Renamed FilteredListCell to SimpleFilteredListCell. Added a more complex version of FilteredListCell with bugs which highlighted a fundamental problem with the observable system. Various other improvements to the observable system.

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

@@ -0,0 +1,7 @@
+package world.phantasmal.core
+
+inline fun assert(value: () -> Boolean) {
+    assert(value) { "An assertion failed." }
+}
+
+expect inline fun assert(value: () -> Boolean, lazyMessage: () -> Any)

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

@@ -0,0 +1,5 @@
+package world.phantasmal.core
+
+actual inline fun assert(value: () -> Boolean, lazyMessage: () -> Any) {
+    // TODO: Figure out a sensible way to do dev assertions in JS.
+}

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

@@ -0,0 +1,11 @@
+@file:JvmName("AssertionsJvm")
+
+package world.phantasmal.core
+
+val ASSERTIONS_ENABLED: Boolean = {}.javaClass.desiredAssertionStatus()
+
+actual inline fun assert(value: () -> Boolean, lazyMessage: () -> Any) {
+    if (ASSERTIONS_ENABLED && !value()) {
+        throw AssertionError(lazyMessage())
+    }
+}

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

@@ -3,9 +3,7 @@ package world.phantasmal.observable
 typealias ChangeObserver<T> = (ChangeEvent<T>) -> Unit
 
 open class ChangeEvent<out T>(
-    /**
-     * The observable's new value.
-     */
+    /** The observable's new value. */
     val value: T,
 ) {
     operator fun component1() = value

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

@@ -8,7 +8,10 @@ interface Dependent {
      * 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.
+     * 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()
 

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

@@ -22,7 +22,7 @@ abstract class AbstractCell<T> : AbstractDependency(), Cell<T> {
         }
     }
 
-    protected fun emitDependencyChanged(event: ChangeEvent<*>?) {
+    protected fun emitDependencyChangedEvent(event: ChangeEvent<*>?) {
         if (mightChangeEmitted) {
             mightChangeEmitted = false
 
@@ -31,4 +31,6 @@ abstract class AbstractCell<T> : AbstractDependency(), Cell<T> {
             }
         }
     }
+
+    override fun toString(): String = "${this::class.simpleName}[$value]"
 }

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

@@ -18,13 +18,15 @@ abstract class AbstractDependentCell<T> : AbstractCell<T>(), Dependent {
             dependenciesActuallyChanged = true
         }
 
-        if (--changingDependencies == 0) {
+        changingDependencies--
+
+        if (changingDependencies == 0) {
             if (dependenciesActuallyChanged) {
                 dependenciesActuallyChanged = false
 
-                dependenciesChanged()
+                dependenciesFinishedChanging()
             } else {
-                emitDependencyChanged(null)
+                emitDependencyChangedEvent(null)
             }
         }
     }
@@ -35,5 +37,9 @@ abstract class AbstractDependentCell<T> : AbstractCell<T>(), Dependent {
         // change set is being completed.
     }
 
-    protected abstract fun dependenciesChanged()
+    /**
+     * Called after a wave of dependencyMightChange notifications followed by an equal amount of
+     * dependencyChanged notifications of which at least one signified an actual change.
+     */
+    protected abstract fun dependenciesFinishedChanging()
 }

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

@@ -193,18 +193,24 @@ infix fun <T : Comparable<T>> Cell<T>.lt(value: T): Cell<Boolean> =
 infix fun <T : Comparable<T>> Cell<T>.lt(other: Cell<T>): Cell<Boolean> =
     map(this, other) { a, b -> a < b }
 
-fun and(vararg cells: Cell<Boolean>): Cell<Boolean> =
-    DependentCell(*cells) { cells.all { it.value } }
-
 infix fun Cell<Boolean>.and(other: Cell<Boolean>): Cell<Boolean> =
     map(this, other) { a, b -> a && b }
 
 infix fun Cell<Boolean>.and(other: Boolean): Cell<Boolean> =
     if (other) this else falseCell()
 
+infix fun Boolean.and(other: Cell<Boolean>): Cell<Boolean> =
+    if (this) other else falseCell()
+
 infix fun Cell<Boolean>.or(other: Cell<Boolean>): Cell<Boolean> =
     map(this, other) { a, b -> a || b }
 
+infix fun Cell<Boolean>.or(other: Boolean): Cell<Boolean> =
+    if (other) trueCell() else this
+
+infix fun Boolean.or(other: Cell<Boolean>): Cell<Boolean> =
+    if (this) trueCell() else other
+
 infix fun Cell<Boolean>.xor(other: Cell<Boolean>): Cell<Boolean> =
     // Use != because of https://youtrack.jetbrains.com/issue/KT-31277.
     map(this, other) { a, b -> a != b }

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

@@ -22,6 +22,6 @@ class DelegatingCell<T>(
         }
 
     override fun emitDependencyChanged() {
-        emitDependencyChanged(ChangeEvent(value))
+        emitDependencyChangedEvent(ChangeEvent(value))
     }
 }

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

@@ -6,16 +6,19 @@ import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
 
 /**
- * Cell of which the value depends on 0 or more other cells.
+ * Cell of which the value depends on 0 or more dependencies.
  */
 class DependentCell<T>(
     private vararg val dependencies: Dependency,
-    private val compute: () -> T
+    private val compute: () -> T,
 ) : AbstractDependentCell<T>() {
 
     private var _value: T? = null
     override val value: T
         get() {
+            // Recompute value every time when we have no dependents. At this point we're not yet a
+            // dependent of our own dependencies, and thus we won't automatically recompute our
+            // value when they change.
             if (dependents.isEmpty()) {
                 _value = compute()
             }
@@ -25,6 +28,9 @@ class DependentCell<T>(
 
     override fun addDependent(dependent: Dependent) {
         if (dependents.isEmpty()) {
+            // Start actually depending on or dependencies when we get our first dependent.
+            // Make sure value is up-to-date here, because from now on `compute` will only be called
+            // when our dependencies change.
             _value = compute()
 
             for (dependency in dependencies) {
@@ -39,20 +45,16 @@ class DependentCell<T>(
         super.removeDependent(dependent)
 
         if (dependents.isEmpty()) {
+            // Stop actually depending on our dependencies when we no longer have any dependents.
             for (dependency in dependencies) {
                 dependency.removeDependent(this)
             }
         }
     }
 
-    override fun dependenciesChanged() {
+    override fun dependenciesFinishedChanging() {
         val newValue = compute()
-
-        if (newValue != _value) {
-            _value = newValue
-            emitDependencyChanged(ChangeEvent(newValue))
-        } else {
-            emitDependencyChanged(null)
-        }
+        _value = newValue
+        emitDependencyChangedEvent(ChangeEvent(newValue))
     }
 }

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

@@ -11,7 +11,7 @@ import world.phantasmal.observable.Dependent
  */
 class FlatteningDependentCell<T>(
     private vararg val dependencies: Dependency,
-    private val compute: () -> Cell<T>
+    private val compute: () -> Cell<T>,
 ) : AbstractDependentCell<T>() {
 
     private var computedCell: Cell<T>? = null
@@ -66,7 +66,7 @@ class FlatteningDependentCell<T>(
         super.dependencyChanged(dependency, event)
     }
 
-    override fun dependenciesChanged() {
+    override fun dependenciesFinishedChanging() {
         if (shouldRecompute) {
             computedCell?.removeDependent(this)
 
@@ -79,12 +79,7 @@ class FlatteningDependentCell<T>(
         }
 
         val newValue = unsafeAssertNotNull(computedCell).value
-
-        if (newValue != _value) {
-            _value = newValue
-            emitDependencyChanged(ChangeEvent(newValue))
-        } else {
-            emitDependencyChanged(null)
-        }
+        _value = newValue
+        emitDependencyChangedEvent(ChangeEvent(newValue))
     }
 }

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

@@ -2,10 +2,19 @@ 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.ChangeObserver
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
+
+class ImmutableCell<T>(override val value: T) : Dependency, Cell<T> {
+    override fun addDependent(dependent: Dependent) {
+        // We don't remember our dependents because we never need to notify them of changes.
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        // Nothing to remove because we don't remember our dependents.
+    }
 
-class ImmutableCell<T>(override val value: T) : AbstractDependency(), Cell<T> {
     override fun observeChange(observer: ChangeObserver<T>): Disposable = nopDisposable()
 
     override fun emitDependencyChanged() {

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

@@ -16,6 +16,6 @@ class SimpleCell<T>(value: T) : AbstractCell<T>(), MutableCell<T> {
         }
 
     override fun emitDependencyChanged() {
-        emitDependencyChanged(ChangeEvent(value))
+        emitDependencyChangedEvent(ChangeEvent(value))
     }
 }

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

@@ -61,15 +61,15 @@ abstract class AbstractDependentListCell<E> :
     override fun observeListChange(observer: ListChangeObserver<E>): Disposable =
         CallbackChangeObserver(this, observer)
 
-    final override fun dependenciesChanged() {
+    final override fun dependenciesFinishedChanging() {
         val oldElements = value
 
         computeElements()
 
-        emitDependencyChanged(
+        emitDependencyChangedEvent(
             ListChangeEvent(
                 elements,
-                listOf(ListChange.Structural(
+                listOf(ListChange(
                     index = 0,
                     prevSize = oldElements.size,
                     removed = oldElements,

