Observables will now always see a consistent view of their dependencies when they change.

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

@@ -1,14 +1,14 @@
 package world.phantasmal.core.unsafe
 
 /**
- * Asserts that receiver is of type T. No runtime check happens in KJS. Should only be used when
- * absolutely certain that receiver is indeed a T.
+ * Asserts that [value] is of type T. No runtime check happens in KJS. Should only be used when it's
+ * absolutely certain that [value] is indeed a T.
  */
-expect inline fun <T> Any?.unsafeCast(): T
+expect inline fun <T> unsafeCast(value: Any?): T
 
 /**
- * Asserts that T is not null. No runtime check happens in KJS. Should only be used when absolutely
- * certain that T is indeed not null.
+ * Asserts that [value] is not null. No runtime check happens in KJS. Should only be used when it's
+ * absolutely certain that [value] is indeed not null.
  */
 @Suppress("NOTHING_TO_INLINE")
-inline fun <T> T?.unsafeAssertNotNull(): T = unsafeCast()
+inline fun <T> unsafeAssertNotNull(value: T?): T = unsafeCast(value)

core/src/jsMain/kotlin/world/phantasmal/core/unsafe/UnsafeCast.kt

@@ -1,6 +1,4 @@
 package world.phantasmal.core.unsafe
 
-import kotlin.js.unsafeCast as kotlinUnsafeCast
-
 @Suppress("NOTHING_TO_INLINE")
-actual inline fun <T> Any?.unsafeCast(): T = kotlinUnsafeCast<T>()
+actual inline fun <T> unsafeCast(value: Any?): T = value.unsafeCast<T>()

core/src/jvmMain/kotlin/world/phantasmal/core/unsafe/UnsafeCast.kt

@@ -3,4 +3,4 @@
 package world.phantasmal.core.unsafe
 
 @Suppress("UNCHECKED_CAST", "NOTHING_TO_INLINE")
-actual inline fun <T> Any?.unsafeCast(): T = this as T
+actual inline fun <T> unsafeCast(value: Any?): T = value as T

lib/src/commonMain/kotlin/world/phantasmal/lib/asm/BytecodeIr.kt

@@ -138,7 +138,7 @@ class Instruction(
             }
         }
 
-        return paramToArgs.unsafeAssertNotNull()[paramIndex]
+        return unsafeAssertNotNull(paramToArgs)[paramIndex]
     }
 
     /**

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

@@ -0,0 +1,13 @@
+package world.phantasmal.observable
+
+abstract class AbstractDependency : Dependency {
+    protected val dependents: MutableList<Dependent> = mutableListOf()
+
+    override fun addDependent(dependent: Dependent) {
+        dependents.add(dependent)
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        dependents.remove(dependent)
+    }
+}

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

@@ -0,0 +1,28 @@
+package world.phantasmal.observable
+
+import world.phantasmal.core.disposable.TrackedDisposable
+import world.phantasmal.core.unsafe.unsafeCast
+
+class CallbackObserver<T, E : ChangeEvent<T>>(
+    private val dependency: Dependency,
+    private val callback: (E) -> Unit,
+) : TrackedDisposable(), Dependent {
+    init {
+        dependency.addDependent(this)
+    }
+
+    override fun dispose() {
+        dependency.removeDependent(this)
+        super.dispose()
+    }
+
+    override fun dependencyMightChange() {
+        // Do nothing.
+    }
+
+    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+        if (event != null) {
+            callback(unsafeCast(event))
+        }
+    }
+}

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

@@ -0,0 +1,14 @@
+package world.phantasmal.observable
+
+interface Dependency {
+    /**
+     * This method is not meant to be called from typical application code. Usually you'll want to
+     * use [Observable.observe].
+     */
+    fun addDependent(dependent: Dependent)
+
+    /**
+     * This method is not meant to be called from typical application code.
+     */
+    fun removeDependent(dependent: Dependent)
+}

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

@@ -0,0 +1,21 @@
+package world.phantasmal.observable
+
+interface Dependent {
+    /**
+     * 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
+     * know that it will actually change, just that it might change. Always call [dependencyChanged]
+     * 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 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.
+     */
+    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<*>?)
+}

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

@@ -2,6 +2,6 @@ package world.phantasmal.observable
 
 import world.phantasmal.core.disposable.Disposable
 
-interface Observable<out T> {
+interface Observable<out T> : Dependency {
     fun observe(observer: Observer<T>): Disposable
 }

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

@@ -4,4 +4,4 @@ open class ChangeEvent<out T>(val value: T) {
     operator fun component1() = value
 }
 
-typealias Observer<T> = (event: ChangeEvent<T>) -> Unit
+typealias Observer<T> = (ChangeEvent<T>) -> Unit

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

@@ -1,20 +1,18 @@
 package world.phantasmal.observable
 
 import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
-
-class SimpleEmitter<T> : Emitter<T> {
-    private val observers = mutableListOf<Observer<T>>()
-
-    override fun observe(observer: Observer<T>): Disposable {
-        observers.add(observer)
-
-        return disposable {
-            observers.remove(observer)
-        }
-    }
 
+class SimpleEmitter<T> : AbstractDependency(), Emitter<T> {
     override fun emit(event: ChangeEvent<T>) {
-        observers.forEach { it(event) }
+        for (dependent in dependents) {
+            dependent.dependencyMightChange()
         }
+
+        for (dependent in dependents) {
+            dependent.dependencyChanged(this, event)
+        }
+    }
+
+    override fun observe(observer: Observer<T>): Disposable =
+        CallbackObserver(this, observer)
 }

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

@@ -1,30 +1,42 @@
 package world.phantasmal.observable.cell
 
 import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
+import world.phantasmal.observable.AbstractDependency
+import world.phantasmal.observable.CallbackObserver
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Observer
 
-abstract class AbstractCell<T> : Cell<T> {
-    protected val observers: MutableList<Observer<T>> = mutableListOf()
+abstract class AbstractCell<T> : AbstractDependency(), Cell<T> {
+    private var mightChangeEmitted = false
 
     final override fun observe(observer: Observer<T>): Disposable =
         observe(callNow = false, observer)
 
     override fun observe(callNow: Boolean, observer: Observer<T>): Disposable {
-        observers.add(observer)
+        val observingCell = CallbackObserver(this, observer)
 
         if (callNow) {
             observer(ChangeEvent(value))
         }
 
-        return disposable {
-            observers.remove(observer)
+        return observingCell
+    }
+
+    protected fun emitMightChange() {
+        if (!mightChangeEmitted) {
+            mightChangeEmitted = true
+
+            for (dependent in dependents) {
+                dependent.dependencyMightChange()
+            }
         }
     }
 
-    protected fun emit() {
-        val event = ChangeEvent(value)
-        observers.forEach { it(event) }
+    protected fun emitChanged(event: ChangeEvent<T>?) {
+        mightChangeEmitted = false
+
+        for (dependent in dependents) {
+            dependent.dependencyChanged(this, event)
+        }
     }
 }

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

@@ -1,71 +1,33 @@
 package world.phantasmal.observable.cell
 
-import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
-import world.phantasmal.core.unsafe.unsafeCast
-import world.phantasmal.observable.Observer
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
 
-/**
- * Starts observing its dependencies when the first observer on this cell is registered. Stops
- * observing its dependencies when the last observer ov this cell is disposed. This way no extra
- * disposables need to be managed when e.g. [map] is used.
- */
-abstract class AbstractDependentCell<T>(
-    private vararg val dependencies: Cell<*>,
-) : AbstractCell<T>() {
-    /**
-     * Is either empty or has a disposable per dependency.
-     */
-    private val dependencyObservers = mutableListOf<Disposable>()
+abstract class AbstractDependentCell<T> : AbstractCell<T>(), Dependent {
+    private var changingDependencies = 0
+    private var dependenciesActuallyChanged = false
 
-    /**
-     * Set to true right before actual observers are added.
-     */
-    protected var hasObservers = false
-
-    protected var _value: T? = null
-
-    override val value: T
-        get() {
-            if (!hasObservers) {
-                _value = computeValue()
+    override fun dependencyMightChange() {
+        changingDependencies++
+        emitMightChange()
     }
 
-            return _value.unsafeCast()
+    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+        if (event != null) {
+            dependenciesActuallyChanged = true
         }
 
-    override fun observe(callNow: Boolean, observer: Observer<T>): Disposable {
-        if (dependencyObservers.isEmpty()) {
-            hasObservers = true
+        if (--changingDependencies == 0) {
+            if (dependenciesActuallyChanged) {
+                dependenciesActuallyChanged = false
 
-            dependencies.forEach { dependency ->
-                dependencyObservers.add(
-                    dependency.observe {
-                        val oldValue = _value
-                        _value = computeValue()
-
-                        if (_value != oldValue) {
-                            emit()
-                        }
-                    }
-                )
-            }
-
-            _value = computeValue()
-        }
-
-        val superDisposable = super.observe(callNow, observer)
-
-        return disposable {
-            superDisposable.dispose()
-
-            if (observers.isEmpty()) {
-                hasObservers = false
-                dependencyObservers.forEach { it.dispose() }
-                dependencyObservers.clear()
+                dependenciesChanged()
+            } else {
+                emitChanged(null)
             }
         }
     }
 
-    protected abstract fun computeValue(): T
+    abstract fun dependenciesChanged()
 }

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

@@ -16,7 +16,7 @@ interface Cell<out T> : Observable<T> {
     operator fun getValue(thisRef: Any?, property: KProperty<*>): T = value
 
     /**
-     * @param callNow Call [observer] immediately with the current [mutableCell].
+     * @param callNow Call [observer] immediately with the current [value].
      */
     fun observe(callNow: Boolean = false, observer: Observer<T>): Disposable
 

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

@@ -59,3 +59,6 @@ fun Cell<String>.isBlank(): Cell<Boolean> =
 
 fun Cell<String>.isNotBlank(): Cell<Boolean> =
     map { it.isNotBlank() }
+
+fun <T> Cell<Cell<T>>.flatten(): Cell<T> =
+    FlatteningDependentCell(this) { this.value }

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

@@ -1,5 +1,7 @@
 package world.phantasmal.observable.cell
 
+import world.phantasmal.observable.ChangeEvent
+
 class DelegatingCell<T>(
     private val getter: () -> T,
     private val setter: (T) -> Unit,
@@ -10,8 +12,11 @@ class DelegatingCell<T>(
             val oldValue = getter()
 
             if (value != oldValue) {
+                emitMightChange()
+
                 setter(value)
-                emit()
+
+                emitChanged(ChangeEvent(value))
             }
         }
 }

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

@@ -1,11 +1,58 @@
 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
+
 /**
  * Cell of which the value depends on 0 or more other cells.
  */
 class DependentCell<T>(
-    vararg dependencies: Cell<*>,
-    private val compute: () -> T,
-) : AbstractDependentCell<T>(*dependencies) {
-    override fun computeValue(): T = compute()
+    private vararg val dependencies: Dependency,
+    private val compute: () -> T
+) : AbstractDependentCell<T>() {
+
+    private var _value: T? = null
+    override val value: T
+        get() {
+            if (dependents.isEmpty()) {
+                _value = compute()
+            }
+
+            return unsafeCast(_value)
+        }
+
+    override fun addDependent(dependent: Dependent) {
+        if (dependents.isEmpty()) {
+            _value = compute()
+
+            for (dependency in dependencies) {
+                dependency.addDependent(this)
+            }
+        }
+
+        super.addDependent(dependent)
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
+
+        if (dependents.isEmpty()) {
+            for (dependency in dependencies) {
+                dependency.removeDependent(this)
+            }
+        }
+    }
+
+    override fun dependenciesChanged() {
+        val newValue = compute()
+
+        if (newValue != _value) {
+            _value = newValue
+            emitChanged(ChangeEvent(newValue))
+        } else {
+            emitChanged(null)
+        }
+    }
 }

