
+    fun addEventAction(event: QuestEventModel, index: Int, action: QuestEventActionModel) {
+        change {
+            setSelectedEvent(event)
+            event.addAction(index, action)
+        }
+    }
+
+    fun removeEventAction(event: QuestEventModel, action: QuestEventActionModel) {
+        change {
+            setSelectedEvent(event)
+            event.removeAction(action)
+        }
+    }
+
+    fun <Action : QuestEventActionModel, T> setEventActionProperty(
+        event: QuestEventModel,
+        action: Action,
+        setter: (Action, T) -> Unit,
+        value: T,
+    ) {
+        change {
+            setSelectedEvent(event)
+            setter(action, value)
+        }
+    }
+
     fun setHighlightedEntity(entity: QuestEntityModel<*, *>?) {
         _highlightedEntity.value = entity
     }
@@ -218,6 +271,63 @@ class QuestEditorStore(
         _selectedEntity.value = entity
     }
 
+    fun addEntity(quest: QuestModel, entity: QuestEntityModel<*, *>) {
+        change {
+            quest.addEntity(entity)
+            setSelectedEntity(entity)
+        }
+    }
+
+    fun removeEntity(quest: QuestModel, entity: QuestEntityModel<*, *>) {
+        change {
+            if (entity == _selectedEntity.value) {
+                _selectedEntity.value = null
+            }
+
+            quest.removeEntity(entity)
+        }
+    }
+
+    fun setEntityPosition(entity: QuestEntityModel<*, *>, sectionId: Int?, position: Vector3) {
+        change {
+            setSelectedEntity(entity)
+            sectionId?.let { setEntitySection(entity, it) }
+            entity.setPosition(position)
+        }
+    }
+
+    fun setEntityRotation(entity: QuestEntityModel<*, *>, rotation: Euler) {
+        change {
+            setSelectedEntity(entity)
+            entity.setRotation(rotation)
+        }
+    }
+
+    fun setEntityWorldRotation(entity: QuestEntityModel<*, *>, rotation: Euler) {
+        change {
+            setSelectedEntity(entity)
+            entity.setWorldRotation(rotation)
+        }
+    }
+
+    fun <Entity : QuestEntityModel<*, *>, T> setEntityProperty(
+        entity: Entity,
+        setter: (Entity, T) -> Unit,
+        value: T,
+    ) {
+        change {
+            setSelectedEntity(entity)
+            setter(entity, value)
+        }
+    }
+
+    fun setEntityProp(entity: QuestEntityModel<*, *>, prop: QuestEntityPropModel, value: Any) {
+        change {
+            setSelectedEntity(entity)
+            prop.setValue(value)
+        }
+    }
+
     suspend fun setMapDesignations(mapDesignations: Map<Int, Int>) {
         currentQuest.value?.let { quest ->
             quest.setMapDesignations(mapDesignations)
@@ -225,6 +335,24 @@ class QuestEditorStore(
         }
     }
 
+    fun setEntitySectionId(entity: QuestEntityModel<*, *>, sectionId: Int) {
+        change {
+            setSelectedEntity(entity)
+            entity.setSectionId(sectionId)
+        }
+    }
+
+    fun setEntitySection(entity: QuestEntityModel<*, *>, section: SectionModel) {
+        change {
+            setSelectedEntity(entity)
+            entity.setSection(section)
+        }
+    }
+
+    /**
+     * Sets [QuestEntityModel.sectionId] and [QuestEntityModel.section] if there's a section with
+     * [sectionId] as ID.
+     */
     fun setEntitySection(entity: QuestEntityModel<*, *>, sectionId: Int) {
         currentQuest.value?.let { quest ->
             val variant = quest.areaVariants.value.find { it.area.id == entity.areaId }
@@ -242,12 +370,12 @@ class QuestEditorStore(
         }
     }
 
-    fun executeAction(action: Action) {
-        pushAction(action)
-        action.execute()
+    fun executeAction(command: Command) {
+        pushAction(command)
+        command.execute()
     }
 
-    fun pushAction(action: Action) {
+    fun pushAction(command: Command) {
         require(questEditingEnabled.value) {
             val reason = when {
                 currentQuest.value == null -> " (no current quest)"
@@ -256,7 +384,7 @@ class QuestEditorStore(
             }
             "Quest editing is disabled at the moment$reason."
         }
-        mainUndo.push(action)
+        mainUndo.push(command)
     }
 
     fun setShowCollisionGeometry(show: Boolean) {

web/src/main/kotlin/world/phantasmal/web/questEditor/undo/TextModelUndo.kt

@@ -1,12 +1,13 @@
 package world.phantasmal.web.questEditor.undo
 
+import kotlinx.browser.window
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.TrackedDisposable
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Observable
 import world.phantasmal.observable.cell.*
 import world.phantasmal.observable.emitter
-import world.phantasmal.web.core.actions.Action
+import world.phantasmal.web.core.commands.Command
 import world.phantasmal.web.core.undo.Undo
 import world.phantasmal.web.core.undo.UndoManager
 import world.phantasmal.web.externals.monacoEditor.IDisposable
@@ -17,8 +18,8 @@ class TextModelUndo(
     private val description: String,
     model: Cell<ITextModel?>,
 ) : Undo, TrackedDisposable() {
-    private val action = object : Action {
-        override val description: String = this@TextModelUndo.description
+    private val command = object : Command {
+        override val description: String get() = this@TextModelUndo.description
 
         override fun execute() {
             _didRedo.emit(ChangeEvent(Unit))
@@ -43,8 +44,8 @@ class TextModelUndo(
     override val canUndo: Cell<Boolean> = _canUndo
     override val canRedo: Cell<Boolean> = _canRedo
 