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

@@ -0,0 +1,218 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.core.unsafe.unsafeCast
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
+
+abstract class AbstractFilteredListCell<E>(
+    protected val list: ListCell<E>,
+) : AbstractListCell<E>(), Dependent {
+
+    /** Keeps track of number of changing dependencies during a change wave. */
+    private var changingDependencies = 0
+
+    /** Set during a change wave when [list] changes. */
+    private var listChangeEvent: ListChangeEvent<E>? = null
+
+    /** Set during a change wave when [predicateDependency] changes. */
+    private var predicateChanged = false
+
+    override val elements = mutableListOf<E>()
+
+    protected abstract val predicateDependency: Dependency
+
+    override val value: List<E>
+        get() {
+            if (dependents.isEmpty()) {
+                recompute()
+            }
+
+            return elementsWrapper
+        }
+
+    override fun addDependent(dependent: Dependent) {
+        val wasEmpty = dependents.isEmpty()
+
+        super.addDependent(dependent)
+
+        if (wasEmpty) {
+            list.addDependent(this)
+            predicateDependency.addDependent(this)
+            recompute()
+        }
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
+
+        if (dependents.isEmpty()) {
+            predicateDependency.removeDependent(this)
+            list.removeDependent(this)
+        }
+    }
+
+    override fun dependencyMightChange() {
+        changingDependencies++
+        emitMightChange()
+    }
+
+    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+        if (dependency === list) {
+            listChangeEvent = unsafeCast(event)
+        } else if (dependency === predicateDependency) {
+            predicateChanged = event != null
+        } else if (event != null) {
+            otherDependencyChanged(dependency)
+        }
+
+        changingDependencies--
+
+        if (changingDependencies == 0) {
+            if (predicateChanged) {
+                // Simply assume the entire list changes and recompute.
+                val removed = elementsWrapper
+
+                ignoreOtherChanges()
+                recompute()
+
+                emitDependencyChangedEvent(
+                    ListChangeEvent(
+                        elementsWrapper,
+                        listOf(ListChange(0, removed.size, removed, elementsWrapper)),
+                    )
+                )
+            } else {
+                // TODO: Conditionally copyAndResetWrapper?
+                copyAndResetWrapper()
+                val filteredChanges = mutableListOf<ListChange<E>>()
+
+                val listChangeEvent = this.listChangeEvent
+
+                if (listChangeEvent != null) {
+                    for (change in listChangeEvent.changes) {
+                        val prevSize = elements.size
+                        // Map the incoming change index to an index into our own elements list.
+                        // TODO: Avoid this loop by storing the index where an element "would" be
+                        //       if it passed the predicate.
+                        var eventIndex = prevSize
+
+                        for (index in change.index..maxDepIndex()) {
+                            val i = mapIndex(index)
+
+                            if (i != -1) {
+                                eventIndex = i
+                                break
+                            }
+                        }
+
+                        // Process removals.
+                        val removed = mutableListOf<E>()
+
+                        for (element in change.removed) {
+                            val index = removeIndexMapping(change.index)
+
+                            if (index != -1) {
+                                elements.removeAt(eventIndex)
+                                removed.add(element)
+                            }
+                        }
+
+                        // Process insertions.
+                        val inserted = mutableListOf<E>()
+                        var insertionIndex = eventIndex
+
+                        for ((i, element) in change.inserted.withIndex()) {
+                            if (applyPredicate(element)) {
+                                insertIndexMapping(change.index + i, insertionIndex, element)
+                                elements.add(insertionIndex, element)
+                                inserted.add(element)
+                                insertionIndex++
+                            } else {
+                                insertIndexMapping(change.index + i, -1, element)
+                            }
+                        }
+
+                        // Shift mapped indices by a certain amount. This amount can be positive,
+                        // negative or zero.
+                        val diff = inserted.size - removed.size
+
+                        if (diff != 0) {
+                            // Indices before the change index stay the same. Newly inserted indices
+                            // are already correct. So we only need to shift everything after the
+                            // new indices.
+                            for (index in (change.index + change.inserted.size)..maxDepIndex()) {
+                                shiftIndexMapping(index, diff)
+                            }
+                        }
+
+                        // Add a list change if something actually changed.
+                        if (removed.isNotEmpty() || inserted.isNotEmpty()) {
+                            filteredChanges.add(
+                                ListChange(
+                                    eventIndex,
+                                    prevSize,
+                                    removed,
+                                    inserted,
+                                )
+                            )
+                        }
+                    }
+                }
+
+                processOtherChanges(filteredChanges)
+
+                if (filteredChanges.isEmpty()) {
+                    emitDependencyChangedEvent(null)
+                } else {
+                    emitDependencyChangedEvent(
+                        ListChangeEvent(elementsWrapper, filteredChanges)
+                    )
+                }
+            }
+
+            // Reset for next change wave.
+            listChangeEvent = null
+            predicateChanged = false
+            resetChangeWaveData()
+        }
+    }
+
+    /** Called when a dependency that's neither [list] nor [predicateDependency] has changed. */
+    protected abstract fun otherDependencyChanged(dependency: Dependency)
+
+    protected abstract fun ignoreOtherChanges()
+
+    protected abstract fun processOtherChanges(filteredChanges: MutableList<ListChange<E>>)
+
+    override fun emitDependencyChanged() {
+        // Nothing to do because AbstractFilteredListCell emits dependencyChanged immediately. We
+        // don't defer this operation because AbstractFilteredListCell only changes when there is no
+        // change set or the current change set is being completed.
+    }
+
+    protected abstract fun applyPredicate(element: E): Boolean
+
+    protected abstract fun maxDepIndex(): Int
+
+    /**
+     * Maps and index into [list] to an index into this list. Returns -1 if the given index does not
+     * point to an element that passes the predicate, i.e. the element is not in this list.
+     */
+    protected abstract fun mapIndex(index: Int): Int
+
+    /**
+     * Removes the element at the given index into [list] from our mapping. Returns the previous
+     * index into our list.
+     */
+    protected abstract fun removeIndexMapping(index: Int): Int
+
+    protected abstract fun insertIndexMapping(depIndex: Int, localIndex: Int, element: E)
+
+    /** Adds [shift] to the local index at [depIndex] if it's not -1. */
+    protected abstract fun shiftIndexMapping(depIndex: Int, shift: Int)
+
+    protected abstract fun recompute()
+
+    protected abstract fun resetChangeWaveData()
+}

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

@@ -27,8 +27,7 @@ class DelegatingList<E>(var backingList: List<E>) : List<E> {
     override fun subList(fromIndex: Int, toIndex: Int): List<E> =
         backingList.subList(fromIndex, toIndex)
 
-    @Suppress("SuspiciousEqualsCombination")
-    override fun equals(other: Any?): Boolean = this === other || other == backingList
+    override fun equals(other: Any?): Boolean = other == backingList
 
     override fun hashCode(): Int = backingList.hashCode()
 

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

@@ -1,217 +1,212 @@
 package world.phantasmal.observable.cell.list
 
+import world.phantasmal.core.assert
+import world.phantasmal.core.unsafe.unsafeCast
 import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
+import world.phantasmal.observable.cell.Cell
 
 class FilteredListCell<E>(
-    private val dependency: ListCell<E>,
-    private val predicate: (E) -> Boolean,
-) : AbstractListCell<E>(), Dependent {
+    list: ListCell<E>,
+    private val predicate: Cell<(E) -> Cell<Boolean>>,
+) : AbstractFilteredListCell<E>(list) {
     /**
-     * Maps the dependency's indices to this list's indices. When an element of the dependency list
+     * Maps the dependency's indices to the corresponding index into this list and the result of the
+     * predicate applied to the element at that index. When an element of the dependency list
      * doesn't pass the predicate, its index in this mapping is set to -1.
      */
-    private val indexMap = mutableListOf<Int>()
+    private val indexMap = mutableListOf<Mapping>()
 
-    override val elements = mutableListOf<E>()
+    private val changedPredicateResults = mutableListOf<Mapping>()
 
-    override val value: List<E>
-        get() {
-            if (dependents.isEmpty()) {
-                recompute()
-            }
-
-            return elementsWrapper
-        }
-
-    override fun addDependent(dependent: Dependent) {
-        if (dependents.isEmpty()) {
-            dependency.addDependent(this)
-            recompute()
-        }
-
-        super.addDependent(dependent)
-    }
+    override val predicateDependency: Dependency
+        get() = predicate
 
     override fun removeDependent(dependent: Dependent) {
         super.removeDependent(dependent)
 
         if (dependents.isEmpty()) {
-            dependency.removeDependent(this)
+            for (mapping in indexMap) {
+                mapping.removeDependent(this)
+            }
         }
     }
 
-    override fun dependencyMightChange() {
-        emitMightChange()
+    override fun otherDependencyChanged(dependency: Dependency) {
+        assert { dependency is FilteredListCell<*>.Mapping }
+
+        changedPredicateResults.add(unsafeCast(dependency))
     }
 
-    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-        if (event is ListChangeEvent<*>) {
-            val prevSize = elements.size
-            val filteredChanges = mutableListOf<ListChange<E>>()
+    override fun ignoreOtherChanges() {
+        changedPredicateResults.clear()
+    }
 
-            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.
-                        @Suppress("UNCHECKED_CAST")
-                        change as ListChange.Structural<E>
+    override fun processOtherChanges(filteredChanges: MutableList<ListChange<E>>) {
+        var shift = 0
 
-                        val removed = mutableListOf<E>()
-                        var eventIndex = -1
+        for ((dependencyIndex, mapping) in indexMap.withIndex()) {
+            if (changedPredicateResults.isEmpty()) {
+                break
+            }
 
-                        change.removed.forEachIndexed { i, element ->
-                            val index = indexMap[change.index + i]
+            if (changedPredicateResults.remove(mapping)) {
+                val result = mapping.predicateResult.value
+                val oldResult = mapping.index != -1
 
-                            if (index != -1) {
-                                removed.add(element)
+                if (result != oldResult) {
+                    val prevSize = elements.size
 
-                                if (eventIndex == -1) {
-                                    eventIndex = index
-                                }
+                    if (result) {
+                        // TODO: Avoid this loop by storing the index where an element "would" be
+                        //       if it passed the predicate.
+                        var insertionIndex = elements.size
+
+                        for (index in (dependencyIndex + 1)..indexMap.lastIndex) {
+                            val i = indexMap[index].index
+
+                            if (i != -1) {
+                                insertionIndex = i + shift
+                                break
                             }
                         }
 
-                        recompute()
+                        val element = list.value[dependencyIndex]
+                        elements.add(insertionIndex, element)
+                        mapping.index = insertionIndex
+                        shift++
 
-                        val inserted = mutableListOf<E>()
+                        filteredChanges.add(ListChange(
+                            insertionIndex,
+                            prevSize,
+                            removed = emptyList(),
+                            inserted = listOf(element),
+                        ))
+                    } else {
+                        val index = mapping.index + shift
+                        val element = elements.removeAt(index)
+                        mapping.index = -1
+                        shift--
 