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

@@ -1,56 +1,90 @@
 package world.phantasmal.observable.cell
 
-import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
 import world.phantasmal.core.unsafe.unsafeAssertNotNull
-import world.phantasmal.observable.Observer
+import world.phantasmal.core.unsafe.unsafeCast
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
 
 /**
  * Similar to [DependentCell], except that this cell's [compute] returns a cell.
  */
 class FlatteningDependentCell<T>(
-    vararg dependencies: Cell<*>,
-    private val compute: () -> Cell<T>,
-) : AbstractDependentCell<T>(*dependencies) {
-    private var computedCell: Cell<T>? = null
-    private var computedCellObserver: Disposable? = null
+    private vararg val dependencies: Dependency,
+    private val compute: () -> Cell<T>
+) : AbstractDependentCell<T>() {
 
+    private var computedCell: Cell<T>? = null
+    private var computedInDeps = false
+    private var shouldRecompute = false
+
+    private var _value: T? = null
     override val value: T
         get() {
-            return if (hasObservers) {
-                computedCell.unsafeAssertNotNull().value
-            } else {
-                super.value
+            if (dependents.isEmpty()) {
+                _value = compute().value
+            }
+
+            return unsafeCast(_value)
+        }
+
+    override fun addDependent(dependent: Dependent) {
+        if (dependents.isEmpty()) {
+            for (dependency in dependencies) {
+                dependency.addDependent(this)
+            }
+
+            computedCell = compute().also { computedCell ->
+                computedCell.addDependent(this)
+                computedInDeps = dependencies.any { it === computedCell }
+                _value = computedCell.value
             }
         }
 
-    override fun observe(callNow: Boolean, observer: Observer<T>): Disposable {
-        val superDisposable = super.observe(callNow, observer)
+        super.addDependent(dependent)
+    }
 
-        return disposable {
-            superDisposable.dispose()
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
 
-            if (!hasObservers) {
-                computedCellObserver?.dispose()
-                computedCellObserver = null
+        if (dependents.isEmpty()) {
+            computedCell?.removeDependent(this)
             computedCell = null
+            computedInDeps = false
+
+            for (dependency in dependencies) {
+                dependency.removeDependent(this)
             }
         }
     }
 
-    override fun computeValue(): T {
-        val computedCell = compute()
-        this.computedCell = computedCell
-
-        computedCellObserver?.dispose()
-
-        if (hasObservers) {
-            computedCellObserver = computedCell.observe { (value) ->
-                _value = value
-                emit()
-            }
+    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+        if ((dependency !== computedCell || computedInDeps) && event != null) {
+            shouldRecompute = true
         }
 
-        return computedCell.value
+        super.dependencyChanged(dependency, event)
+    }
+
+    override fun dependenciesChanged() {
+        if (shouldRecompute) {
+            computedCell?.removeDependent(this)
+
+            computedCell = compute().also { computedCell ->
+                computedCell.addDependent(this)
+                computedInDeps = dependencies.any { it === computedCell }
+            }
+
+            shouldRecompute = false
+        }
+
+        val newValue = unsafeAssertNotNull(computedCell).value
+
+        if (newValue != _value) {
+            _value = newValue
+            emitChanged(ChangeEvent(newValue))
+        } else {
+            emitChanged(null)
+        }
     }
 }

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

@@ -1,11 +1,16 @@
 package world.phantasmal.observable.cell
 
+import world.phantasmal.observable.ChangeEvent
+
 class SimpleCell<T>(value: T) : AbstractCell<T>(), MutableCell<T> {
     override var value: T = value
         set(value) {
             if (value != field) {
+                emitMightChange()
+
                 field = value
-                emit()
+
+                emitChanged(ChangeEvent(value))
             }
         }
 }

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

@@ -2,10 +2,11 @@ package world.phantasmal.observable.cell
 
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.nopDisposable
+import world.phantasmal.observable.AbstractDependency
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Observer
 
-class StaticCell<T>(override val value: T) : Cell<T> {
+class StaticCell<T>(override val value: T) : AbstractDependency(), Cell<T> {
     override fun observe(callNow: Boolean, observer: Observer<T>): Disposable {
         if (callNow) {
             observer(ChangeEvent(value))

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

@@ -1,131 +1,66 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
+import world.phantasmal.observable.CallbackObserver
+import world.phantasmal.observable.Dependent
 import world.phantasmal.observable.Observer
-import world.phantasmal.observable.cell.AbstractCell
+import world.phantasmal.observable.cell.AbstractDependentCell
 import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.DependentCell
+import world.phantasmal.observable.cell.not
 
-/**
- * Starts observing its dependencies when the first observer on this cell is registered. Stops
- * observing its dependencies when the last observer on this cell is disposed. This way no extra
- * disposables need to be managed when e.g. [map] is used.
- */
-abstract class AbstractDependentListCell<E>(
-    private vararg val dependencies: Cell<*>,
-) : AbstractListCell<E>(extractObservables = null) {
-    private val _size = SizeCell()
-
-    /**
-     * Is either empty or has a disposable per dependency.
-     */
-    private val dependencyObservers = mutableListOf<Disposable>()
+abstract class AbstractDependentListCell<E> :
+    AbstractDependentCell<List<E>>(),
+    ListCell<E>,
+    Dependent {
 
     protected abstract val elements: List<E>
 
-    /**
-     * Set to true right before actual observers are added.
-     */
-    protected var hasObservers = false
-
     override val value: List<E>
         get() {
-            if (!hasObservers) {
+            if (dependents.isEmpty()) {
                 computeElements()
             }
 
             return elements
         }
 
-    override val size: Cell<Int> = _size
+    @Suppress("LeakingThis")
+    final override val size: Cell<Int> = DependentCell(this) { elements.size }
 
-    override fun observe(callNow: Boolean, observer: Observer<List<E>>): Disposable {
-        initDependencyObservers()
+    final override val empty: Cell<Boolean> = size.map { it == 0 }
 
-        val superDisposable = super.observe(callNow, observer)
+    final override val notEmpty: Cell<Boolean> = !empty
 
-        return disposable {
-            superDisposable.dispose()
-            disposeDependencyObservers()
-        }
-    }
+    final override fun observe(callNow: Boolean, observer: Observer<List<E>>): Disposable =
+        observeList(callNow, observer as ListObserver<E>)
 
     override fun observeList(callNow: Boolean, observer: ListObserver<E>): Disposable {
-        initDependencyObservers()
+        val observingCell = CallbackObserver(this, observer)
 
-        val superDisposable = super.observeList(callNow, observer)
-
-        return disposable {
-            superDisposable.dispose()
-            disposeDependencyObservers()
-        }
-    }
-
-    protected abstract fun computeElements()
-
-    protected open fun lastObserverRemoved() {
-        dependencyObservers.forEach { it.dispose() }
-        dependencyObservers.clear()
-    }
-
-    private fun initDependencyObservers() {
-        if (dependencyObservers.isEmpty()) {
-            hasObservers = true
-
-            dependencies.forEach { dependency ->
-                dependencyObservers.add(
-                    dependency.observe {
-                        val removed = elements
-                        computeElements()
-                        finalizeUpdate(ListChangeEvent.Change(0, removed, elements))
-                    }
+        if (callNow) {
+            observer(
+                ListChangeEvent(
+                    value,
+                    listOf(
+                        ListChange.Structural(index = 0, removed = emptyList(), inserted = value),
+                    ),
+                )
             )
         }
 
+        return observingCell
+    }
+
+    final override fun dependenciesChanged() {
+        val oldElements = value
+
         computeElements()
-        }
+
+        emitChanged(
+            ListChangeEvent(elements, listOf(ListChange.Structural(0, oldElements, elements)))
+        )
     }
 
-    private fun disposeDependencyObservers() {
-        if (observers.isEmpty() && listObservers.isEmpty() && _size.publicObservers.isEmpty()) {
-            hasObservers = false
-            lastObserverRemoved()
-        }
-    }
-
-    override fun finalizeUpdate(event: ListChangeEvent<E>) {
-        if (event is ListChangeEvent.Change && event.removed.size != event.inserted.size) {
-            _size.publicEmit()
-        }
-
-        super.finalizeUpdate(event)
-    }
-
-    private inner class SizeCell : AbstractCell<Int>() {
-        override val value: Int
-            get() {
-                if (!hasObservers) {
-                    computeElements()
-                }
-
-                return elements.size
-            }
-
-        val publicObservers = super.observers
-
-        override fun observe(callNow: Boolean, observer: Observer<Int>): Disposable {
-            initDependencyObservers()
-
-            val superDisposable = super.observe(callNow, observer)
-
-            return disposable {
-                superDisposable.dispose()
-                disposeDependencyObservers()
-            }
-        }
-
-        fun publicEmit() {
-            super.emit()
-        }
-    }
+    protected abstract fun computeElements()
 }

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

@@ -1,140 +1,38 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
-import world.phantasmal.core.unsafe.unsafeAssertNotNull
-import world.phantasmal.observable.ChangeEvent
-import world.phantasmal.observable.Observable
+import world.phantasmal.observable.CallbackObserver
 import world.phantasmal.observable.Observer
 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>(
-    private val extractObservables: ObservablesExtractor<E>?,
-) : AbstractCell<List<E>>(), ListCell<E> {
-    /**
-     * Internal observers which observe observables related to this list's elements so that their
-     * changes can be propagated via ElementChange events.
-     */
-    private val elementObservers = mutableListOf<ElementObserver>()
+abstract class AbstractListCell<E> : AbstractCell<List<E>>(), ListCell<E> {
+    @Suppress("LeakingThis")
+    final override val size: Cell<Int> = DependentCell(this) { value.size }
 
-    /**
-     * External list observers which are observing this list.
-     */
-    protected val listObservers = mutableListOf<ListObserver<E>>()
+    final override val empty: Cell<Boolean> = size.map { it == 0 }
 
-    override val empty: Cell<Boolean> by lazy { size.map { it == 0 } }
+    final override val notEmpty: Cell<Boolean> = !empty
 
-    override val notEmpty: Cell<Boolean> by lazy { !empty }
-
-    override fun get(index: Int): E =
-        value[index]
-
-    override fun observe(callNow: Boolean, observer: Observer<List<E>>): Disposable {
-        if (elementObservers.isEmpty() && extractObservables != null) {
-            replaceElementObservers(0, elementObservers.size, value)
-        }
-
-        observers.add(observer)
-
-        if (callNow) {
-            observer(ChangeEvent(value))
-        }
-
-        return disposable {
-            observers.remove(observer)
-            disposeElementObserversIfNecessary()
-        }
-    }
+    final override fun observe(callNow: Boolean, observer: Observer<List<E>>): Disposable =
+        observeList(callNow, observer as ListObserver<E>)
 
     override fun observeList(callNow: Boolean, observer: ListObserver<E>): Disposable {
-        if (elementObservers.isEmpty() && extractObservables != null) {
-            replaceElementObservers(0, elementObservers.size, value)
-        }
-
-        listObservers.add(observer)
+        val observingCell = CallbackObserver(this, observer)
 
         if (callNow) {
-            observer(ListChangeEvent.Change(0, emptyList(), value))
-        }
-
-        return disposable {
-            listObservers.remove(observer)
-            disposeElementObserversIfNecessary()
-        }
-    }
-
-    override fun firstOrNull(): Cell<E?> =
-        DependentCell(this) { value.firstOrNull() }
-
-    /**
-     * Does the following in the given order:
-     * - Updates element observers
-     * - Emits ListChangeEvent
-     * - Emits ChangeEvent
-     */
-    protected open fun finalizeUpdate(event: ListChangeEvent<E>) {
-        if (
-            (listObservers.isNotEmpty() || observers.isNotEmpty()) &&
-            extractObservables != null &&
-            event is ListChangeEvent.Change
-        ) {
-            replaceElementObservers(event.index, event.removed.size, event.inserted)
-        }
-
-        listObservers.forEach { observer: ListObserver<E> ->
-            observer(event)
-        }
-
-        emit()
-    }
-
-    private fun replaceElementObservers(from: Int, amountRemoved: Int, insertedElements: List<E>) {
-        repeat(amountRemoved) {
-            elementObservers.removeAt(from).observers.forEach { it.dispose() }
-        }
-
-        var index = from
-
-        elementObservers.addAll(
-            from,
-            insertedElements.map { element ->
-                ElementObserver(
-                    index++,
-                    element,
-                    extractObservables.unsafeAssertNotNull()(element)
+            observer(
+                ListChangeEvent(
+                    value,
+                    listOf(
+                        ListChange.Structural(index = 0, removed = emptyList(), inserted = value),
+                    ),
+                )
             )
         }
-        )
 