-    override val firstUndo: Cell<Action?> = canUndo.map { if (it) action else null }
-    override val firstRedo: Cell<Action?> = canRedo.map { if (it) action else null }
+    override val firstUndo: Cell<Command?> = canUndo.map { if (it) command else null }
+    override val firstRedo: Cell<Command?> = canRedo.map { if (it) command else null }
 
     override val atSavePoint: Cell<Boolean> = savePointVersionId eq currentVersionId
 
@@ -63,56 +64,61 @@ class TextModelUndo(
     }
 
     private fun onModelChange(model: ITextModel?) {
-        modelChangeObserver?.dispose()
+        // TODO: Remove this hack.
+        window.setTimeout({
+            if (disposed) return@setTimeout
 
-        if (model == null) {
-            reset()
-            return
-        }
+            modelChangeObserver?.dispose()
 
-        _canUndo.value = false
-        _canRedo.value = false
-
-        val initialVersionId = model.getAlternativeVersionId()
-        currentVersionId.value = initialVersionId
-        savePointVersionId.value = initialVersionId
-        var lastVersionId = initialVersionId
-
-        modelChangeObserver = model.onDidChangeContent {
-            val versionId = model.getAlternativeVersionId()
-            val prevVersionId = currentVersionId.value!!
-
-            if (versionId < prevVersionId) {
-                // Undoing.
-                _canRedo.value = true
-
-                if (versionId == initialVersionId) {
-                    _canUndo.value = false
-                }
-            } else {
-                // Redoing.
-                if (versionId <= lastVersionId) {
-                    if (versionId == lastVersionId) {
-                        _canRedo.value = false
-                    }
-                } else {
-                    _canRedo.value = false
-
-                    if (prevVersionId > lastVersionId) {
-                        lastVersionId = prevVersionId
-                    }
-                }
-
-                _canUndo.value = true
+            if (model == null) {
+                reset()
+                return@setTimeout
             }
 
-            currentVersionId.value = versionId
-        }
+            _canUndo.value = false
+            _canRedo.value = false
+
+            val initialVersionId = model.getAlternativeVersionId()
+            currentVersionId.value = initialVersionId
+            savePointVersionId.value = initialVersionId
+            var lastVersionId = initialVersionId
+
+            modelChangeObserver = model.onDidChangeContent {
+                val versionId = model.getAlternativeVersionId()
+                val prevVersionId = currentVersionId.value!!
+
+                if (versionId < prevVersionId) {
+                    // Undoing.
+                    _canRedo.value = true
+
+                    if (versionId == initialVersionId) {
+                        _canUndo.value = false
+                    }
+                } else {
+                    if (versionId <= lastVersionId) {
+                        // Redoing.
+                        if (versionId == lastVersionId) {
+                            _canRedo.value = false
+                        }
+                    } else {
+                        _canRedo.value = false
+
+                        if (prevVersionId > lastVersionId) {
+                            lastVersionId = prevVersionId
+                        }
+                    }
+
+                    _canUndo.value = true
+                }
+
+                currentVersionId.value = versionId
+            }
+        }, 0)
     }
 
     override fun undo(): Boolean =
         if (canUndo.value) {
-            action.undo()
+            command.undo()
             true
         } else {
             false
@@ -120,7 +126,7 @@ class TextModelUndo(
 
     override fun redo(): Boolean =
         if (canRedo.value) {
-            action.execute()
+            command.execute()
             true
         } else {
             false

web/src/main/kotlin/world/phantasmal/web/questEditor/widgets/AsmEditorWidget.kt

@@ -1,5 +1,6 @@
 package world.phantasmal.web.questEditor.widgets
 
+import kotlinx.browser.window
 import org.w3c.dom.Node
 import world.phantasmal.core.disposable.disposable
 import world.phantasmal.web.externals.monacoEditor.*
@@ -62,20 +63,32 @@ class AsmEditorWidget(private val ctrl: AsmEditorController) : Widget() {
             // Undo/redo.
             observe(ctrl.didUndo) {
                 editor.focus()
-                editor.trigger(
-                    source = AsmEditorWidget::class.simpleName,
-                    handlerId = "undo",
-                    payload = undefined,
-                )
+
+                // TODO: Remove this hack.
+                window.setTimeout({
+                    if (disposed) return@setTimeout
+
+                    editor.trigger(
+                        source = AsmEditorWidget::class.simpleName,
+                        handlerId = "undo",
+                        payload = undefined,
+                    )
+                }, 0)
             }
 
             observe(ctrl.didRedo) {
                 editor.focus()
-                editor.trigger(
-                    source = AsmEditorWidget::class.simpleName,
-                    handlerId = "redo",
-                    payload = undefined,
-                )
+
+                // TODO: Remove this hack.
+                window.setTimeout({
+                    if (disposed) return@setTimeout
+
+                    editor.trigger(
+                        source = AsmEditorWidget::class.simpleName,
+                        handlerId = "redo",
+                        payload = undefined,
+                    )
+                }, 0)
             }
 
             editor.onDidFocusEditorWidget(ctrl::makeUndoCurrent)