-                        change.inserted.forEachIndexed { i, element ->
-                            val index = indexMap[change.index + i]
-
-                            if (index != -1) {
-                                inserted.add(element)
-
-                                if (eventIndex == -1) {
-                                    eventIndex = index
-                                }
-                            }
-                        }
-
-                        if (removed.isNotEmpty() || inserted.isNotEmpty()) {
-                            check(eventIndex != -1)
-                            filteredChanges.add(
-                                ListChange.Structural(
-                                    eventIndex,
-                                    prevSize,
-                                    removed,
-                                    inserted
-                                )
-                            )
-                        }
-                    }
-                    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).
-                        @Suppress("UNCHECKED_CAST")
-                        change as ListChange.Element<E>
-
-                        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 (change.index + 1)..indexMap.lastIndex) {
-                                    val thisIdx = indexMap[depIdx]
-
-                                    if (thisIdx != -1) {
-                                        insertIndex = thisIdx
-                                        break
-                                    }
-                                }
-
-                                copyAndResetWrapper()
-                                elements.add(insertIndex, change.updated)
-                                indexMap[change.index] = insertIndex
-
-                                for (depIdx in (change.index + 1)..indexMap.lastIndex) {
-                                    val thisIdx = indexMap[depIdx]
-
-                                    if (thisIdx != -1) {
-                                        indexMap[depIdx]++
-                                    }
-                                }
-
-                                filteredChanges.add(
-                                    ListChange.Structural(
-                                        insertIndex,
-                                        prevSize,
-                                        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 structural change.
-                                copyAndResetWrapper()
-                                elements.removeAt(index)
-                                indexMap[change.index] = -1
-
-                                for (depIdx in (change.index + 1)..indexMap.lastIndex) {
-                                    val thisIdx = indexMap[depIdx]
-
-                                    if (thisIdx != -1) {
-                                        indexMap[depIdx]--
-                                    }
-                                }
-
-                                filteredChanges.add(
-                                    ListChange.Structural(
-                                        index,
-                                        prevSize,
-                                        removed = listOf(change.updated),
-                                        inserted = emptyList(),
-                                    )
-                                )
-                            } else {
-                                // Otherwise just propagate the element change.
-                                filteredChanges.add(ListChange.Element(index, change.updated))
-                            }
-                        }
+                        filteredChanges.add(ListChange(
+                            index,
+                            prevSize,
+                            removed = listOf(element),
+                            inserted = emptyList(),
+                        ))
                     }
+                } else if (oldResult) {
+                    mapping.index += shift
                 }
-            }
-
-            if (filteredChanges.isEmpty()) {
-                emitDependencyChanged(null)
             } else {
-                emitDependencyChanged(ListChangeEvent(elementsWrapper, filteredChanges))
+                mapping.index += shift
             }
-        } else {
-            emitDependencyChanged(null)
+        }
+
+        // Can still contain changed mappings at this point if e.g. an element was removed after its
+        // predicate result changed.
+        changedPredicateResults.clear()
+    }
+
+    override fun applyPredicate(element: E): Boolean =
+        predicate.value(element).value
+
+    override fun maxDepIndex(): Int =
+        indexMap.lastIndex
+
+    override fun mapIndex(index: Int): Int =
+        indexMap[index].index
+
+    override fun removeIndexMapping(index: Int): Int {
+        val mapping = indexMap.removeAt(index)
+        mapping.removeDependent(this)
+        return mapping.index
+    }
+
+    override fun insertIndexMapping(depIndex: Int, localIndex: Int, element: E) {
+        val mapping = Mapping(predicate.value(element), localIndex)
+        mapping.addDependent(this)
+        indexMap.add(depIndex, mapping)
+    }
+
+    override fun shiftIndexMapping(depIndex: Int, shift: Int) {
+        val mapping = indexMap[depIndex]
+
+        if (mapping.index != -1) {
+            mapping.index += shift
         }
     }
 
-    override fun emitDependencyChanged() {
-        // Nothing to do because FilteredListCell emits dependencyChanged immediately. We don't
-        // defer this operation because FilteredListCell only changes when there is no transaction
-        // or the current transaction is being committed.
-    }
-
-    private fun recompute() {
+    override fun recompute() {
         copyAndResetWrapper()
         elements.clear()
+
+        for (mapping in indexMap) {
+            mapping.removeDependent(this)
+        }
+
         indexMap.clear()
 
-        dependency.value.forEach { element ->
-            if (predicate(element)) {
-                elements.add(element)
-                indexMap.add(elements.lastIndex)
-            } else {
-                indexMap.add(-1)
+        // Cache value here to facilitate loop unswitching.
+        val hasDependents = dependents.isNotEmpty()
+        val pred = predicate.value
+
+        for (element in list.value) {
+            val predicateResult = pred(element)
+
+            val index =
+                if (predicateResult.value) {
+                    elements.add(element)
+                    elements.lastIndex
+                } else {
+                    -1
+                }
+
+            if (hasDependents) {
+                val mapping = Mapping(predicateResult, index)
+                mapping.addDependent(this)
+                indexMap.add(mapping)
             }
         }
     }
+
+    override fun resetChangeWaveData() {
+        changedPredicateResults.clear()
+    }
+
+    private inner class Mapping(
+        val predicateResult: Cell<Boolean>,
+        /**
+         * The index into [elements] if the element passes the predicate. -1 If the element does not
+         * pass the predicate.
+         */
+        var index: Int,
+    ) : Dependent, Dependency {
+        override fun dependencyMightChange() {
+            this@FilteredListCell.dependencyMightChange()
+        }
+
+        override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+            assert { dependency === predicateResult }
+
+            this@FilteredListCell.dependencyChanged(this, event)
+        }
+
+        override fun addDependent(dependent: Dependent) {
+            assert { dependent === this@FilteredListCell }
+
+            predicateResult.addDependent(this)
+        }
+
+        override fun removeDependent(dependent: Dependent) {
+            assert { dependent === this@FilteredListCell }
+
+            predicateResult.removeDependent(this)
+        }
+
+        override fun emitDependencyChanged() {
+            // Nothing to do.
+        }
+    }
 }

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

@@ -2,20 +2,29 @@ package world.phantasmal.observable.cell.list
 
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.nopDisposable
-import world.phantasmal.observable.AbstractDependency
 import world.phantasmal.observable.ChangeObserver
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
 import world.phantasmal.observable.cell.Cell
 import world.phantasmal.observable.cell.cell
 import world.phantasmal.observable.cell.falseCell
 import world.phantasmal.observable.cell.trueCell
 
-class ImmutableListCell<E>(private val elements: List<E>) : AbstractDependency(), ListCell<E> {
+class ImmutableListCell<E>(private val elements: List<E>) : Dependency, ListCell<E> {
     override val size: Cell<Int> = cell(elements.size)
     override val empty: Cell<Boolean> = if (elements.isEmpty()) trueCell() else falseCell()
     override val notEmpty: Cell<Boolean> = if (elements.isNotEmpty()) trueCell() else falseCell()
 
     override val value: List<E> = elements
 
+    override fun addDependent(dependent: Dependent) {
+        // We don't remember our dependents because we never need to notify them of changes.
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        // Nothing to remove because we don't remember our dependents.
+    }
+
     override fun get(index: Int): E =
         elements[index]
 

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

@@ -1,7 +1,9 @@
 package world.phantasmal.observable.cell.list
 
+import world.phantasmal.observable.Observable
 import world.phantasmal.observable.cell.Cell
 import world.phantasmal.observable.cell.DependentCell
+import world.phantasmal.observable.cell.ImmutableCell
 
 private val EMPTY_LIST_CELL = ImmutableListCell<Nothing>(emptyList())
 
@@ -12,11 +14,20 @@ fun <E> listCell(vararg elements: E): ListCell<E> = ImmutableListCell(elements.t
 fun <E> emptyListCell(): ListCell<E> = EMPTY_LIST_CELL
 
 /** Returns a mutable list cell containing [elements]. */
-fun <E> mutableListCell(
-    vararg elements: E,
-    extractDependencies: DependenciesExtractor<E>? = null,
-): MutableListCell<E> =
-    SimpleListCell(mutableListOf(*elements), extractDependencies)
+fun <E> mutableListCell(vararg elements: E): MutableListCell<E> =
+    SimpleListCell(mutableListOf(*elements))
+
+/**
+ * Returns a cell that changes whenever this list cell is structurally changed or when its
+ * individual elements change.
+ *
+ * @param extractObservables Called on each element to determine which element changes should be
+ * observed.
+ */
+fun <E> ListCell<E>.dependingOnElements(
+    extractObservables: (element: E) -> Array<out Observable<*>>,
+): Cell<List<E>> =
+    ListElementsDependentCell(this, extractObservables)
 
 fun <E, R> ListCell<E>.listMap(transform: (E) -> R): ListCell<R> =
     DependentListCell(this) { value.map(transform) }
@@ -31,13 +42,16 @@ fun <E> ListCell<E>.sumOf(selector: (E) -> Int): Cell<Int> =
     DependentCell(this) { value.sumOf(selector) }
 
 fun <E> ListCell<E>.filtered(predicate: (E) -> Boolean): ListCell<E> =
-    FilteredListCell(this, predicate)
+    SimpleFilteredListCell(this, ImmutableCell(predicate))
 
 fun <E> ListCell<E>.filtered(predicate: Cell<(E) -> Boolean>): ListCell<E> =
-    DependentListCell(this, predicate) { value.filter(predicate.value) }
+    SimpleFilteredListCell(this, predicate)
 
-fun <E> ListCell<E>.sortedWith(comparator: Cell<Comparator<E>>): ListCell<E> =
-    DependentListCell(this, comparator) { value.sortedWith(comparator.value) }
+fun <E> ListCell<E>.filteredCell(predicate: (E) -> Cell<Boolean>): ListCell<E> =
+    FilteredListCell(this, ImmutableCell(predicate))
+
+fun <E> ListCell<E>.filteredCell(predicate: Cell<(E) -> Cell<Boolean>>): ListCell<E> =
+    FilteredListCell(this, predicate)
 
 fun <E> ListCell<E>.firstOrNull(): Cell<E?> =
     DependentCell(this) { value.firstOrNull() }

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

@@ -7,39 +7,19 @@ class ListChangeEvent<out E>(
     val changes: List<ListChange<E>>,
 ) : ChangeEvent<List<E>>(value)
 