-        val shift = insertedElements.size - amountRemoved
-
-        while (index < elementObservers.size) {
-            elementObservers[index++].index += shift
-        }
-    }
-
-    private fun disposeElementObserversIfNecessary() {
-        if (listObservers.isEmpty() && observers.isEmpty()) {
-            elementObservers.forEach { elementObserver: ElementObserver ->
-                elementObserver.observers.forEach { it.dispose() }
-            }
-
-            elementObservers.clear()
-        }
-    }
-
-    private inner class ElementObserver(
-        var index: Int,
-        element: E,
-        observables: Array<Observable<*>>,
-    ) {
-        val observers: Array<Disposable> = Array(observables.size) {
-            observables[it].observe {
-                finalizeUpdate(ListChangeEvent.ElementChange(index, element))
-            }
-        }
+        return observingCell
     }
 }

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

@@ -1,20 +1,42 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.core.unsafe.unsafeAssertNotNull
+import world.phantasmal.observable.Dependent
 import world.phantasmal.observable.cell.Cell
 
 /**
  * ListCell of which the value depends on 0 or more other cells.
  */
 class DependentListCell<E>(
-    vararg dependencies: Cell<*>,
+    private vararg val dependencies: Cell<*>,
     private val computeElements: () -> List<E>,
-) : AbstractDependentListCell<E>(*dependencies) {
-    private var _elements: List<E>? = null
+) : AbstractDependentListCell<E>() {
 
-    override val elements: List<E> get() = _elements.unsafeAssertNotNull()
+    override var elements: List<E> = emptyList()
+        private set
+
+    override fun addDependent(dependent: Dependent) {
+        if (dependents.isEmpty()) {
+            computeElements()
+
+            for (dependency in dependencies) {
+                dependency.addDependent(this)
+            }
+        }
+
+        super.addDependent(dependent)
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
+
+        if (dependents.isEmpty()) {
+            for (dependency in dependencies) {
+                dependency.removeDependent(this)
+            }
+        }
+    }
 
     override fun computeElements() {
-        _elements = computeElements.invoke()
+        elements = computeElements.invoke()
     }
 }

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

@@ -1,28 +1,16 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
-import world.phantasmal.observable.Observer
-import world.phantasmal.observable.cell.AbstractCell
-import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
 
-// TODO: This class shares 95% of its code with AbstractDependentListCell.
 class FilteredListCell<E>(
     private val dependency: ListCell<E>,
     private val predicate: (E) -> Boolean,
-) : AbstractListCell<E>(extractObservables = null) {
-    private val _size = SizeCell()
-
-    /**
-     * Set to true right before actual observers are added.
-     */
-    private var hasObservers = false
-
-    private var dependencyObserver: Disposable? = null
-
+) : AbstractListCell<E>(), Dependent {
     /**
      * Maps the dependency's indices to this list's indices. When an element of the dependency list
-     * doesn't pass the predicate, it's index in this mapping is set to -1.
+     * doesn't pass the predicate, its index in this mapping is set to -1.
      */
     private val indexMap = mutableListOf<Int>()
 
@@ -30,66 +18,52 @@ class FilteredListCell<E>(
 
     override val value: List<E>
         get() {
-            if (!hasObservers) {
+            if (dependents.isEmpty()) {
                 recompute()
             }
 
             return elements
         }
 
-    override val size: Cell<Int> = _size
+    override fun addDependent(dependent: Dependent) {
+        if (dependents.isEmpty()) {
+            dependency.addDependent(this)
+            recompute()
+        }
 
-    override fun observe(callNow: Boolean, observer: Observer<List<E>>): Disposable {
-        initDependencyObservers()
+        super.addDependent(dependent)
+    }
 
-        val superDisposable = super.observe(callNow, observer)
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
 
-        return disposable {
-            superDisposable.dispose()
-            disposeDependencyObservers()
+        if (dependents.isEmpty()) {
+            dependency.removeDependent(this)
         }
     }
 
-    override fun observeList(callNow: Boolean, observer: ListObserver<E>): Disposable {
-        initDependencyObservers()
-
-        val superDisposable = super.observeList(callNow, observer)
-
-        return disposable {
-            superDisposable.dispose()
-            disposeDependencyObservers()
-        }
+    override fun dependencyMightChange() {
+        emitMightChange()
     }
 
-    private fun recompute() {
-        elements = ListWrapper(mutableListOf())
-        indexMap.clear()
+    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+        if (event is ListChangeEvent<*>) {
+            val filteredChanges = mutableListOf<ListChange<E>>()
 
-        dependency.value.forEach { element ->
-            if (predicate(element)) {
-                elements.mutate { add(element) }
-                indexMap.add(elements.lastIndex)
-            } else {
-                indexMap.add(-1)
-            }
-        }
-    }
-
-    private fun initDependencyObservers() {
-        if (dependencyObserver == null) {
-            hasObservers = true
-
-            dependencyObserver = dependency.observeList { event ->
-                when (event) {
-                    is ListChangeEvent.Change -> {
+            for (change in event.changes) {
+                when (change) {
+                    is ListChange.Structural -> {
                         // Figure out which elements should be removed from this list, then simply
                         // recompute the entire filtered list and finally figure out which elements
-                        // have been added. Emit a Change event if something actually changed.
+                        // have been added. Emit a change event if something actually changed.
+                        @Suppress("UNCHECKED_CAST")
+                        change as ListChange.Structural<E>
+
                         val removed = mutableListOf<E>()
                         var eventIndex = -1
 
-                        event.removed.forEachIndexed { i, element ->
-                            val index = indexMap[event.index + i]
+                        change.removed.forEachIndexed { i, element ->
+                            val index = indexMap[change.index + i]
 
                             if (index != -1) {
                                 removed.add(element)
@@ -104,8 +78,8 @@ class FilteredListCell<E>(
 
                         val inserted = mutableListOf<E>()
 
-                        event.inserted.forEachIndexed { i, element ->
-                            val index = indexMap[event.index + i]
+                        change.inserted.forEachIndexed { i, element ->
+                            val index = indexMap[change.index + i]
 
                             if (index != -1) {
                                 inserted.add(element)
@@ -118,23 +92,31 @@ class FilteredListCell<E>(
 
                         if (removed.isNotEmpty() || inserted.isNotEmpty()) {
                             check(eventIndex != -1)
-                            finalizeUpdate(ListChangeEvent.Change(eventIndex, removed, inserted))
+                            filteredChanges.add(
+                                ListChange.Structural(
+                                    eventIndex,
+                                    removed,
+                                    inserted
+                                )
+                            )
                         }
                     }
-
-                    is ListChangeEvent.ElementChange -> {
-                        // Emit a Change or ElementChange event based on whether the updated element
+                    is ListChange.Element -> {
+                        // Emit a structural or element change based on whether the updated element
                         // passes the predicate test and whether it was already in the elements list
                         // (i.e. whether it passed the predicate test before the update).
-                        val index = indexMap[event.index]
+                        @Suppress("UNCHECKED_CAST")
+                        change as ListChange.Element<E>
 
-                        if (predicate(event.updated)) {
+                        val index = indexMap[change.index]
+
+                        if (predicate(change.updated)) {
                             if (index == -1) {
                                 // If the element now passed the test and previously didn't pass,
                                 // insert it and emit a Change event.
                                 var insertIndex = elements.size
 
-                                for (depIdx in (event.index + 1)..indexMap.lastIndex) {
+                                for (depIdx in (change.index + 1)..indexMap.lastIndex) {
                                     val thisIdx = indexMap[depIdx]
 
                                     if (thisIdx != -1) {
@@ -143,10 +125,10 @@ class FilteredListCell<E>(
                                     }
                                 }
 
-                                elements = elements.mutate { add(insertIndex, event.updated) }
-                                indexMap[event.index] = insertIndex
+                                elements = elements.mutate { add(insertIndex, change.updated) }
+                                indexMap[change.index] = insertIndex
 
-                                for (depIdx in (event.index + 1)..indexMap.lastIndex) {
+                                for (depIdx in (change.index + 1)..indexMap.lastIndex) {
                                     val thisIdx = indexMap[depIdx]
 
                                     if (thisIdx != -1) {
@@ -154,25 +136,25 @@ class FilteredListCell<E>(
                                     }
                                 }
 
-                                finalizeUpdate(ListChangeEvent.Change(
+                                filteredChanges.add(
+                                    ListChange.Structural(
                                         insertIndex,
-                                    emptyList(),
-                                    listOf(event.updated),
-                                ))
-                            } else {
-                                // Otherwise just propagate the ElementChange event.
-                                finalizeUpdate(
-                                    ListChangeEvent.ElementChange(index, event.updated)
+                                        removed = emptyList(),
+                                        inserted = listOf(change.updated),
                                     )
+                                )
+                            } else {
+                                // Otherwise just propagate the element change.
+                                filteredChanges.add(ListChange.Element(index, change.updated))
                             }
                         } else {
                             if (index != -1) {
                                 // If the element now doesn't pass the test and it previously did
-                                // pass, remove it and emit a Change event.
+                                // pass, remove it and emit a structural change.
                                 elements = elements.mutate { removeAt(index) }
-                                indexMap[event.index] = -1
+                                indexMap[change.index] = -1
 
-                                for (depIdx in (event.index + 1)..indexMap.lastIndex) {
+                                for (depIdx in (change.index + 1)..indexMap.lastIndex) {
                                     val thisIdx = indexMap[depIdx]
 
                                     if (thisIdx != -1) {
@@ -180,67 +162,45 @@ class FilteredListCell<E>(
                                     }
                                 }
 
-                                finalizeUpdate(ListChangeEvent.Change(
+                                filteredChanges.add(
+                                    ListChange.Structural(
                                         index,
-                                    listOf(event.updated),
-                                    emptyList(),
-                                ))
-                            } else {
-                                // Otherwise just propagate the ElementChange event.
-                                finalizeUpdate(
-                                    ListChangeEvent.ElementChange(index, event.updated)
+                                        removed = listOf(change.updated),
+                                        inserted = emptyList(),
                                     )
+                                )
+                            } else {
+                                // Otherwise just propagate the element change.
+                                filteredChanges.add(ListChange.Element(index, change.updated))
                             }
                         }
                     }
                 }
             }
 
-            recompute()
+            if (filteredChanges.isEmpty()) {
+                emitChanged(null)
+            } else {
+                emitChanged(ListChangeEvent(elements, filteredChanges))
+            }
+        } else {
+            emitChanged(null)
         }
     }
 