-sealed class ListChange<out E> {
-    /**
-     * Represents a structural change to a list cell. E.g. an element is inserted or removed.
-     */
-    class Structural<out E>(
-        val index: Int,
-        val prevSize: Int,
-        /**
-         * The elements that were removed from the list at [index].
-         *
-         * 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 [ChangeEvent]'s [inserted] list, it may or may not
-         * be mutated when the originating [ListCell] is mutated.
-         */
-        val inserted: List<E>,
-    ) : ListChange<E>() {
-        val allRemoved: Boolean get() = removed.size == prevSize
-    }
-
-    /**
-     * Represents a change to an element in a list cell. Will only be emitted if the list is
-     * configured to do so.
-     */
-    class Element<E>(
-        val index: Int,
-        val updated: E,
-    ) : ListChange<E>()
+/**
+ * Represents a structural change to a list cell. E.g. an element is inserted or removed.
+ */
+class ListChange<out E>(
+    val index: Int,
+    val prevSize: Int,
+    /** The elements that were removed from the list at [index]. */
+    val removed: List<E>,
+    /** The elements that were inserted into the list at [index]. */
+    val inserted: List<E>,
+) {
+    /** True when this change resulted in the removal of all elements from the list. */
+    val allRemoved: Boolean get() = removed.size == prevSize
 }
 
 typealias ListChangeObserver<E> = (ListChangeEvent<E>) -> Unit

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

@@ -0,0 +1,140 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.core.splice
+import world.phantasmal.core.unsafe.unsafeCast
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.Dependent
+import world.phantasmal.observable.Observable
+import world.phantasmal.observable.cell.AbstractCell
+
+/**
+ * Depends on a [ListCell] and zero or more [Observable]s per element in the list.
+ */
+class ListElementsDependentCell<E>(
+    private val list: ListCell<E>,
+    private val extractObservables: (element: E) -> Array<out Observable<*>>,
+) : AbstractCell<List<E>>(), Dependent {
+    /** An array of dependencies per [list] element, extracted by [extractObservables]. */
+    private val elementDependencies = mutableListOf<Array<out Dependency>>()
+
+    /** Keeps track of how many of our dependencies are about to (maybe) change. */
+    private var changingDependencies = 0
+
+    /**
+     * Set to true once one of our dependencies has actually changed. Reset to false whenever
+     * [changingDependencies] hits 0 again.
+     */
+    private var dependenciesActuallyChanged = false
+
+    private var listChangeEvent: ListChangeEvent<E>? = null
+
+    override val value: List<E>
+        get() = list.value
+
+    override fun addDependent(dependent: Dependent) {
+        if (dependents.isEmpty()) {
+            // Once we have our first dependent, we start depending on our own dependencies.
+            list.addDependent(this)
+
+            for (element in list.value) {
+                val dependencies = extractObservables(element)
+
+                for (dependency in dependencies) {
+                    dependency.addDependent(this)
+                }
+
+                elementDependencies.add(dependencies)
+            }
+        }
+
+        super.addDependent(dependent)
+    }
+
+    override fun removeDependent(dependent: Dependent) {
+        super.removeDependent(dependent)
+
+        if (dependents.isEmpty()) {
+            // At this point we have no more dependents, so we can stop depending on our own
+            // dependencies.
+            for (dependencies in elementDependencies) {
+                for (dependency in dependencies) {
+                    dependency.removeDependent(this)
+                }
+            }
+
+            elementDependencies.clear()
+            list.removeDependent(this)
+        }
+    }
+
+    override fun dependencyMightChange() {
+        changingDependencies++
+        emitMightChange()
+    }
+
+    override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+        if (event != null) {
+            dependenciesActuallyChanged = true
+
+            // Simply store all list changes when the changing dependency is our list dependency. We
+            // don't update our dependencies yet to avoid receiving dependencyChanged notifications
+            // from newly inserted dependencies for which we haven't received any
+            // dependencyMightChange notifications and to avoid *NOT* receiving dependencyChanged
+            // notifications from removed dependencies for which we *HAVE* received
+            // dependencyMightChange notifications.
+            if (dependency === list) {
+                listChangeEvent = unsafeCast(event)
+            }
+        }
+
+        changingDependencies--
+
+        if (changingDependencies == 0) {
+            // All of our dependencies have finished changing.
+
+            // At this point we can remove this dependent from the removed elements' dependencies
+            // and add it to the newly inserted elements' dependencies.
+            listChangeEvent?.let { listChangeEvent ->
+                for (change in listChangeEvent.changes) {
+                    for (i in change.index until (change.index + change.removed.size)) {
+                        for (elementDependency in elementDependencies[i]) {
+                            elementDependency.removeDependent(this)
+                        }
+                    }
+
+                    val inserted = change.inserted.map(extractObservables)
+
+                    elementDependencies.splice(
+                        startIndex = change.index,
+                        amount = change.removed.size,
+                        elements = inserted,
+                    )
+
+                    for (elementDependencies in inserted) {
+                        for (elementDependency in elementDependencies) {
+                            elementDependency.addDependent(this)
+                        }
+                    }
+                }
+            }
+
+            // Reset for the next change wave.
+            listChangeEvent = null
+
+            if (dependenciesActuallyChanged) {
+                dependenciesActuallyChanged = false
+
+                emitDependencyChangedEvent(ChangeEvent(list.value))
+            } else {
+                emitDependencyChangedEvent(null)
+            }
+        }
+    }
+
+    override fun emitDependencyChanged() {
+        // Nothing to do because ListElementsDependentCell emits dependencyChanged immediately. We
+        // don't defer this operation because ListElementsDependentCell only changes when there is
+        // no transaction or the current transaction is being committed.
+    }
+}

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

@@ -0,0 +1,80 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.Dependency
+import world.phantasmal.observable.cell.Cell
+
+class SimpleFilteredListCell<E>(
+    list: ListCell<E>,
+    private val predicate: Cell<(E) -> Boolean>,
+) : AbstractFilteredListCell<E>(list) {
+    /**
+     * Maps the dependency's indices to this list's indices. When an element of the dependency list
+     * doesn't pass the predicate, its index in this mapping is set to -1.
+     *
+     * This is not a performance improvement but a requirement. We can't determine an element's
+     * index into our own [elements] list by using e.g. indexOf because a list can contain the same
+     * element multiple times.
+     */
+    private val indexMap = mutableListOf<Int>()
+
+    override val predicateDependency: Dependency
+        get() = predicate
+
+    override fun otherDependencyChanged(dependency: Dependency) {
+        // Unreachable code path.
+        error("Unexpected dependency.")
+    }
+
+    override fun ignoreOtherChanges() {
+        // Nothing to ignore.
+    }
+
+    override fun processOtherChanges(filteredChanges: MutableList<ListChange<E>>) {
+        // Nothing to process.
+    }
+
+    override fun applyPredicate(element: E): Boolean =
+        predicate.value(element)
+
+    override fun maxDepIndex(): Int =
+        indexMap.lastIndex
+
+    override fun mapIndex(index: Int): Int =
+        indexMap[index]
+
+    override fun removeIndexMapping(index: Int): Int =
+        indexMap.removeAt(index)
+
+    override fun insertIndexMapping(depIndex: Int, localIndex: Int, element: E) {
+        indexMap.add(depIndex, localIndex)
+    }
+
+    override fun shiftIndexMapping(depIndex: Int, shift: Int) {
+        val i = indexMap[depIndex]
+
+        if (i != -1) {
+            indexMap[depIndex] = i + shift
+        }
+    }
+
+    override fun recompute() {
+        copyAndResetWrapper()
+        elements.clear()
+        indexMap.clear()
+
+        val pred = predicate.value
+
+        for (element in list.value) {
+            if (pred(element)) {
+                elements.add(element)
+                indexMap.add(elements.lastIndex)
+            } else {
+                indexMap.add(-1)
+            }
+        }
+    }
+
+    override fun resetChangeWaveData() {
+        // Nothing to reset.
+    }
+}

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

@@ -1,32 +1,15 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.replaceAll
-import world.phantasmal.core.unsafe.unsafeAssertNotNull
-import world.phantasmal.observable.ChangeEvent
 import world.phantasmal.observable.ChangeManager
-import world.phantasmal.observable.Dependency
-import world.phantasmal.observable.Dependent
-
-typealias DependenciesExtractor<E> = (element: E) -> Array<Dependency>
 
 /**
  * @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>(
     override val elements: MutableList<E>,
-    private val extractDependencies: DependenciesExtractor<E>? = null,
 ) : AbstractListCell<E>(), MutableListCell<E> {
 
-    /**
-     * 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 changes = mutableListOf<ListChange<E>>()
 
     override var value: List<E>
@@ -45,18 +28,12 @@ class SimpleListCell<E>(
         copyAndResetWrapper()
         val removed = elements.set(index, element)
 
-        if (dependents.isNotEmpty() && extractDependencies != null) {
-            elementDependents[index].dispose()
-            elementDependents[index] = ElementDependent(index, element)
-        }
-
-        changes.add(ListChange.Structural(
+        finalizeChange(
             index,
             prevSize = elements.size,
             removed = listOf(removed),
             inserted = listOf(element),
-        ))
-        ChangeManager.changed(this)
+        )
 
         return removed
     }
@@ -68,7 +45,7 @@ class SimpleListCell<E>(
         copyAndResetWrapper()
         elements.add(element)
 
-        finalizeStructuralChange(
+        finalizeChange(
             index,
             prevSize = index,
             removed = emptyList(),
@@ -84,7 +61,7 @@ class SimpleListCell<E>(
         copyAndResetWrapper()
         elements.add(index, element)
 
-        finalizeStructuralChange(index, prevSize, removed = emptyList(), inserted = listOf(element))
+        finalizeChange(index, prevSize, removed = emptyList(), inserted = listOf(element))
     }
 
     override fun remove(element: E): Boolean {
@@ -107,7 +84,7 @@ class SimpleListCell<E>(
         copyAndResetWrapper()
         val removed = elements.removeAt(index)
 
-        finalizeStructuralChange(index, prevSize, removed = listOf(removed), inserted = emptyList())
+        finalizeChange(index, prevSize, removed = listOf(removed), inserted = emptyList())
         return removed
     }
 
@@ -120,7 +97,7 @@ class SimpleListCell<E>(
         copyAndResetWrapper()
         this.elements.replaceAll(elements)
 
-        finalizeStructuralChange(index = 0, prevSize, removed, inserted = elementsWrapper)
+        finalizeChange(index = 0, prevSize, removed, inserted = elementsWrapper)
     }
 
     override fun replaceAll(elements: Sequence<E>) {
@@ -132,7 +109,7 @@ class SimpleListCell<E>(
         copyAndResetWrapper()
         this.elements.replaceAll(elements)
 
-        finalizeStructuralChange(index = 0, prevSize, removed, inserted = elementsWrapper)
+        finalizeChange(index = 0, prevSize, removed, inserted = elementsWrapper)
     }
 
     override fun splice(fromIndex: Int, removeCount: Int, newElement: E) {
@@ -149,7 +126,7 @@ class SimpleListCell<E>(
         repeat(removeCount) { elements.removeAt(fromIndex) }
         elements.add(fromIndex, newElement)
 
-        finalizeStructuralChange(fromIndex, prevSize, removed, inserted = listOf(newElement))
+        finalizeChange(fromIndex, prevSize, removed, inserted = listOf(newElement))
     }
 
     override fun clear() {
@@ -161,7 +138,7 @@ class SimpleListCell<E>(
         copyAndResetWrapper()
         elements.clear()
 