-    private fun disposeDependencyObservers() {
-        if (observers.isEmpty() && listObservers.isEmpty() && _size.publicObservers.isEmpty()) {
-            hasObservers = false
-            dependencyObserver?.dispose()
-            dependencyObserver = null
+    private fun recompute() {
+        val newElements = mutableListOf<E>()
+        indexMap.clear()
+
+        dependency.value.forEach { element ->
+            if (predicate(element)) {
+                newElements.add(element)
+                indexMap.add(newElements.lastIndex)
+            } else {
+                indexMap.add(-1)
             }
         }
 
-    override fun finalizeUpdate(event: ListChangeEvent<E>) {
-        if (event is ListChangeEvent.Change && event.removed.size != event.inserted.size) {
-            _size.publicEmit()
-        }
-
-        super.finalizeUpdate(event)
-    }
-
-    private inner class SizeCell : AbstractCell<Int>() {
-        override val value: Int
-            get() {
-                if (!hasObservers) {
-                    recompute()
-                }
-
-                return elements.size
-            }
-
-        val publicObservers = super.observers
-
-        override fun observe(callNow: Boolean, observer: Observer<Int>): Disposable {
-            initDependencyObservers()
-
-            val superDisposable = super.observe(callNow, observer)
-
-            return disposable {
-                superDisposable.dispose()
-                disposeDependencyObservers()
-            }
-        }
-
-        fun publicEmit() {
-            super.emit()
-        }
+        elements = ListWrapper(newElements)
     }
 }

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

@@ -1,38 +1,75 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.unsafe.unsafeAssertNotNull
-import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
 
 /**
  * Similar to [DependentListCell], except that this cell's [computeElements] returns a [ListCell].
  */
 class FlatteningDependentListCell<E>(
-    vararg dependencies: Cell<*>,
+    private vararg val dependencies: Dependency,
     private val computeElements: () -> ListCell<E>,
-) : AbstractDependentListCell<E>(*dependencies) {
-    private var computedCell: ListCell<E>? = null
-    private var computedCellObserver: Disposable? = null
+) : AbstractDependentListCell<E>() {
 
-    override val elements: List<E> get() = computedCell.unsafeAssertNotNull().value
+    private var computedCell: ListCell<E>? = null
+    private var computedInDeps = false
+    private var shouldRecompute = false
+
+    override var elements: List<E> = emptyList()
+        private set
+
+    override fun addDependent(dependent: Dependent) {
+        if (dependents.isEmpty()) {
+            for (dependency in dependencies) {
+                dependency.addDependent(this)
+            }
+
+            computedCell = computeElements.invoke().also { computedCell ->
+                computedCell.addDependent(this)
+                computedInDeps = dependencies.any { it === computedCell }
+                elements = computedCell.value.toList()
+            }
+        }
+
+        super.addDependent(dependent)
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
+
+        if (dependents.isEmpty()) {
+            computedCell?.removeDependent(this)
+            computedCell = null
+            computedInDeps = false
+
+            for (dependency in dependencies) {
+                dependency.removeDependent(this)
+            }
+        }
+    }
+
+    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+        if ((dependency !== computedCell || computedInDeps) && event != null) {
+            shouldRecompute = true
+        }
+
+        super.dependencyChanged(dependency, event)
+    }
 
     override fun computeElements() {
-        computedCell = computeElements.invoke()
+        if (shouldRecompute || dependents.isEmpty()) {
+            computedCell?.removeDependent(this)
 
-        computedCellObserver?.dispose()
-
-        computedCellObserver =
-            if (hasObservers) {
-                computedCell.unsafeAssertNotNull().observeList(observer = ::finalizeUpdate)
-            } else {
-                null
-            }
+            computedCell = computeElements.invoke().also { computedCell ->
+                computedCell.addDependent(this)
+                computedInDeps = dependencies.any { it === computedCell }
             }
 
-    override fun lastObserverRemoved() {
-        super.lastObserverRemoved()
+            shouldRecompute = false
+        }
 
-        computedCellObserver?.dispose()
-        computedCellObserver = null
+        elements = unsafeAssertNotNull(computedCell).value.toList()
     }
 }

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

@@ -1,49 +0,0 @@
-package world.phantasmal.observable.cell.list
-
-import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
-import world.phantasmal.core.unsafe.unsafeCast
-import world.phantasmal.observable.Observer
-import world.phantasmal.observable.cell.AbstractCell
-
-class FoldedCell<T, R>(
-    private val dependency: ListCell<T>,
-    private val initial: R,
-    private val operation: (R, T) -> R,
-) : AbstractCell<R>() {
-    private var dependencyDisposable: Disposable? = null
-    private var _value: R? = null
-
-    override val value: R
-        get() {
-            return if (dependencyDisposable == null) {
-                computeValue()
-            } else {
-                _value.unsafeCast()
-            }
-        }
-
-    override fun observe(callNow: Boolean, observer: Observer<R>): Disposable {
-        val superDisposable = super.observe(callNow, observer)
-
-        if (dependencyDisposable == null) {
-            _value = computeValue()
-
-            dependencyDisposable = dependency.observe {
-                _value = computeValue()
-                emit()
-            }
-        }
-
-        return disposable {
-            superDisposable.dispose()
-
-            if (observers.isEmpty()) {
-                dependencyDisposable?.dispose()
-                dependencyDisposable = null
-            }
-        }
-    }
-
-    private fun computeValue(): R = dependency.value.fold(initial, operation)
-}

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

@@ -2,6 +2,7 @@ package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.DependentCell
 
 interface ListCell<out E> : Cell<List<E>> {
     /**
@@ -16,23 +17,24 @@ interface ListCell<out E> : Cell<List<E>> {
 
     val notEmpty: Cell<Boolean>
 
-    operator fun get(index: Int): E
+    operator fun get(index: Int): E = value[index]
 
     fun observeList(callNow: Boolean = false, observer: ListObserver<E>): Disposable
 
     fun <R> fold(initialValue: R, operation: (R, E) -> R): Cell<R> =
-        FoldedCell(this, initialValue, operation)
+        DependentCell(this) { value.fold(initialValue, operation) }
 
     fun all(predicate: (E) -> Boolean): Cell<Boolean> =
-        fold(true) { acc, el -> acc && predicate(el) }
+        DependentCell(this) { value.all(predicate) }
 
-    fun sumBy(selector: (E) -> Int): Cell<Int> =
-        fold(0) { acc, el -> acc + selector(el) }
+    fun sumOf(selector: (E) -> Int): Cell<Int> =
+        DependentCell(this) { value.sumOf(selector) }
 
     fun filtered(predicate: (E) -> Boolean): ListCell<E> =
         FilteredListCell(this, predicate)
 
-    fun firstOrNull(): Cell<E?>
+    fun firstOrNull(): Cell<E?> =
+        DependentCell(this) { value.firstOrNull() }
 
     operator fun contains(element: @UnsafeVariance E): Boolean = element in value
 }

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

@@ -10,8 +10,9 @@ fun <E> emptyListCell(): ListCell<E> = EMPTY_LIST_CELL
 
 fun <E> mutableListCell(
     vararg elements: E,
-    extractObservables: ObservablesExtractor<E>? = null,
-): MutableListCell<E> = SimpleListCell(mutableListOf(*elements), extractObservables)
+    extractDependencies: DependenciesExtractor<E>? = null,
+): MutableListCell<E> =
+    SimpleListCell(mutableListOf(*elements), extractDependencies)
 
 fun <T1, T2, R> flatMapToList(
     c1: Cell<T1>,

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

@@ -1,37 +1,42 @@
 package world.phantasmal.observable.cell.list
 
-sealed class ListChangeEvent<out E> {
-    abstract val index: Int
+import world.phantasmal.observable.ChangeEvent
 
+class ListChangeEvent<out E>(
+    value: List<E>,
+    val changes: List<ListChange<E>>,
+) : ChangeEvent<List<E>>(value)
+
+sealed class ListChange<out E> {
     /**
-     * Represents a structural change to the list. E.g. an element is inserted or removed.
+     * Represents a structural change to a list cell. E.g. an element is inserted or removed.
      */
-    class Change<E>(
-        override val index: Int,
+    class Structural<out E>(
+        val index: Int,
         /**
          * The elements that were removed from the list at [index].
          *
-         * Do not keep long-lived references to a [Change]'s [removed] list, it may or may not be
-         * mutated when the originating [ListCell] is mutated.
+         * Do not keep long-lived references to a [ChangeEvent]'s [removed] list, it may or may not
+         * be mutated when the originating [ListCell] is mutated.
          */
         val removed: List<E>,
         /**
          * The elements that were inserted into the list at [index].
          *
-         * Do not keep long-lived references to a [Change]'s [inserted] list, it may or may not be
-         * mutated when the originating [ListCell] is mutated.
+         * Do not keep long-lived references to a [ChangeEvent]'s [inserted] list, it may or may not
+         * be mutated when the originating [ListCell] is mutated.
          */
         val inserted: List<E>,
-    ) : ListChangeEvent<E>()
+    ) : ListChange<E>()
 
     /**
-     * Represents a change to an element in the list. Will only be emitted if the list is configured
-     * to do so.
+     * Represents a change to an element in a list cell. Will only be emitted if the list is
+     * configured to do so.
      */
-    class ElementChange<E>(
-        override val index: Int,
+    class Element<E>(
+        val index: Int,
         val updated: E,
-    ) : ListChangeEvent<E>()
+    ) : ListChange<E>()
 }
 
-typealias ListObserver<E> = (change: ListChangeEvent<E>) -> Unit
+typealias ListObserver<E> = (ListChangeEvent<E>) -> Unit

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

@@ -17,7 +17,7 @@ interface MutableListCell<E> : ListCell<E>, MutableCell<List<E>> {
 
     fun replaceAll(elements: Sequence<E>)
 
-    fun splice(from: Int, removeCount: Int, newElement: E)
+    fun splice(fromIndex: Int, removeCount: Int, newElement: E)
 
     fun clear()
 

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

@@ -1,23 +1,33 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.observable.Observable
-import world.phantasmal.observable.cell.Cell
-import world.phantasmal.observable.cell.MutableCell
-import world.phantasmal.observable.cell.mutableCell
+import world.phantasmal.core.disposable.Disposable
+import world.phantasmal.core.unsafe.unsafeAssertNotNull
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
 
-typealias ObservablesExtractor<E> = (element: E) -> Array<Observable<*>>
+typealias DependenciesExtractor<E> = (element: E) -> Array<Dependency>
 
 /**
- * @param elements The backing list for this [ListCell]
- * @param extractObservables Extractor function called on each element in this list, changes to the
- * returned observables will be propagated via ElementChange events
+ * @param elements The backing list for this [ListCell].
+ * @param extractDependencies Extractor function called on each element in this list, changes to
+ * the returned dependencies will be propagated via [ListChange.Element]s in a [ListChangeEvent]
+ * event.
  */
 class SimpleListCell<E>(
     elements: MutableList<E>,
-    extractObservables: ObservablesExtractor<E>? = null,
-) : AbstractListCell<E>(extractObservables), MutableListCell<E> {
+    private val extractDependencies: DependenciesExtractor<E>? = null,
+) : AbstractListCell<E>(), MutableListCell<E> {
+
     private var elements = ListWrapper(elements)
-    private val _size: MutableCell<Int> = mutableCell(elements.size)
+
+    /**
+     * Dependents of dependencies related to this list's elements. Allows us to propagate changes to
+     * elements via [ListChangeEvent]s.
+     */
+    private val elementDependents = mutableListOf<ElementDependent>()
+    private var changingElements = 0
+    private var elementListChanges = mutableListOf<ListChange.Element<E>>()
 
     override var value: List<E>
         get() = elements
@@ -25,27 +35,47 @@ class SimpleListCell<E>(
             replaceAll(value)
         }
 