-        finalizeStructuralChange(index = 0, prevSize, removed, inserted = emptyList())
+        finalizeChange(index = 0, prevSize, removed, inserted = emptyList())
     }
 
     override fun sortWith(comparator: Comparator<E>) {
@@ -177,7 +154,7 @@ class SimpleListCell<E>(
             throwable = e
         }
 
-        finalizeStructuralChange(
+        finalizeChange(
             index = 0,
             prevSize = elements.size,
             removed,
@@ -189,32 +166,10 @@ class SimpleListCell<E>(
         }
     }
 
-    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()
-        }
-    }
-
     override fun emitDependencyChanged() {
         val currentChanges = changes
         changes = mutableListOf()
-        emitDependencyChanged(ListChangeEvent(elementsWrapper, currentChanges))
+        emitDependencyChangedEvent(ListChangeEvent(elementsWrapper, currentChanges))
     }
 
     private fun checkIndex(index: Int, maxIndex: Int) {
@@ -225,75 +180,13 @@ class SimpleListCell<E>(
         }
     }
 
-    private fun finalizeStructuralChange(
+    private fun finalizeChange(
         index: Int,
         prevSize: Int,
         removed: List<E>,
         inserted: List<E>,
     ) {
-        if (dependents.isNotEmpty() && 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
-            }
-        }
-
-        changes.add(ListChange.Structural(index, prevSize, removed, inserted))
+        changes.add(ListChange(index, prevSize, removed, inserted))
         ChangeManager.changed(this)
     }
-
-    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
-                    changes.add(ListChange.Element(index, element))
-                }
-
-                if (--changingElements == 0) {
-                    ChangeManager.changed(this@SimpleListCell)
-                }
-            }
-        }
-    }
 }

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

@@ -42,6 +42,10 @@ interface DependencyTests : ObservableTestSuite {
     interface Provider {
         val dependency: Dependency
 
+        /**
+         * Makes [dependency] emit [Dependent.dependencyMightChange] followed by
+         * [Dependent.dependencyChanged] with a non-null event.
+         */
         fun emit()
     }
 }

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

@@ -51,12 +51,12 @@ interface CellTests : ObservableTests {
     fun emits_correct_value_in_change_events() = test {
         val p = createProvider()
 
-        var prevValue: Any?
-        var observedValue: Any? = null
+        var prevValue: Snapshot?
+        var observedValue: Snapshot? = null
 
         disposer.add(p.observable.observeChange { changeEvent ->
             assertNull(observedValue)
-            observedValue = changeEvent.value
+            observedValue = changeEvent.value.snapshot()
         })
 
         repeat(3) {
@@ -69,7 +69,7 @@ interface CellTests : ObservableTests {
             // it should be equal to the cell's current value.
             assertNotNull(observedValue)
             assertNotEquals(prevValue, observedValue)
-            assertEquals(p.observable.value, observedValue)
+            assertEquals(p.observable.value.snapshot(), observedValue)
         }
     }
 
@@ -82,20 +82,20 @@ interface CellTests : ObservableTests {
     fun reflects_changes_without_observers() = test {
         val p = createProvider()
 
-        var old: Any?
+        var old: Snapshot?
 
         repeat(5) {
             // Value should change after emit.
-            old = p.observable.value
+            old = p.observable.value.snapshot()
 
             p.emit()
 
-            val new = p.observable.value
+            val new = p.observable.value.snapshot()
 
             assertNotEquals(old, new)
 
             // Value should not change when emit hasn't been called since the last access.
-            assertEquals(new, p.observable.value)
+            assertEquals(new, p.observable.value.snapshot())
         }
     }
 
@@ -120,10 +120,10 @@ interface CellTests : ObservableTests {
     @Test
     fun propagates_changes_to_mapped_cell() = test {
         val p = createProvider()
-        val mapped = p.observable.map { it.hashCode() }
+        val mapped = p.observable.map { it.snapshot() }
         val initialValue = mapped.value
 
-        var observedValue: Int? = null
+        var observedValue: Snapshot? = null
 
         disposer.add(mapped.observeChange { changeEvent ->
             assertNull(observedValue)
@@ -140,10 +140,10 @@ interface CellTests : ObservableTests {
     fun propagates_changes_to_flat_mapped_cell() = test {
         val p = createProvider()
 
-        val mapped = p.observable.flatMap { ImmutableCell(it.hashCode()) }
+        val mapped = p.observable.flatMap { ImmutableCell(it.snapshot()) }
         val initialValue = mapped.value
 
-        var observedValue: Int? = null
+        var observedValue: Snapshot? = null
 
         disposer.add(mapped.observeChange {
             assertNull(observedValue)
@@ -160,3 +160,16 @@ interface CellTests : ObservableTests {
         override val observable: Cell<Any>
     }
 }
+
+/** See [snapshot]. */
+private typealias Snapshot = String
+
+/**
+ * We use toString to create "snapshots" of values throughout the tests. Most of the time cells will
+ * actually have a new value after emitting a change event, but this is not always the case with
+ * more complex cells or cells that point to complex values. So instead of keeping references to
+ * values and comparing them with == (or using e.g. assertEquals), we compare snapshots.
+ *
+ * This of course assumes that all values have sensible toString implementations.
+ */
+private fun Any?.snapshot(): Snapshot = toString()

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

@@ -1,21 +1,61 @@
 package world.phantasmal.observable.cell
 
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.Dependency
 import world.phantasmal.observable.Dependent
-import kotlin.test.Test
-import kotlin.test.assertEquals
-import kotlin.test.assertTrue
+import kotlin.test.*
 
 interface CellWithDependenciesTests : CellTests {
-    override fun createProvider(): Provider
+    fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ): Cell<Any>
+
+    @Test
+    fun emits_precisely_once_when_all_of_its_dependencies_emit() = test {
+        val root = SimpleCell(5)
+        val branch1 = DependentCell(root) { root.value * 2 }
+        val branch2 = DependentCell(root) { root.value * 3 }
+        val branch3 = DependentCell(root) { root.value * 4 }
+        val leaf = createWithDependencies(branch1, branch2, branch3)
+        var dependencyMightChangeCalled = false
+        var dependencyChangedCalled = false
+
+        leaf.addDependent(object : Dependent {
+            override fun dependencyMightChange() {
+                assertFalse(dependencyMightChangeCalled)
+                assertFalse(dependencyChangedCalled)
+                dependencyMightChangeCalled = true
+            }
+
+            override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
+                assertTrue(dependencyMightChangeCalled)
+                assertFalse(dependencyChangedCalled)
+                assertEquals(leaf, dependency)
+                assertNotNull(event)
+                dependencyChangedCalled = true
+            }
+        })
+
+        repeat(5) { index ->
+            dependencyMightChangeCalled = false
+            dependencyChangedCalled = false
+
+            root.value += 1
+
+            assertTrue(dependencyMightChangeCalled, "repetition $index")
+            assertTrue(dependencyChangedCalled, "repetition $index")
+        }
+    }
 
     @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)
+        val branch1 = DependentCell(root) { root.value * 2 }
+        val branch2 = DependentCell(root) { root.value * 3 }
+        val branch3 = DependentCell(root) { root.value * 4 }
+        val leaf = createWithDependencies(branch1, branch2, branch3)
 
         var observedChanges = 0
 
@@ -30,29 +70,31 @@ interface CellWithDependenciesTests : CellTests {
 
     @Test
     fun doesnt_register_as_dependent_of_its_dependencies_until_it_has_dependents_itself() = test {
-        val p = createProvider()
+        val dependency1 = TestCell()
+        val dependency2 = TestCell()
+        val dependency3 = TestCell()
 
-        val dependency = object : AbstractCell<Int>() {
-            val publicDependents: List<Dependent> = dependents
+        val cell = createWithDependencies(dependency1, dependency2, dependency3)
 
-            override val value: Int = 5
-
-            override fun emitDependencyChanged() {
-                // Not going to change.
-                throw NotImplementedError()
-            }
-        }
-
-        val cell = p.createWithDependencies(dependency)
-
-        assertTrue(dependency.publicDependents.isEmpty())
+        assertTrue(dependency1.publicDependents.isEmpty())
+        assertTrue(dependency2.publicDependents.isEmpty())
+        assertTrue(dependency3.publicDependents.isEmpty())
 
         disposer.add(cell.observeChange { })
 
-        assertEquals(1, dependency.publicDependents.size)
+        assertEquals(1, dependency1.publicDependents.size)
+        assertEquals(1, dependency2.publicDependents.size)
+        assertEquals(1, dependency3.publicDependents.size)
     }
 
-    interface Provider : CellTests.Provider {
-        fun createWithDependencies(vararg dependencies: Cell<Int>): Cell<Any>
+    private class TestCell : AbstractCell<Int>() {
+        val publicDependents: List<Dependent> = dependents
+
+        override val value: Int = 5
+
+        override fun emitDependencyChanged() {
+            // Not going to change.
+            throw NotImplementedError()
+        }
     }
 }

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

@@ -8,7 +8,16 @@ class DependentCellTests : RegularCellTests, CellWithDependenciesTests {
         return DependentCell(dependency) { dependency.value }
     }
 
-    class Provider : CellTests.Provider, CellWithDependenciesTests.Provider {
+    override fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ) =
+        DependentCell(dependency1, dependency2, dependency3) {
+            dependency1.value + dependency2.value + dependency3.value
+        }
+
+    class Provider : CellTests.Provider {
         private val dependencyCell = SimpleCell(1)
 
         override val observable = DependentCell(dependencyCell) { 2 * dependencyCell.value }
@@ -16,8 +25,5 @@ class DependentCellTests : RegularCellTests, CellWithDependenciesTests {
         override fun emit() {
             dependencyCell.value += 2
         }
-
-        override fun createWithDependencies(vararg dependencies: Cell<Int>) =
-            DependentCell(*dependencies) { dependencies.sumOf { it.value } }
     }
 }

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

@@ -14,7 +14,16 @@ class FlatteningDependentCellTransitiveDependencyEmitsTests :
         return FlatteningDependentCell(dependency) { dependency.value }
     }
 
-    class Provider : CellTests.Provider, CellWithDependenciesTests.Provider {
+    override fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ) =
+        FlatteningDependentCell(dependency1, dependency2, dependency3) {
+            ImmutableCell(dependency1.value + dependency2.value + dependency3.value)
+        }
+
+    class Provider : CellTests.Provider {
         // The transitive dependency can change.
         private val transitiveDependency = SimpleCell(5)
 
@@ -28,8 +37,5 @@ class FlatteningDependentCellTransitiveDependencyEmitsTests :
             // Update the transitive dependency.
             transitiveDependency.value += 5
         }
-
-        override fun createWithDependencies(vararg dependencies: Cell<Int>): Cell<Any> =
-            FlatteningDependentCell(*dependencies) { ImmutableCell(dependencies.sumOf { it.value }) }
     }
 }

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

@@ -0,0 +1,306 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.ChangeManager
+import world.phantasmal.observable.cell.CellWithDependenciesTests
+import world.phantasmal.observable.cell.ImmutableCell
+import world.phantasmal.observable.cell.SimpleCell
+import kotlin.test.*
+
+interface AbstractFilteredListCellTests : ListCellTests, CellWithDependenciesTests {
+    @Test
+    fun contains_only_values_that_match_the_predicate() = test {
+        val dep = SimpleListCell(mutableListOf("a", "b"))
+        val list = SimpleFilteredListCell(dep, predicate = ImmutableCell { 'a' in it })
+
+        assertEquals(1, list.value.size)
+        assertEquals("a", list.value[0])
+
+        dep.add("foo")
+        dep.add("bar")
+
+        assertEquals(2, list.value.size)
+        assertEquals("a", list.value[0])
+        assertEquals("bar", list.value[1])
+
+        dep.add("quux")
+        dep.add("qaax")
+
+        assertEquals(3, list.value.size)
+        assertEquals("a", list.value[0])
+        assertEquals("bar", list.value[1])
+        assertEquals("qaax", list.value[2])
+    }
+
+    @Test
+    fun only_emits_when_necessary() = test {
+        val dep = SimpleListCell<Int>(mutableListOf())
+        val list = SimpleFilteredListCell(dep, predicate = ImmutableCell { it % 2 == 0 })
+        var changes = 0
+        var listChanges = 0
+
+        disposer.add(list.observeChange {
+            changes++
+        })
+        disposer.add(list.observeListChange {
+            listChanges++
+        })
+
+        dep.add(1)
+        dep.add(3)
+        dep.add(5)
+
+        assertEquals(0, changes)
+        assertEquals(0, listChanges)
+
+        dep.add(0)
+        dep.add(2)
+        dep.add(4)
+
+        assertEquals(3, changes)
+        assertEquals(3, listChanges)
+    }
+
+    @Test
+    fun emits_correct_change_events() = test {
+        val dep = SimpleListCell<Int>(mutableListOf())
+        val list = SimpleFilteredListCell(dep, predicate = ImmutableCell { it % 2 == 0 })
+        var event: ListChangeEvent<Int>? = null
+
+        disposer.add(list.observeListChange {
+            assertNull(event)
+            event = it
+        })
+
+        run {
+            dep.replaceAll(listOf(1, 2, 3, 4, 5))
+
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            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
+
+        run {
+            dep.splice(2, 2, 10)
+
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertEquals(1, c.index)
+            assertEquals(1, c.removed.size)
+            assertEquals(4, c.removed[0])
+            assertEquals(1, c.inserted.size)
+            assertEquals(10, c.inserted[0])
+        }
+    }
+
+    @Test
+    fun value_changes_and_emits_when_predicate_changes() = test {
+        val predicate: SimpleCell<(Int) -> Boolean> = SimpleCell { it % 2 == 0 }
+        val list = SimpleFilteredListCell(ImmutableListCell(listOf(1, 2, 3, 4, 5)), predicate)
+        var event: ListChangeEvent<Int>? = null
+
+        disposer.add(list.observeListChange {
+            assertNull(event)
+            event = it
+        })
+
+        run {
+            // Change predicate.
+            predicate.value = { it % 2 == 1 }
+
+            // Value changes.
+            assertEquals(listOf(1, 3, 5), list.value)
+
+            // An event was emitted.
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertEquals(0, c.index)
+            assertEquals(listOf(2, 4), c.removed)
+            assertEquals(listOf(1, 3, 5), c.inserted)
+        }
+
+        event = null
+
+        run {
+            // Change predicate.
+            predicate.value = { it % 2 == 0 }
+
+            // Value changes.
+            assertEquals(listOf(2, 4), list.value)
+
+            // An event was emitted.
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertEquals(0, c.index)
+            assertEquals(listOf(1, 3, 5), c.removed)
+            assertEquals(listOf(2, 4), c.inserted)
+        }
+    }
+
+    @Test
+    fun emits_correctly_when_multiple_changes_happen_at_once() = test {
+        val dependency = object : AbstractListCell<Int>() {
+            private val changes: MutableList<Pair<Int, Int>> = mutableListOf()
+            override val elements: MutableList<Int> = mutableListOf()
+            override val value = elements
+
+            override fun emitDependencyChanged() {
+                emitDependencyChangedEvent(ListChangeEvent(
+                    elementsWrapper,
+                    changes.map { (index, newElement) ->
+                        ListChange(
+                            index = index,
+                            prevSize = index,
+                            removed = emptyList(),
+                            inserted = listOf(newElement),
+                        )
+                    }
+                ))
+                changes.clear()
+            }
+
+            fun makeChanges(newElements: List<Int>) {
+                emitMightChange()
+
+                for (newElement in newElements) {
+                    changes.add(Pair(elements.size, newElement))
+                    elements.add(newElement)
+                }
+
+                ChangeManager.changed(this)
+            }
+        }
+
+        val list = SimpleFilteredListCell(dependency, ImmutableCell { true })
+        var event: ListChangeEvent<Int>? = null
+
+        disposer.add(list.observeListChange {
+            assertNull(event)
+            event = it
+        })
+
+        for (i in 1..3) {
+            event = null
+
+            // Make two changes at once everytime.
+            val change0 = i * 13
+            val change1 = i * 17
+            val changes = listOf(change0, change1)
+            val oldList = list.value.toList()
+
+            dependency.makeChanges(changes)
+
+            // These checks are very implementation-specific. At some point the filtered list might,
+            // for example, emit an event with a single change instead of two changes and then this
+            // test will incorrectly fail.
+            val e = event
+            assertNotNull(e)
+            assertEquals(oldList + changes, e.value)
+            assertEquals(2, e.changes.size)
+
+            val lc0 = e.changes[0]
+            assertEquals(oldList.size, lc0.index)
+            assertEquals(oldList.size, lc0.prevSize)
+            assertTrue(lc0.removed.isEmpty())
+            assertEquals(listOf(change0), lc0.inserted)
+
+            val lc1 = e.changes[1]
+            assertEquals(oldList.size + 1, lc1.index)
+            assertEquals(oldList.size + 1, lc1.prevSize)
+            assertTrue(lc1.removed.isEmpty())
+            assertEquals(listOf(change1), lc1.inserted)
+        }
+    }
+
+    @Test
+    fun emits_correctly_when_dependency_contains_same_element_twice() = test {
+        val x = "x"
+        val y = "y"
+        val z = "z"
+        val dependency = SimpleListCell(mutableListOf(x, y, z, x, y, z))
+        val list = SimpleFilteredListCell(dependency, SimpleCell { it != y })
+        var event: ListChangeEvent<String>? = null
+
+        disposer.add(list.observeListChange {
+            assertNull(event)
+            event = it
+        })
+
+        assertEquals(listOf(x, z, x, z), list.value)
+
+        run {
+            // Remove second x element.
+            dependency.removeAt(3)
+
+            // Value changes.
+            assertEquals(listOf(x, z, z), list.value)
+
+            // An event was emitted.
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertEquals(2, c.index)
+            assertEquals(listOf(x), c.removed)
+            assertTrue(c.inserted.isEmpty())
+        }
+
+        event = null
+
+        run {
+            // Remove first x element.
+            dependency.removeAt(0)
+
+            // Value changes.
+            assertEquals(listOf(z, z), list.value)
+
+            // An event was emitted.
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertEquals(0, c.index)
+            assertEquals(listOf(x), c.removed)
+            assertTrue(c.inserted.isEmpty())
+        }
+
+        event = null
+
+        run {
+            // Remove second z element.
+            dependency.removeAt(3)
+
+            // Value changes.
+            assertEquals(listOf(z), list.value)
+
+            // An event was emitted.
+            val e = event
+            assertNotNull(e)
+            assertEquals(1, e.changes.size)
+
+            val c = e.changes.first()
+            assertEquals(1, c.index)
+            assertEquals(listOf(z), c.removed)
+            assertTrue(c.inserted.isEmpty())
+        }
+    }
+}

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

@@ -8,7 +8,16 @@ class DependentListCellTests : ListCellTests, CellWithDependenciesTests {
 
     override fun createListProvider(empty: Boolean) = Provider(empty)
 
-    class Provider(empty: Boolean) : ListCellTests.Provider, CellWithDependenciesTests.Provider {
+    override fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ) =
+        DependentListCell(dependency1, dependency2, dependency3) {
+            listOf(dependency1.value, dependency2.value, dependency3.value)
+        }
+
+    class Provider(empty: Boolean) : ListCellTests.Provider {
         private val dependencyCell =
             SimpleListCell(if (empty) mutableListOf() else mutableListOf(5))
 
@@ -18,8 +27,5 @@ class DependentListCellTests : ListCellTests, CellWithDependenciesTests {
         override fun addElement() {
             dependencyCell.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/FilteredListCellListDependencyEmitsTests.kt

@@ -0,0 +1,41 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.DependentCell
+import world.phantasmal.observable.cell.ImmutableCell
+
+// TODO: A test suite that tests FilteredListCell while its predicate dependency is changing.
+// TODO: A test suite that tests FilteredListCell while the predicate results are changing.
+class FilteredListCellListDependencyEmitsTests : AbstractFilteredListCellTests {
+    override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
+        private val dependencyCell =
+            SimpleListCell(if (empty) mutableListOf(5) else mutableListOf(5, 10))
+
+        override val observable =
+            FilteredListCell(
+                list = dependencyCell,
+                predicate = ImmutableCell { ImmutableCell(it % 2 == 0) },
+            )
+
+        override fun addElement() {
+            dependencyCell.add(4)
+        }
+    }
+
+    override fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ) =
+        FilteredListCell(
+            list = DependentListCell(dependency1, computeElements = {
+                listOf(dependency1.value)
+            }),
+            predicate = DependentCell(dependency2, compute = {
+                fun predicate(element: Int) =
+                    DependentCell(dependency3, compute = { element < dependency2.value })
+
+                ::predicate
+            }),
+        )
+}