-    override val size: Cell<Int> = _size
-
     override operator fun get(index: Int): E =
         elements[index]
 
     override operator fun set(index: Int, element: E): E {
+        checkIndex(index, elements.lastIndex)
+        emitMightChange()
+
         val removed: E
         elements = elements.mutate { removed = set(index, element) }
-        finalizeUpdate(ListChangeEvent.Change(index, listOf(removed), listOf(element)))
+
+        if (extractDependencies != null) {
+            elementDependents[index].dispose()
+            elementDependents[index] = ElementDependent(index, element)
+        }
+
+        emitChanged(
+            ListChangeEvent(
+                elements,
+                listOf(ListChange.Structural(index, listOf(removed), listOf(element))),
+            ),
+        )
+
         return removed
     }
 
     override fun add(element: E) {
+        emitMightChange()
+
         val index = elements.size
         elements = elements.mutate { add(index, element) }
-        finalizeUpdate(ListChangeEvent.Change(index, emptyList(), listOf(element)))
+
+        finalizeStructuralChange(index, emptyList(), listOf(element))
     }
 
     override fun add(index: Int, element: E) {
+        checkIndex(index, elements.size)
+        emitMightChange()
+
         elements = elements.mutate { add(index, element) }
-        finalizeUpdate(ListChangeEvent.Change(index, emptyList(), listOf(element)))
+
+        finalizeStructuralChange(index, emptyList(), listOf(element))
     }
 
     override fun remove(element: E): Boolean {
@@ -60,46 +90,180 @@ class SimpleListCell<E>(
     }
 
     override fun removeAt(index: Int): E {
+        checkIndex(index, elements.lastIndex)
+        emitMightChange()
+
         val removed: E
         elements = elements.mutate { removed = removeAt(index) }
-        finalizeUpdate(ListChangeEvent.Change(index, listOf(removed), emptyList()))
+
+        finalizeStructuralChange(index, listOf(removed), emptyList())
         return removed
     }
 
     override fun replaceAll(elements: Iterable<E>) {
+        emitMightChange()
+
         val removed = this.elements
         this.elements = ListWrapper(elements.toMutableList())
-        finalizeUpdate(ListChangeEvent.Change(0, removed, this.elements))
+
+        finalizeStructuralChange(0, removed, this.elements)
     }
 
     override fun replaceAll(elements: Sequence<E>) {
+        emitMightChange()
+
         val removed = this.elements
         this.elements = ListWrapper(elements.toMutableList())
-        finalizeUpdate(ListChangeEvent.Change(0, removed, this.elements))
+
+        finalizeStructuralChange(0, removed, this.elements)
     }
 
-    override fun splice(from: Int, removeCount: Int, newElement: E) {
-        val removed = ArrayList(elements.subList(from, from + removeCount))
+    override fun splice(fromIndex: Int, removeCount: Int, newElement: E) {
+        val removed = ArrayList(elements.subList(fromIndex, fromIndex + removeCount))
+
+        emitMightChange()
+
         elements = elements.mutate {
-            repeat(removeCount) { removeAt(from) }
-            add(from, newElement)
+            repeat(removeCount) { removeAt(fromIndex) }
+            add(fromIndex, newElement)
         }
-        finalizeUpdate(ListChangeEvent.Change(from, removed, listOf(newElement)))
+
+        finalizeStructuralChange(fromIndex, removed, listOf(newElement))
     }
 
     override fun clear() {
+        emitMightChange()
+
         val removed = elements
         elements = ListWrapper(mutableListOf())
-        finalizeUpdate(ListChangeEvent.Change(0, removed, emptyList()))
+
+        finalizeStructuralChange(0, removed, emptyList())
     }
 
     override fun sortWith(comparator: Comparator<E>) {
+        emitMightChange()
+
+        var throwable: Throwable? = null
+
+        try {
             elements = elements.mutate { sortWith(comparator) }
-        finalizeUpdate(ListChangeEvent.Change(0, elements, elements))
+        } catch (e: Throwable) {
+            throwable = e
         }
 
-    override fun finalizeUpdate(event: ListChangeEvent<E>) {
-        _size.value = elements.size
-        super.finalizeUpdate(event)
+        finalizeStructuralChange(0, elements, elements)
+
+        if (throwable != null) {
+            throw throwable
+        }
+    }
+
+    override fun addDependent(dependent: Dependent) {
+        if (dependents.isEmpty() && extractDependencies != null) {
+            for ((index, element) in elements.withIndex()) {
+                elementDependents.add(ElementDependent(index, element))
+            }
+        }
+
+        super.addDependent(dependent)
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
+
+        if (dependents.isEmpty()) {
+            for (elementDependent in elementDependents) {
+                elementDependent.dispose()
+            }
+
+            elementDependents.clear()
+        }
+    }
+
+    private fun checkIndex(index: Int, maxIndex: Int) {
+        if (index !in 0..maxIndex) {
+            throw IndexOutOfBoundsException(
+                "Index $index out of bounds for length ${elements.size}",
+            )
+        }
+    }
+
+    private fun finalizeStructuralChange(index: Int, removed: List<E>, inserted: List<E>) {
+        if (extractDependencies != null) {
+            repeat(removed.size) {
+                elementDependents.removeAt(index).dispose()
+            }
+
+            for ((i, element) in inserted.withIndex()) {
+                val elementIdx = index + i
+                elementDependents.add(elementIdx, ElementDependent(elementIdx, element))
+            }
+
+            val shift = inserted.size - removed.size
+
+            for (i in (index + inserted.size)..elementDependents.lastIndex) {
+                elementDependents[i].index += shift
+            }
+        }
+
+        emitChanged(
+            ListChangeEvent(
+                elements,
+                listOf(ListChange.Structural(index, removed, inserted)),
+            ),
+        )
+    }
+
+    private inner class ElementDependent(
+        var index: Int,
+        private val element: E,
+    ) : Dependent, Disposable {
+        private val dependencies = unsafeAssertNotNull(extractDependencies)(element)
+        private var changingDependencies = 0
+        private var dependenciesActuallyChanged = false
+
+        init {
+            for (dependency in dependencies) {
+                dependency.addDependent(this)
+            }
+        }
+
+        override fun dispose() {
+            for (dependency in dependencies) {
+                dependency.removeDependent(this)
+            }
+        }
+
+        override fun dependencyMightChange() {
+            if (changingDependencies++ == 0) {
+                changingElements++
+                emitMightChange()
+            }
+        }
+
+        override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+            if (event != null) {
+                dependenciesActuallyChanged = true
+            }
+
+            if (--changingDependencies == 0) {
+                if (dependenciesActuallyChanged) {
+                    dependenciesActuallyChanged = false
+                    elementListChanges.add(ListChange.Element(index, element))
+                }
+
+                if (--changingElements == 0) {
+                    try {
+                        if (elementListChanges.isNotEmpty()) {
+                            emitChanged(ListChangeEvent(value, elementListChanges))
+                        } else {
+                            emitChanged(null)
+                        }
+                    } finally {
+                        elementListChanges = mutableListOf()
+                    }
+                }
+            }
+        }
     }
 }

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

@@ -2,12 +2,14 @@ package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.nopDisposable
+import world.phantasmal.core.unsafe.unsafeAssertNotNull
+import world.phantasmal.observable.AbstractDependency
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Observer
 import world.phantasmal.observable.cell.*
 
-class StaticListCell<E>(private val elements: List<E>) : ListCell<E> {
-    private val firstOrNull = StaticCell(elements.firstOrNull())
+class StaticListCell<E>(private val elements: List<E>) : AbstractDependency(), ListCell<E> {
+    private var firstOrNull: Cell<E?>? = null
 
     override val size: Cell<Int> = cell(elements.size)
     override val empty: Cell<Boolean> = if (elements.isEmpty()) trueCell() else falseCell()
@@ -30,11 +32,17 @@ class StaticListCell<E>(private val elements: List<E>) : ListCell<E> {
 
     override fun observeList(callNow: Boolean, observer: ListObserver<E>): Disposable {
         if (callNow) {
-            observer(ListChangeEvent.Change(0, emptyList(), value))
+            observer(ListChangeEvent(value, listOf(ListChange.Structural(0, emptyList(), value))))
         }
 
         return nopDisposable()
     }
 
-    override fun firstOrNull(): Cell<E?> = firstOrNull
+    override fun firstOrNull(): Cell<E?> {
+        if (firstOrNull == null) {
+            firstOrNull = StaticCell(elements.firstOrNull())
+        }
+
+        return unsafeAssertNotNull(firstOrNull)
+    }
 }

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

@@ -12,7 +12,7 @@ interface ObservableTests : ObservableTestSuite {
     fun createProvider(): Provider
 
     @Test
-    fun observable_calls_observers_when_events_are_emitted() = test {
+    fun calls_observers_when_events_are_emitted() = test {
         val p = createProvider()
         var changes = 0
 
@@ -34,7 +34,7 @@ interface ObservableTests : ObservableTestSuite {
     }
 
     @Test
-    fun observable_does_not_call_observers_after_they_are_disposed() = test {
+    fun does_not_call_observers_after_they_are_disposed() = test {
         val p = createProvider()
         var changes = 0
 

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

@@ -11,6 +11,27 @@ import kotlin.test.*
 interface CellTests : ObservableTests {
     override fun createProvider(): Provider
 
+    @Test
+    fun value_is_accessible_without_observers() = test {
+        val p = createProvider()
+
+        assertNotNull(p.observable.value)
+    }
+
+    @Test
+    fun value_is_accessible_with_observers() = test {
+        val p = createProvider()
+
+        var observedValue: Any? = null
+
+        disposer.add(p.observable.observe(callNow = true) {
+            observedValue = it.value
+        })
+
+        assertNotNull(observedValue)
+        assertNotNull(p.observable.value)
+    }
+
     @Test
     fun propagates_changes_to_mapped_cell() = test {
         val p = createProvider()

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

@@ -0,0 +1,53 @@
+package world.phantasmal.observable.cell
+
+import world.phantasmal.observable.Dependent
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+interface CellWithDependenciesTests : CellTests {
+    override fun createProvider(): Provider
+
+    @Test
+    fun is_recomputed_once_even_when_many_dependencies_change() = test {
+        val p = createProvider()
+
+        val root = SimpleCell(5)
+        val branch1 = root.map { it * 2 }
+        val branch2 = root.map { it * 4 }
+        val leaf = p.createWithDependencies(branch1, branch2)
+
+        var observedChanges = 0
+
+        disposer.add(leaf.observe { observedChanges++ })
+
+        // Change root, which results in both branches changing and thus two dependencies of leaf
+        // changing.
+        root.value = 7
+
+        assertEquals(1, observedChanges)
+    }
+
+    @Test
+    fun doesnt_register_as_dependent_of_its_dependencies_until_it_has_dependents_itself() = test {
+        val p = createProvider()
+
+        val dependency = object : AbstractCell<Int>() {
+            val publicDependents: List<Dependent> = dependents
+
+            override val value: Int = 5
+        }
+
+        val cell = p.createWithDependencies(dependency)
+
+        assertTrue(dependency.publicDependents.isEmpty())
+
+        disposer.add(cell.observe { })
+
+        assertEquals(1, dependency.publicDependents.size)
+    }
+
+    interface Provider : CellTests.Provider {
+        fun createWithDependencies(vararg dependencies: Cell<Int>): Cell<Any>
+    }
+}

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

@@ -2,7 +2,7 @@ package world.phantasmal.observable.cell
 
 class DelegatingCellTests : RegularCellTests, MutableCellTests<Int> {
     override fun createProvider() = object : MutableCellTests.Provider<Int> {
-        private var v = 0
+        private var v = 17
 
         override val observable = DelegatingCell({ v }, { v = it })
 