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

@@ -1,179 +0,0 @@
-package world.phantasmal.observable.cell.list
-
-import world.phantasmal.observable.cell.SimpleCell
-import kotlin.test.*
-
-class FilteredListCellTests : ListCellTests {
-    override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
-        private val dependencyCell =
-            SimpleListCell(if (empty) mutableListOf(5) else mutableListOf(5, 10))
-
-        override val observable = FilteredListCell(dependencyCell, predicate = { it % 2 == 0 })
-
-        override fun addElement() {
-            dependencyCell.add(4)
-        }
-    }
-
-    @Test
-    fun contains_only_values_that_match_the_predicate() = test {
-        val dep = SimpleListCell(mutableListOf("a", "b"))
-        val list = FilteredListCell(dep, predicate = { 'a' in it })
-
-        assertEquals(1, list.value.size)
-        assertEquals("a", list.value[0])
-
-        dep.add("foo")
-        dep.add("bar")
-
-        assertEquals(2, list.value.size)
-        assertEquals("a", list.value[0])
-        assertEquals("bar", list.value[1])
-
-        dep.add("quux")
-        dep.add("qaax")
-
-        assertEquals(3, list.value.size)
-        assertEquals("a", list.value[0])
-        assertEquals("bar", list.value[1])
-        assertEquals("qaax", list.value[2])
-    }
-
-    @Test
-    fun only_emits_when_necessary() = test {
-        val dep = SimpleListCell<Int>(mutableListOf())
-        val list = FilteredListCell(dep, predicate = { it % 2 == 0 })
-        var changes = 0
-        var listChanges = 0
-
-        disposer.add(list.observeChange {
-            changes++
-        })
-        disposer.add(list.observeListChange {
-            listChanges++
-        })
-
-        dep.add(1)
-        dep.add(3)
-        dep.add(5)
-
-        assertEquals(0, changes)
-        assertEquals(0, listChanges)
-
-        dep.add(0)
-        dep.add(2)
-        dep.add(4)
-
-        assertEquals(3, changes)
-        assertEquals(3, listChanges)
-    }
-
-    @Test
-    fun emits_correct_change_events() = test {
-        val dep = SimpleListCell<Int>(mutableListOf())
-        val list = FilteredListCell(dep, predicate = { it % 2 == 0 })
-        var event: ListChangeEvent<Int>? = null
-
-        disposer.add(list.observeListChange {
-            assertNull(event)
-            event = it
-        })
-
-        dep.replaceAll(listOf(1, 2, 3, 4, 5))
-
-        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)
-
-        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 [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_element_changes() = test {
-        val dep = SimpleListCell(
-            mutableListOf(SimpleCell(1), SimpleCell(2), SimpleCell(3), SimpleCell(4)),
-            extractDependencies = { arrayOf(it) },
-        )
-        val list = FilteredListCell(dep, predicate = { it.value % 2 == 0 })
-        var event: ListChangeEvent<SimpleCell<Int>>? = null
-
-        disposer.add(list.observeListChange {
-            assertNull(event)
-            event = it
-        })
-
-        for (i in 0 until dep.size.value) {
-            event = null
-
-            // 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
-
-            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, c.removed.size)
-                assertEquals(1, c.inserted.size)
-                assertEquals(newValue, c.inserted[0].value)
-            } else {
-                assertEquals(1, c.removed.size)
-                assertEquals(0, c.inserted.size)
-                assertEquals(newValue, c.removed[0].value)
-            }
-        }
-
-        for (i in 0 until dep.size.value) {
-            event = null
-
-            // Change a value, but keep even numbers even and odd numbers odd. List should emit an
-            // ElementChange event.
-            val newValue = dep[i].value + 2
-            dep[i].value = newValue
-
-            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/FlatteningDependentListCellTransitiveDependencyEmitsTests.kt

@@ -1,6 +1,7 @@
 package world.phantasmal.observable.cell.list
 
 import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.CellTests
 import world.phantasmal.observable.cell.CellWithDependenciesTests
 import world.phantasmal.observable.cell.ImmutableCell
 
@@ -15,7 +16,16 @@ class FlatteningDependentListCellTransitiveDependencyEmitsTests :
 
     override fun createListProvider(empty: Boolean) = Provider(empty)
 
-    class Provider(empty: Boolean) : ListCellTests.Provider, CellWithDependenciesTests.Provider {
+    override fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ) =
+        FlatteningDependentListCell(dependency1, dependency2, dependency3) {
+            ImmutableListCell(listOf(dependency1.value, dependency2.value, dependency3.value))
+        }
+
+    class Provider(empty: Boolean) : ListCellTests.Provider, CellTests.Provider {
         // The transitive dependency can change.
         private val transitiveDependency =
             SimpleListCell(if (empty) mutableListOf() else mutableListOf(7))
@@ -30,10 +40,5 @@ class FlatteningDependentListCellTransitiveDependencyEmitsTests :
             // Update the transitive dependency.
             transitiveDependency.add(4)
         }
-
-        override fun createWithDependencies(vararg dependencies: Cell<Int>): Cell<Any> =
-            FlatteningDependentListCell(*dependencies) {
-                ImmutableListCell(dependencies.map { it.value })
-            }
     }
 }

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

@@ -211,8 +211,8 @@ interface ListCellTests : CellTests {
             p.addElement()
 
             assertNotNull(firstOrNull.value)
-            // Observer should not be called when adding elements at the end of the list.
-            assertNull(observedValue)
+            // Observer may or may not be called when adding elements at the end of the list.
+            assertTrue(observedValue == null || observedValue == firstOrNull.value)
         }
     }
 

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

@@ -0,0 +1,43 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.cell.*
+
+/**
+ * In these tests, the direct list cell dependency of the [ListElementsDependentCell] doesn't
+ * change, but its elements do change.
+ */
+class ListElementsDependentCellElementEmitsTests : CellWithDependenciesTests {
+
+    override fun createProvider() = object : CellTests.Provider {
+        // One transitive dependency can change.
+        private val transitiveDependency = Element(2)
+
+        // The direct dependency of the list under test can't change.
+        private val directDependency: ListCell<Element> =
+            ImmutableListCell(listOf(Element(1), transitiveDependency, Element(3)))
+
+        override val observable =
+            ListElementsDependentCell(directDependency) { arrayOf(it.int, it.double, it.string) }
+
+        override fun emit() {
+            transitiveDependency.int.value++
+        }
+    }
+
+    override fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ) =
+        ListElementsDependentCell(
+            ImmutableListCell(listOf(dependency1, dependency2, dependency3))
+        ) { arrayOf(it) }
+
+    private class Element(value: Int) {
+        val int: MutableCell<Int> = SimpleCell(value)
+        val double: Cell<Double> = DependentCell(int) { int.value.toDouble() }
+        val string: Cell<String> = DependentCell(int) { int.value.toString() }
+
+        override fun toString(): String = "Element[int=$int, double=$double, string=$string]"
+    }
+}

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

@@ -0,0 +1,25 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.CellTests
+import world.phantasmal.observable.cell.ImmutableCell
+
+/**
+ * In these tests, the direct list cell dependency of the [ListElementsDependentCell] changes, while
+ * its elements don't change.
+ */
+class ListElementsDependentCellListCellEmitsTests : CellTests {
+
+    override fun createProvider() = object : CellTests.Provider {
+        // The direct dependency of the list under test changes, its elements are immutable.
+        private val directDependency: SimpleListCell<Cell<Int>> =
+            SimpleListCell(mutableListOf(ImmutableCell(1), ImmutableCell(2), ImmutableCell(3)))
+
+        override val observable =
+            ListElementsDependentCell(directDependency) { arrayOf(it) }
+
+        override fun emit() {
+            directDependency.add(ImmutableCell(directDependency.value.size + 1))
+        }
+    }
+}

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

@@ -0,0 +1,56 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.ChangeEvent
+import world.phantasmal.observable.cell.SimpleCell
+import world.phantasmal.observable.test.ObservableTestSuite
+import kotlin.test.Test
+import kotlin.test.assertNotNull
+import kotlin.test.assertNull
+
+/**
+ * Standard tests are done by [ListElementsDependentCellElementEmitsTests] and
+ * [ListElementsDependentCellListCellEmitsTests].
+ */
+class ListElementsDependentCellTests : ObservableTestSuite {
+    @Test
+    fun element_changes_are_correctly_propagated() = test {
+        val list = SimpleListCell(
+            mutableListOf(
+                SimpleCell("a"),
+                SimpleCell("b"),
+                SimpleCell("c")
+            )
+        )
+
+        val cell = ListElementsDependentCell(list) { arrayOf(it) }
+
+        var event: ChangeEvent<*>? = null
+
+        disposer.add(cell.observeChange {
+            assertNull(event)
+            event = it
+        })
+
+        // The cell should not emit events when an old element is changed.
+        run {
+            val removed = list.removeAt(1)
+            event = null
+
+            removed.value += "-1"
+
+            assertNull(event)
+        }
+
+        // The cell should emit events when any of the current elements are changed.
+        list.add(SimpleCell("d"))
+
+        for (element in list.value) {
+            event = null
+
+            element.value += "-2"
+
+            val e = event
+            assertNotNull(e)
+        }
+    }
+}

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

@@ -34,7 +34,6 @@ interface MutableListCellTests<T : Any> : ListCellTests, MutableCellTests<List<T
             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)
@@ -57,7 +56,6 @@ interface MutableListCellTests<T : Any> : ListCellTests, MutableCellTests<List<T
             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)
@@ -81,7 +79,6 @@ interface MutableListCellTests<T : Any> : ListCellTests, MutableCellTests<List<T
             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)

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

@@ -0,0 +1,39 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.DependentCell
+import world.phantasmal.observable.cell.ImmutableCell
+
+/**
+ * In these tests the list dependency of the [SimpleListCell] changes and the predicate
+ * dependency does not.
+ */
+class SimpleFilteredListCellListDependencyEmitsTests : AbstractFilteredListCellTests {
+    override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
+        private val dependencyCell =
+            SimpleListCell(if (empty) mutableListOf(5) else mutableListOf(5, 10))
+
+        override val observable = SimpleFilteredListCell(
+            list = dependencyCell,
+            predicate = ImmutableCell { it % 2 == 0 },
+        )
+
+        override fun addElement() {
+            dependencyCell.add(4)
+        }
+    }
+
+    override fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ) =
+        SimpleFilteredListCell(
+            list = DependentListCell(dependency1, dependency2) {
+                listOf(dependency1.value, dependency2.value)
+            },
+            predicate = DependentCell(dependency3) {
+                { it < dependency3.value }
+            },
+        )
+}