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

@@ -1,18 +1,23 @@
 package world.phantasmal.observable.cell
 
-class DependentCellTests : RegularCellTests {
-    override fun createProvider() = object : CellTests.Provider {
-        val dependency = SimpleCell(0)
+class DependentCellTests : RegularCellTests, CellWithDependenciesTests {
+    override fun createProvider() = Provider()
+
+    override fun <T> createWithValue(value: T): DependentCell<T> {
+        val dependency = SimpleCell(value)
+        return DependentCell(dependency) { dependency.value }
+    }
+
+    class Provider : CellTests.Provider, CellWithDependenciesTests.Provider {
+        private val dependency = SimpleCell(1)
 
         override val observable = DependentCell(dependency) { 2 * dependency.value }
 
         override fun emit() {
             dependency.value += 2
         }
-    }
 
-    override fun <T> createWithValue(value: T): DependentCell<T> {
-        val dependency = SimpleCell(value)
-        return DependentCell(dependency) { dependency.value }
+        override fun createWithDependencies(vararg dependencies: Cell<Int>) =
+            DependentCell(*dependencies) { dependencies.sumOf { it.value } }
     }
 }

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

@@ -3,13 +3,23 @@ package world.phantasmal.observable.cell
 /**
  * In these tests the dependency of the [FlatteningDependentCell]'s direct dependency changes.
  */
-class FlatteningDependentCellTransitiveDependencyEmitsTests : RegularCellTests {
-    override fun createProvider() = object : CellTests.Provider {
+class FlatteningDependentCellTransitiveDependencyEmitsTests :
+    RegularCellTests,
+    CellWithDependenciesTests {
+
+    override fun createProvider() = Provider()
+
+    override fun <T> createWithValue(value: T): FlatteningDependentCell<T> {
+        val dependency = StaticCell(StaticCell(value))
+        return FlatteningDependentCell(dependency) { dependency.value }
+    }
+
+    class Provider : CellTests.Provider, CellWithDependenciesTests.Provider {
         // The transitive dependency can change.
-        val transitiveDependency = SimpleCell(5)
+        private val transitiveDependency = SimpleCell(5)
 
         // The direct dependency of the cell under test can't change.
-        val directDependency = StaticCell(transitiveDependency)
+        private val directDependency = StaticCell(transitiveDependency)
 
         override val observable =
             FlatteningDependentCell(directDependency) { directDependency.value }
@@ -18,10 +28,8 @@ class FlatteningDependentCellTransitiveDependencyEmitsTests : RegularCellTests {
             // Update the transitive dependency.
             transitiveDependency.value += 5
         }
-    }
 
-    override fun <T> createWithValue(value: T): FlatteningDependentCell<T> {
-        val dependency = StaticCell(StaticCell(value))
-        return FlatteningDependentCell(dependency) { dependency.value }
+        override fun createWithDependencies(vararg dependencies: Cell<Int>): Cell<Any> =
+            FlatteningDependentCell(*dependencies) { StaticCell(dependencies.sumOf { it.value }) }
     }
 }

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

@@ -32,8 +32,6 @@ interface RegularCellTests : CellTests {
 
             // Value should not change when emit hasn't been called since the last access.
             assertEquals(new, p.observable.value)
-
-            old = new
         }
     }
 

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

@@ -1,13 +1,23 @@
 package world.phantasmal.observable.cell.list
 
-class DependentListCellTests : ListCellTests {
-    override fun createProvider() = object : ListCellTests.Provider {
-        private val dependency = SimpleListCell<Int>(mutableListOf())
+import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.CellWithDependenciesTests
+
+class DependentListCellTests : ListCellTests, CellWithDependenciesTests {
+    override fun createProvider() = createListProvider(empty = true)
+
+    override fun createListProvider(empty: Boolean) = Provider(empty)
+
+    class Provider(empty: Boolean) : ListCellTests.Provider, CellWithDependenciesTests.Provider {
+        private val dependency = SimpleListCell(if (empty) mutableListOf() else mutableListOf(5))
 
         override val observable = DependentListCell(dependency) { dependency.value.map { 2 * it } }
 
         override fun addElement() {
             dependency.add(4)
         }
+
+        override fun createWithDependencies(vararg dependencies: Cell<Int>): Cell<Any> =
+            DependentListCell(*dependencies) { dependencies.map { it.value } }
     }
 }

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

@@ -1,13 +1,12 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.observable.cell.SimpleCell
-import kotlin.test.Test
-import kotlin.test.assertEquals
-import kotlin.test.assertNull
+import kotlin.test.*
 
 class FilteredListCellTests : ListCellTests {
-    override fun createProvider() = object : ListCellTests.Provider {
-        private val dependency = SimpleListCell<Int>(mutableListOf())
+    override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
+        private val dependency =
+            SimpleListCell(if (empty) mutableListOf(5) else mutableListOf(5, 10))
 
         override val observable = FilteredListCell(dependency, predicate = { it % 2 == 0 })
 
@@ -82,37 +81,49 @@ class FilteredListCellTests : ListCellTests {
 
         dep.replaceAll(listOf(1, 2, 3, 4, 5))
 
-        (event as ListChangeEvent.Change).let { e ->
-            assertEquals(0, e.index)
-            assertEquals(0, e.removed.size)
-            assertEquals(2, e.inserted.size)
-            assertEquals(2, e.inserted[0])
-            assertEquals(4, e.inserted[1])
+        run {
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertTrue(c is ListChange.Structural)
+            assertEquals(0, c.index)
+            assertEquals(0, c.removed.size)
+            assertEquals(2, c.inserted.size)
+            assertEquals(2, c.inserted[0])
+            assertEquals(4, c.inserted[1])
         }
 
         event = null
 
         dep.splice(2, 2, 10)
 
-        (event as ListChangeEvent.Change).let { e ->
-            assertEquals(1, e.index)
-            assertEquals(1, e.removed.size)
-            assertEquals(4, e.removed[0])
-            assertEquals(1, e.inserted.size)
-            assertEquals(10, e.inserted[0])
+        run {
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertTrue(c is ListChange.Structural)
+            assertEquals(1, c.index)
+            assertEquals(1, c.removed.size)
+            assertEquals(4, c.removed[0])
+            assertEquals(1, c.inserted.size)
+            assertEquals(10, c.inserted[0])
         }
     }
 
     /**
-     * When the dependency of a [FilteredListCell] emits ElementChange events, the
-     * [FilteredListCell] should emit either Change events or ElementChange events, depending on
-     * whether the predicate result has changed.
+     * When the dependency of a [FilteredListCell] emits [ListChange.Element] changes, the
+     * [FilteredListCell] should emit either [ListChange.Structural] or [ListChange.Element]
+     * changes, depending on whether the predicate result has changed.
      */
     @Test
-    fun emits_correct_events_when_dependency_emits_ElementChange_events() = test {
+    fun emits_correct_events_when_dependency_emits_element_changes() = test {
         val dep = SimpleListCell(
             mutableListOf(SimpleCell(1), SimpleCell(2), SimpleCell(3), SimpleCell(4)),
-            extractObservables = { arrayOf(it) },
+            extractDependencies = { arrayOf(it) },
         )
         val list = FilteredListCell(dep, predicate = { it.value % 2 == 0 })
         var event: ListChangeEvent<SimpleCell<Int>>? = null
@@ -125,20 +136,25 @@ class FilteredListCellTests : ListCellTests {
         for (i in 0 until dep.size.value) {
             event = null
 
-            // Make an even number odd or an odd number even. List should emit a Change event.
+            // Make an even number odd or an odd number even. List should emit a structural change.
             val newValue = dep[i].value + 1
             dep[i].value = newValue
 
-            (event as ListChangeEvent.Change).let { e ->
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertTrue(c is ListChange.Structural)
+
             if (newValue % 2 == 0) {
-                    assertEquals(0, e.removed.size)
-                    assertEquals(1, e.inserted.size)
-                    assertEquals(newValue, e.inserted[0].value)
+                assertEquals(0, c.removed.size)
+                assertEquals(1, c.inserted.size)
+                assertEquals(newValue, c.inserted[0].value)
             } else {
-                    assertEquals(1, e.removed.size)
-                    assertEquals(0, e.inserted.size)
-                    assertEquals(newValue, e.removed[0].value)
-                }
+                assertEquals(1, c.removed.size)
+                assertEquals(0, c.inserted.size)
+                assertEquals(newValue, c.removed[0].value)
             }
         }
 
@@ -150,7 +166,14 @@ class FilteredListCellTests : ListCellTests {
             val newValue = dep[i].value + 2
             dep[i].value = newValue
 
-            assertEquals(newValue, (event as ListChangeEvent.ElementChange).updated.value)
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertTrue(c is ListChange.Element)
+
+            assertEquals(newValue, c.updated.value)
         }
     }
 }

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

@@ -6,9 +6,9 @@ import world.phantasmal.observable.cell.SimpleCell
  * In these tests the direct dependency of the [FlatteningDependentListCell] changes.
  */
 class FlatteningDependentListCellDirectDependencyEmitsTests : ListCellTests {
-    override fun createProvider() = object : ListCellTests.Provider {
+    override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
         // The transitive dependency can't change.
-        private val transitiveDependency = StaticListCell<Int>(emptyList())
+        private val transitiveDependency = StaticListCell(if (empty) emptyList() else listOf(7))
 
         // The direct dependency of the list under test can change.
         private val dependency = SimpleCell<ListCell<Int>>(transitiveDependency)

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

@@ -1,14 +1,24 @@
 package world.phantasmal.observable.cell.list
 
+import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.CellWithDependenciesTests
 import world.phantasmal.observable.cell.StaticCell
 
 /**
  * In these tests the dependency of the [FlatteningDependentListCell]'s direct dependency changes.
  */
-class FlatteningDependentListCellTransitiveDependencyEmitsTests : ListCellTests {
-    override fun createProvider() = object : ListCellTests.Provider {
+class FlatteningDependentListCellTransitiveDependencyEmitsTests :
+    ListCellTests,
+    CellWithDependenciesTests {
+
+    override fun createProvider() = createListProvider(empty = true)
+
+    override fun createListProvider(empty: Boolean) = Provider(empty)
+
+    class Provider(empty: Boolean) : ListCellTests.Provider, CellWithDependenciesTests.Provider {
         // The transitive dependency can change.
-        private val transitiveDependency = SimpleListCell(mutableListOf<Int>())
+        private val transitiveDependency =
+            SimpleListCell(if (empty) mutableListOf() else mutableListOf(7))
 
         // The direct dependency of the list under test can't change.
         private val dependency = StaticCell<ListCell<Int>>(transitiveDependency)
@@ -20,5 +30,10 @@ class FlatteningDependentListCellTransitiveDependencyEmitsTests : ListCellTests
             // Update the transitive dependency.
             transitiveDependency.add(4)
         }
+
+        override fun createWithDependencies(vararg dependencies: Cell<Int>): Cell<Any> =
+            FlatteningDependentListCell(*dependencies) {
+                StaticListCell(dependencies.map { it.value })
+            }
     }
 }

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

@@ -8,7 +8,30 @@ import kotlin.test.*
  * [ListCell] implementation.
  */
 interface ListCellTests : CellTests {
-    override fun createProvider(): Provider
+    override fun createProvider(): Provider = createListProvider(empty = true)
+
+    fun createListProvider(empty: Boolean): Provider
+
+    @Test
+    fun list_value_is_accessible_without_observers() = test {
+        val p = createListProvider(empty = false)
+
+        assertTrue(p.observable.value.isNotEmpty())
+    }
+
+    @Test
+    fun list_value_is_accessible_with_observers() = test {
+        val p = createListProvider(empty = false)
+
+        var observedValue: List<*>? = null
+
+        disposer.add(p.observable.observe(callNow = true) {
+            observedValue = it.value
+        })
+
+        assertTrue(observedValue!!.isNotEmpty())
+        assertTrue(p.observable.value.isNotEmpty())
+    }
 
     @Test
     fun calls_list_observers_when_changed() = test {
@@ -28,7 +51,7 @@ interface ListCellTests : CellTests {
 
             p.addElement()
 
-            assertTrue(event is ListChangeEvent.Change<*>)
+            assertNotNull(event)
         }
     }
 
@@ -100,7 +123,7 @@ interface ListCellTests : CellTests {
     fun sumBy() = test {
         val p = createProvider()
 
-        val sum = p.observable.sumBy { 1 }
+        val sum = p.observable.sumOf { 1 }
 
         var observedValue: Int? = null
 

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

@@ -1,10 +1,7 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.observable.cell.MutableCellTests
-import kotlin.test.Test
-import kotlin.test.assertEquals
-import kotlin.test.assertNull
-import kotlin.test.assertTrue
+import kotlin.test.*
 
 /**
  * Test suite for all [MutableListCell] implementations. There is a subclass of this suite for every
@@ -17,58 +14,79 @@ interface MutableListCellTests<T : Any> : ListCellTests, MutableCellTests<List<T
     fun add() = test {
         val p = createProvider()
 
-        var change: ListChangeEvent<T>? = null
+        var changeEvent: ListChangeEvent<T>? = null
 
         disposer.add(p.observable.observeList {
-            assertNull(change)
-            change = it
+            assertNull(changeEvent)
+            changeEvent = it
         })
 
         // Insert once.
         val v1 = p.createElement()
         p.observable.add(v1)
 
+        run {
             assertEquals(1, p.observable.size.value)
             assertEquals(v1, p.observable[0])
-        val c1 = change
-        assertTrue(c1 is ListChangeEvent.Change<T>)
-        assertEquals(0, c1.index)
-        assertTrue(c1.removed.isEmpty())
-        assertEquals(1, c1.inserted.size)
-        assertEquals(v1, c1.inserted[0])
+
+            val e = changeEvent
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c0 = e.changes[0]
+            assertTrue(c0 is ListChange.Structural)
+            assertEquals(0, c0.index)
+            assertTrue(c0.removed.isEmpty())
+            assertEquals(1, c0.inserted.size)
+            assertEquals(v1, c0.inserted[0])
+        }
 
         // Insert a second time.
-        change = null
+        changeEvent = null
 
         val v2 = p.createElement()
         p.observable.add(v2)
 
+        run {
             assertEquals(2, p.observable.size.value)
             assertEquals(v1, p.observable[0])
             assertEquals(v2, p.observable[1])
-        val c2 = change
-        assertTrue(c2 is ListChangeEvent.Change<T>)
-        assertEquals(1, c2.index)
-        assertTrue(c2.removed.isEmpty())
-        assertEquals(1, c2.inserted.size)
-        assertEquals(v2, c2.inserted[0])
+
+            val e = changeEvent
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c0 = e.changes[0]
+            assertTrue(c0 is ListChange.Structural)
+            assertEquals(1, c0.index)
+            assertTrue(c0.removed.isEmpty())
+            assertEquals(1, c0.inserted.size)
+            assertEquals(v2, c0.inserted[0])
+        }
 
         // Insert at index.
-        change = null
+        changeEvent = null
 
         val v3 = p.createElement()
         p.observable.add(1, v3)
 
+        run {
             assertEquals(3, p.observable.size.value)
             assertEquals(v1, p.observable[0])
             assertEquals(v3, p.observable[1])
             assertEquals(v2, p.observable[2])
-        val c3 = change
-        assertTrue(c3 is ListChangeEvent.Change<T>)
-        assertEquals(1, c3.index)
-        assertTrue(c3.removed.isEmpty())
-        assertEquals(1, c3.inserted.size)
-        assertEquals(v3, c3.inserted[0])
+
+            val e = changeEvent
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c0 = e.changes[0]
+            assertTrue(c0 is ListChange.Structural)
+            assertEquals(1, c0.index)
+            assertTrue(c0.removed.isEmpty())
+            assertEquals(1, c0.inserted.size)
+            assertEquals(v3, c0.inserted[0])
+        }
     }
 
     interface Provider<T : Any> : ListCellTests.Provider, MutableCellTests.Provider<List<T>> {

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

@@ -1,13 +1,16 @@
 package world.phantasmal.observable.cell.list
 
-import kotlin.test.Test
-import kotlin.test.assertEquals
+import world.phantasmal.observable.cell.SimpleCell
+import world.phantasmal.testUtils.TestContext
+import kotlin.test.*
 
 class SimpleListCellTests : MutableListCellTests<Int> {
-    override fun createProvider() = object : MutableListCellTests.Provider<Int> {
+    override fun createProvider() = createListProvider(empty = true)
+
+    override fun createListProvider(empty: Boolean) = object : MutableListCellTests.Provider<Int> {
         private var nextElement = 0
 
-        override val observable = SimpleListCell(mutableListOf<Int>())
+        override val observable = SimpleListCell(if (empty) mutableListOf() else mutableListOf(-13))
 
         override fun addElement() {
             observable.add(createElement())
@@ -28,4 +31,147 @@ class SimpleListCellTests : MutableListCellTests<Int> {
         assertEquals(2, list[1])
         assertEquals(3, list[2])
     }
+
+    @Test
+    fun add_with_index() = test {
+        val list = SimpleListCell(mutableListOf<String>())
+
+        list.add(0, "b")
+        list.add(1, "c")
+        list.add(0, "a")
+
+        assertEquals(3, list.size.value)
+        assertEquals("a", list[0])
+        assertEquals("b", list[1])
+        assertEquals("c", list[2])
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_set() = test {
+        testElementChangePropagation {
+            val old = it[1]
+            it[1] = SimpleCell("new")
+            listOf(old)
+        }
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_add() = test {
+        testElementChangePropagation {
+            it.add(SimpleCell("new"))
+            emptyList()
+        }
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_add_with_index() = test {
+        testElementChangePropagation {
+            it.add(1, SimpleCell("new"))
+            emptyList()
+        }
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_remove() = test {
+        testElementChangePropagation {
+            val removed = it[1]
+            it.remove(removed)
+            listOf(removed)
+        }
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_removeAt() = test {
+        testElementChangePropagation {
+            listOf(it.removeAt(2))
+        }
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_replaceAll() = test {
+        testElementChangePropagation {
+            val removed = it.value.toList()
+            it.replaceAll(listOf(SimpleCell("new a"), SimpleCell("new b")))
+            removed
+        }
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_replaceAll_with_sequence() = test {
+        testElementChangePropagation {
+            val removed = it.value.toList()
+            it.replaceAll(sequenceOf(SimpleCell("new a"), SimpleCell("new b")))
+            removed
+        }
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_splice() = test {
+        testElementChangePropagation {
+            val removed = it.value.toList().drop(1)
+            it.splice(1, 2, SimpleCell("new"))
+            removed
+        }
+    }
+
+    @Test
+    fun element_changes_propagate_correctly_after_clear() = test {
+        testElementChangePropagation {
+            val removed = it.value.toList()
+            it.clear()
+            removed
+        }
+    }
+
+    /**
+     * Creates a list with 3 SimpleCells as elements, calls [updateList] with this list and then
+     * checks that changes to old elements don't affect the list and changes to new elements do
+     * affect the list.
+     *
+     * @param updateList Function that changes the list and returns the removed elements if any.
+     */
+    private fun TestContext.testElementChangePropagation(
+        updateList: (SimpleListCell<SimpleCell<String>>) -> List<SimpleCell<String>>
+    ) {
+        val list = SimpleListCell(
+            mutableListOf(
+                SimpleCell("a"),
+                SimpleCell("b"),
+                SimpleCell("c")
+            )
+        ) { arrayOf(it) }
+
+        var event: ListChangeEvent<SimpleCell<String>>? = null
+
+        disposer.add(list.observeList {
+            assertNull(event)
+            event = it
+        })
+
+        val removed = updateList(list)
+
+        event = null
+
+        // The list should not emit events when an old element is changed.
+        for (element in removed) {
+            element.value += "-1"
+
+            assertNull(event)
+        }
+
+        // The list should emit events when any of the current elements are changed.
+        for ((index, element) in list.value.withIndex()) {
+            event = null
+
+            element.value += "-2"
+
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+            val c = e.changes.first()
+            assertTrue(c is ListChange.Element)
+            assertEquals(index, c.index)
+            assertEquals(element, c.updated)
+        }
+    }
 }

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

@@ -5,7 +5,7 @@ import world.phantasmal.observable.cell.list.mutableListCell
 import world.phantasmal.web.core.actions.Action
 
 class UndoManager {
-    private val undos = mutableListCell<Undo>(NopUndo) { arrayOf(it.atSavePoint) }
+    private val undos = mutableListCell<Undo>(NopUndo)
     private val _current = mutableCell<Undo>(NopUndo)
 
     val current: Cell<Undo> = _current
@@ -19,7 +19,9 @@ class UndoManager {
      * True if all undos are at the most recent save point. I.e., true if there are no changes to
      * save.
      */
-    val allAtSavePoint: Cell<Boolean> = undos.all { it.atSavePoint.value }
+    // TODO: Optimize this once ListCell supports more performant method for this use-case.
+    val allAtSavePoint: Cell<Boolean> =
+        undos.fold(trueCell()) { acc, undo -> acc and undo.atSavePoint }.flatten()
 
     fun addUndo(undo: Undo) {
         undos.add(undo)

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

@@ -100,7 +100,7 @@ class HuntOptimizerStore(
             val wantedItems = wantedItemPersister.loadWantedItems(server)
 
             withContext(Dispatchers.Main) {
-                _wantedItems.value = wantedItems
+                _wantedItems.replaceAll(wantedItems)
             }
         }
     }

web/src/main/kotlin/world/phantasmal/web/questEditor/QuestEditor.kt

@@ -45,21 +45,26 @@ class QuestEditor(
 
         // Stores
         val areaStore = addDisposable(AreaStore(areaAssetLoader))
-        val questEditorStore = addDisposable(QuestEditorStore(
+        val questEditorStore = addDisposable(
+            QuestEditorStore(
                 questLoader,
                 uiStore,
                 areaStore,
                 undoManager,
-        ))
+                initializeNewQuest = true,
+            )
+        )
         val asmStore = addDisposable(AsmStore(questEditorStore, undoManager))
 
         // Controllers
         val questEditorController = addDisposable(QuestEditorController(questEditorUiPersister))
-        val toolbarController = addDisposable(QuestEditorToolbarController(
+        val toolbarController = addDisposable(
+            QuestEditorToolbarController(
                 uiStore,
                 areaStore,
                 questEditorStore,
-        ))
+            )
+        )
         val questInfoController = addDisposable(QuestInfoController(questEditorStore))
         val npcCountsController = addDisposable(NpcCountsController(questEditorStore))
         val entityInfoController = addDisposable(EntityInfoController(areaStore, questEditorStore))
@@ -70,12 +75,14 @@ class QuestEditor(
         val eventsController = addDisposable(EventsController(questEditorStore))
 
         // Rendering
-        val renderer = addDisposable(QuestRenderer(
+        val renderer = addDisposable(
+            QuestRenderer(
                 areaAssetLoader,
                 entityAssetLoader,
                 questEditorStore,
                 createThreeRenderer,
-        ))
+            )
+        )
         val entityImageRenderer =
             addDisposable(EntityImageRenderer(entityAssetLoader, createThreeRenderer))
 

web/src/main/kotlin/world/phantasmal/web/questEditor/loading/AreaAssetLoader.kt

@@ -53,8 +53,6 @@ class AreaAssetLoader(private val assetLoader: AssetLoader) : DisposableContaine
 
                 addSectionsToCollisionGeometry(collisionObj3d, renderObj3d)
 
-//                cullRenderGeometry(collisionObj3d, renderObj3d)
-
                 Geom(sections, renderObj3d, collisionObj3d)
             },
             { geom ->

web/src/main/kotlin/world/phantasmal/web/questEditor/rendering/QuestMeshManager.kt

@@ -7,6 +7,7 @@ import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.DisposableSupervisedScope
 import world.phantasmal.lib.Episode
 import world.phantasmal.observable.cell.list.ListCell
+import world.phantasmal.observable.cell.list.ListChange
 import world.phantasmal.observable.cell.list.ListChangeEvent
 import world.phantasmal.web.questEditor.loading.AreaAssetLoader
 import world.phantasmal.web.questEditor.loading.EntityAssetLoader
@@ -68,17 +69,21 @@ abstract class QuestMeshManager protected constructor(
         }
     }
 
-    private fun npcsChanged(change: ListChangeEvent<QuestNpcModel>) {
-        if (change is ListChangeEvent.Change) {
+    private fun npcsChanged(event: ListChangeEvent<QuestNpcModel>) {
+        for (change in event.changes) {
+            if (change is ListChange.Structural) {
                 change.removed.forEach(npcMeshManager::remove)
                 change.inserted.forEach(npcMeshManager::add)
             }
         }
+    }
 
-    private fun objectsChanged(change: ListChangeEvent<QuestObjectModel>) {
-        if (change is ListChangeEvent.Change) {
+    private fun objectsChanged(event: ListChangeEvent<QuestObjectModel>) {
+        for (change in event.changes) {
+            if (change is ListChange.Structural) {
                 change.removed.forEach(objectMeshManager::remove)
                 change.inserted.forEach(objectMeshManager::add)
             }
         }
+    }
 }

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

@@ -22,6 +22,7 @@ class QuestEditorStore(
     uiStore: UiStore,
     private val areaStore: AreaStore,
     private val undoManager: UndoManager,
+    initializeNewQuest: Boolean,
 ) : Store() {
     private val _devMode = mutableCell(false)
     private val _currentQuest = mutableCell<QuestModel?>(null)
@@ -102,8 +103,10 @@ class QuestEditorStore(
             }
         }
 
+        if (initializeNewQuest) {
             scope.launch { setCurrentQuest(getDefaultQuest(Episode.I)) }
         }
+    }
 
     override fun dispose() {
         runner.stop()

web/src/test/kotlin/world/phantasmal/web/questEditor/controllers/EventsControllerTests.kt

@@ -0,0 +1,77 @@
+package world.phantasmal.web.questEditor.controllers
+
+import world.phantasmal.web.questEditor.models.QuestEventActionModel
+import world.phantasmal.web.test.WebTestSuite
+import world.phantasmal.web.test.createQuestModel
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class EventsControllerTests : WebTestSuite {
+    @Test
+    fun addEvent() = testAsync {
+        // Setup.
+        val store = components.questEditorStore
+        val quest = createQuestModel(mapDesignations = mapOf(1 to 0))
+        store.setCurrentQuest(quest)
+        store.setCurrentArea(quest.areaVariants.value.first().area)
+        store.makeMainUndoCurrent()
+
+        val ctrl = disposer.add(EventsController(store))
+
+        // Add an event.
+        ctrl.addEvent()
+
+        assertEquals(1, quest.events.value.size)
+
+        // Undo.
+        assertTrue(store.canUndo.value)
+        assertFalse(store.canRedo.value)
+
+        store.undo()
+
+        assertTrue(quest.events.value.isEmpty())
+
+        // Redo.
+        assertFalse(store.canUndo.value)
+        assertTrue(store.canRedo.value)
+
+        store.redo()
+
+        assertEquals(1, quest.events.value.size)
+    }
+
+    @Test
+    fun addAction() = testAsync {
+        // Setup.
+        val store = components.questEditorStore
+        val quest = createQuestModel(mapDesignations = mapOf(1 to 0))
+        store.setCurrentQuest(quest)
+        store.setCurrentArea(quest.areaVariants.value.first().area)
+        store.makeMainUndoCurrent()
+
+        val ctrl = disposer.add(EventsController(store))
+
+        // Add an event and an action.
+        ctrl.addEvent()
+        val event = ctrl.events.value.first()
+        ctrl.addAction(event, QuestEventActionModel.Door.Unlock.SHORT_NAME)
+
+        // Undo.
+        assertTrue(store.canUndo.value)
+        assertFalse(store.canRedo.value)
+
+        store.undo()
+
+        assertTrue(event.actions.value.isEmpty())
+
+        // Redo.
+        assertTrue(store.canUndo.value) // Can still undo event creation at this point.
+        assertTrue(store.canRedo.value)
+
+        store.redo()
+
+        assertEquals(1, event.actions.value.size)
+    }
+}

web/src/test/kotlin/world/phantasmal/web/test/TestComponents.kt

@@ -63,7 +63,7 @@ class TestComponents(private val ctx: TestContext) {
     var areaStore: AreaStore by default { AreaStore(areaAssetLoader) }
 
     var questEditorStore: QuestEditorStore by default {
-        QuestEditorStore(questLoader, uiStore, areaStore, undoManager)
+        QuestEditorStore(questLoader, uiStore, areaStore, undoManager, initializeNewQuest = false)
     }
 
     // Rendering

web/src/test/kotlin/world/phantasmal/web/test/TestModels.kt

@@ -4,18 +4,21 @@ import world.phantasmal.lib.Episode
 import world.phantasmal.lib.asm.BytecodeIr
 import world.phantasmal.lib.fileFormats.quest.NpcType
 import world.phantasmal.lib.fileFormats.quest.QuestNpc
+import world.phantasmal.web.questEditor.models.QuestEventModel
 import world.phantasmal.web.questEditor.models.QuestModel
 import world.phantasmal.web.questEditor.models.QuestNpcModel
 import world.phantasmal.web.questEditor.models.QuestObjectModel
 
-fun createQuestModel(
+fun WebTestContext.createQuestModel(
     id: Int = 1,
     name: String = "Test",
     shortDescription: String = name,
     longDescription: String = name,
     episode: Episode = Episode.I,
+    mapDesignations: Map<Int, Int> = emptyMap(),
     npcs: List<QuestNpcModel> = emptyList(),
     objects: List<QuestObjectModel> = emptyList(),
+    events: List<QuestEventModel> = emptyList(),
     bytecodeIr: BytecodeIr = BytecodeIr(emptyList()),
 ): QuestModel =
     QuestModel(
@@ -25,14 +28,15 @@ fun createQuestModel(
         shortDescription,
         longDescription,
         episode,
-        emptyMap(),
+        mapDesignations,
         npcs.toMutableList(),
         objects.toMutableList(),
-        events = mutableListOf(),
+        events.toMutableList(),
         datUnknowns = emptyList(),
         bytecodeIr,
         UIntArray(0),
-    ) { _, _, _ -> null }
+        components.areaStore::getVariant,
+    )
 
 fun createQuestNpcModel(type: NpcType, episode: Episode): QuestNpcModel =
     QuestNpcModel(

webui/src/main/kotlin/world/phantasmal/webui/dom/Dom.kt

@@ -12,6 +12,7 @@ import world.phantasmal.core.disposable.Disposer
 import world.phantasmal.core.disposable.disposable
 import world.phantasmal.observable.cell.Cell
 import world.phantasmal.observable.cell.list.ListCell
+import world.phantasmal.observable.cell.list.ListChange
 import world.phantasmal.observable.cell.list.ListChangeEvent
 
 fun <E : Event> EventTarget.disposableListener(
@@ -255,8 +256,9 @@ private fun <T> bindChildrenTo(
     childrenRemoved: (index: Int, count: Int) -> Unit,
     after: (ListChangeEvent<T>) -> Unit,
 ): Disposable =
-    list.observeList(callNow = true) { change: ListChangeEvent<T> ->
-        if (change is ListChangeEvent.Change) {
+    list.observeList(callNow = true) { event: ListChangeEvent<T> ->
+        for (change in event.changes) {
+            if (change is ListChange.Structural) {
                 repeat(change.removed.size) {
                     parent.removeChild(parent.childNodes[change.index].unsafeCast<Node>())
                 }
@@ -275,6 +277,7 @@ private fun <T> bindChildrenTo(
                     parent.insertBefore(frag, parent.childNodes[change.index])
                 }
             }
-
-        after(change)
+        }
+
+        after(event)
     }

webui/src/main/kotlin/world/phantasmal/webui/dom/HTMLElementSizeCell.kt

@@ -1,10 +1,9 @@
 package world.phantasmal.webui.dom
 
 import org.w3c.dom.HTMLElement
-import world.phantasmal.core.disposable.Disposable
-import world.phantasmal.core.disposable.disposable
 import world.phantasmal.core.unsafe.unsafeAssertNotNull
-import world.phantasmal.observable.Observer
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependent
 import world.phantasmal.observable.cell.AbstractCell
 
 data class Size(val width: Double, val height: Double)
@@ -12,14 +11,9 @@ data class Size(val width: Double, val height: Double)
 class HTMLElementSizeCell(element: HTMLElement? = null) : AbstractCell<Size>() {
     private var resizeObserver: dynamic = null
 
-    /**
-     * Set to true right before actual observers are added.
-     */
-    private var hasObservers = false
-
     private var _value: Size? = null
 
-    var element: HTMLElement? = null
+    var element: HTMLElement? = element
         set(element) {
             if (resizeObserver != null) {
                 if (field != null) {
@@ -34,24 +28,17 @@ class HTMLElementSizeCell(element: HTMLElement? = null) : AbstractCell<Size>() {
             field = element
         }
 
-    init {
-        // Ensure we call the setter with element.
-        this.element = element
-    }
-
     override val value: Size
         get() {
-            if (!hasObservers) {
+            if (dependents.isEmpty()) {
                 _value = getSize()
             }
 
-            return _value.unsafeAssertNotNull()
+            return unsafeAssertNotNull(_value)
         }
 
-    override fun observe(callNow: Boolean, observer: Observer<Size>): Disposable {
-        if (!hasObservers) {
-            hasObservers = true
-
+    override fun addDependent(dependent: Dependent) {
+        if (dependents.isEmpty()) {
             if (resizeObserver == null) {
                 @Suppress("UNUSED_VARIABLE")
                 val resize = ::resizeCallback
@@ -65,15 +52,14 @@ class HTMLElementSizeCell(element: HTMLElement? = null) : AbstractCell<Size>() {
             _value = getSize()
         }
 
-        val superDisposable = super.observe(callNow, observer)
-
-        return disposable {
-            superDisposable.dispose()
-
-            if (observers.isEmpty()) {
-                hasObservers = false
-                resizeObserver.disconnect()
+        super.addDependent(dependent)
     }
+
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
+
+        if (dependents.isEmpty()) {
+            resizeObserver.disconnect()
         }
     }
 
@@ -83,12 +69,16 @@ class HTMLElementSizeCell(element: HTMLElement? = null) : AbstractCell<Size>() {
             ?: Size(0.0, 0.0)
 
     private fun resizeCallback(entries: Array<dynamic>) {
-        entries.forEach { entry ->
-            _value = Size(
+        val entry = entries.first()
+        val newValue = Size(
             entry.contentRect.width.unsafeCast<Double>(),
-                entry.contentRect.height.unsafeCast<Double>()
+            entry.contentRect.height.unsafeCast<Double>(),
         )
-            emit()
+
+        if (newValue != _value) {
+            emitMightChange()
+            _value = newValue
+            emitChanged(ChangeEvent(newValue))
         }
     }
 }