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

@@ -0,0 +1,41 @@
+package world.phantasmal.observable.cell.list
+
+import world.phantasmal.observable.cell.Cell
+import world.phantasmal.observable.cell.DependentCell
+import world.phantasmal.observable.cell.SimpleCell
+
+/**
+ * In these tests the predicate dependency of the [SimpleListCell] changes and the list dependency
+ * does not.
+ */
+class SimpleFilteredListCellPredicateDependencyEmitsTests : AbstractFilteredListCellTests {
+    override fun createListProvider(empty: Boolean) = object : ListCellTests.Provider {
+        private var maxValue = if (empty) 0 else 1
+        private val predicateCell = SimpleCell<(Int) -> Boolean> { it <= maxValue }
+
+        override val observable = SimpleFilteredListCell(
+            list = ImmutableListCell((1..20).toList()),
+            predicate = predicateCell,
+        )
+
+        override fun addElement() {
+            maxValue++
+            val max = maxValue
+            predicateCell.value = { it <= max }
+        }
+    }
+
+    override fun createWithDependencies(
+        dependency1: Cell<Int>,
+        dependency2: Cell<Int>,
+        dependency3: Cell<Int>,
+    ) =
+        SimpleFilteredListCell(
+            list = DependentListCell(dependency1, dependency2) {
+                listOf(dependency1.value, dependency2.value)
+            },
+            predicate = DependentCell(dependency3) {
+                { it < dependency3.value }
+            },
+        )
+}

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

@@ -1,9 +1,10 @@
 package world.phantasmal.observable.cell.list
 
-import world.phantasmal.observable.cell.SimpleCell
 import world.phantasmal.observable.test.assertListCellEquals
-import world.phantasmal.testUtils.TestContext
-import kotlin.test.*
+import kotlin.test.Test
+import kotlin.test.assertFailsWith
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
 
 class SimpleListCellTests : MutableListCellTests<Int> {
     override fun createProvider() = createListProvider(empty = true)
@@ -31,15 +32,8 @@ class SimpleListCellTests : MutableListCellTests<Int> {
 
     @Test
     fun set() = test {
-        testSet(SimpleListCell(mutableListOf("a", "b", "c")))
-    }
+        val list = SimpleListCell(mutableListOf("a", "b", "c"))
 
-    @Test
-    fun set_with_extractDependencies() = test {
-        testSet(SimpleListCell(mutableListOf("a", "b", "c")) { arrayOf() })
-    }
-
-    private fun testSet(list: SimpleListCell<String>) {
         list[1] = "test"
         list[2] = "test2"
         assertFailsWith<IndexOutOfBoundsException> {
@@ -147,133 +141,4 @@ class SimpleListCellTests : MutableListCellTests<Int> {
         // List should remain unchanged after invalid calls.
         assertListCellEquals(listOf(101, 0, 1, 2, 100, 8, 9, 102), list)
     }
-
-    @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.observeListChange {
-            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/huntOptimizer/controllers/MethodsForEpisodeController.kt

@@ -1,9 +1,6 @@
 package world.phantasmal.web.huntOptimizer.controllers
 
-import world.phantasmal.observable.cell.list.ListCell
-import world.phantasmal.observable.cell.list.filtered
-import world.phantasmal.observable.cell.list.listCell
-import world.phantasmal.observable.cell.list.sortedWith
+import world.phantasmal.observable.cell.list.*
 import world.phantasmal.observable.cell.mutableCell
 import world.phantasmal.psolib.Episode
 import world.phantasmal.psolib.fileFormats.quest.NpcType
@@ -27,9 +24,13 @@ class MethodsForEpisodeController(
     override val fixedColumns = 2
 
     override val values: ListCell<HuntMethodModel> by lazy {
-        huntMethodStore.methods
+        val methodsForEpisode = huntMethodStore.methods
             .filtered { it.episode == episode }
-            .sortedWith(comparator)
+            .dependingOnElements { arrayOf(it.time) }
+
+        mapToList(methodsForEpisode, comparator) { list, cmp ->
+            list.sortedWith(cmp)
+        }
     }
 
     override val loadingStatus: LoadingStatusCell = huntMethodStore.methodsStatus

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

@@ -26,7 +26,7 @@ class HuntMethodStore(
     private val assetLoader: AssetLoader,
     private val huntMethodPersister: HuntMethodPersister,
 ) : Store() {
-    private val _methods = mutableListCell<HuntMethodModel> { arrayOf(it.time) }
+    private val _methods = mutableListCell<HuntMethodModel>()
     private val _methodsStatus = LoadingStatusCellImpl(scope, "methods", ::loadMethods)
 
     /** Hunting methods supported by the current server. */

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

@@ -12,8 +12,10 @@ import world.phantasmal.core.unsafe.UnsafeMap
 import world.phantasmal.core.unsafe.UnsafeSet
 import world.phantasmal.observable.cell.Cell
 import world.phantasmal.observable.cell.list.ListCell
+import world.phantasmal.observable.cell.list.dependingOnElements
 import world.phantasmal.observable.cell.list.mutableListCell
 import world.phantasmal.observable.cell.mutableCell
+import world.phantasmal.observable.observe
 import world.phantasmal.psolib.fileFormats.quest.NpcType
 import world.phantasmal.web.core.models.Server
 import world.phantasmal.web.core.stores.EnemyDropTable
@@ -50,7 +52,7 @@ class HuntOptimizerStore(
     private val itemDropStore: ItemDropStore,
 ) : Store() {
     private val _huntableItems = mutableListCell<ItemType>()
-    private val _wantedItems = mutableListCell<WantedItemModel> { arrayOf(it.amount) }
+    private val _wantedItems = mutableListCell<WantedItemModel>()
     private val _optimizationResult = mutableCell(OptimizationResultModel(emptyList(), emptyList()))
     private var wantedItemsPersistenceObserver: Disposable? = null
 
@@ -58,6 +60,7 @@ class HuntOptimizerStore(
         observeNow(uiStore.server) { server ->
             _huntableItems.clear()
 
+            // There's a race condition here.
             scope.launch {
                 val dropTable = itemDropStore.getEnemyDropTable(server)
 
@@ -76,9 +79,18 @@ class HuntOptimizerStore(
     }
 
     val optimizationResult: Cell<OptimizationResultModel> by lazy {
-        observeNow(wantedItems, huntMethodStore.methods) { wantedItems, huntMethods ->
+        observeNow(
+            _wantedItems.dependingOnElements { arrayOf(it.amount) },
+            huntMethodStore.methods.dependingOnElements { arrayOf(it.time) },
+        ) { wantedItems, huntMethods ->
+            // There's a race condition here.
             scope.launch(Dispatchers.Default) {
-                _optimizationResult.value = optimize(wantedItems, huntMethods)
+                val dropTable = itemDropStore.getEnemyDropTable(uiStore.server.value)
+                val result = optimize(wantedItems, huntMethods, dropTable)
+
+                withContext(Dispatchers.Main) {
+                    _optimizationResult.value = result
+                }
             }
         }
 
@@ -91,7 +103,7 @@ class HuntOptimizerStore(
     }
 
     fun addWantedItem(itemType: ItemType) {
-        if (wantedItems.value.none { it.itemType == itemType }) {
+        if (_wantedItems.value.none { it.itemType == itemType }) {
             _wantedItems.add(WantedItemModel(itemType, 1))
         }
     }
@@ -114,20 +126,20 @@ class HuntOptimizerStore(
                 _wantedItems.replaceAll(wantedItems)
 
                 // Wanted items are loaded, start observing them and persist whenever they change.
-                wantedItemsPersistenceObserver = _wantedItems.observeChange {
-                    val items = it.value
-
-                    scope.launch(Dispatchers.Main) {
-                        wantedItemPersister.persistWantedItems(items, server)
+                wantedItemsPersistenceObserver =
+                    _wantedItems.dependingOnElements { arrayOf(it.amount) }.observe { items ->
+                        scope.launch(Dispatchers.Main) {
+                            wantedItemPersister.persistWantedItems(items, server)
+                        }
                     }
-                }
             }
         }
     }
 
-    private suspend fun optimize(
+    private fun optimize(
         wantedItems: List<WantedItemModel>,
         methods: List<HuntMethodModel>,
+        dropTable: EnemyDropTable,
     ): OptimizationResultModel {
         logger.debug { "Optimization start." }
 
@@ -139,8 +151,6 @@ class HuntOptimizerStore(
             return OptimizationResultModel(emptyList(), emptyList())
         }
 
-        val dropTable = itemDropStore.getEnemyDropTable(uiStore.server.value)
-
         // Add a constraint per wanted item.
         val constraints: dynamic = obj {}
 

web/src/main/kotlin/world/phantasmal/web/questEditor/models/QuestModel.kt

@@ -36,9 +36,9 @@ class QuestModel(
     private val _shortDescription = mutableCell("")
     private val _longDescription = mutableCell("")
     private val _mapDesignations = mutableCell(mapDesignations)
-    private val _npcs = SimpleListCell(npcs) { arrayOf(it.sectionInitialized, it.wave) }
-    private val _objects = SimpleListCell(objects) { arrayOf(it.sectionInitialized) }
-    private val _events = SimpleListCell(events) { arrayOf(it.id) }
+    private val _npcs = SimpleListCell(npcs)
+    private val _objects = SimpleListCell(objects)
+    private val _events = SimpleListCell(events)
 
     val id: Cell<Int> = _id
     val language: Cell<Int> = _language

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

@@ -1,8 +1,11 @@
 package world.phantasmal.web.questEditor.rendering
 
+import world.phantasmal.observable.cell.and
+import world.phantasmal.observable.cell.eq
 import world.phantasmal.observable.cell.flatMapNull
 import world.phantasmal.observable.cell.list.emptyListCell
-import world.phantasmal.observable.cell.list.filtered
+import world.phantasmal.observable.cell.list.filteredCell
+import world.phantasmal.observable.cell.or
 import world.phantasmal.web.questEditor.loading.AreaAssetLoader
 import world.phantasmal.web.questEditor.loading.EntityAssetLoader
 import world.phantasmal.web.questEditor.stores.QuestEditorStore
@@ -28,10 +31,10 @@ class QuestEditorMeshManager(
         ) { quest, area, wave ->
             loadNpcMeshes(
                 if (quest != null && area != null) {
-                    quest.npcs.filtered {
-                        it.sectionInitialized.value &&
-                                it.areaId == area.id &&
-                                (wave == null || it.wave.value == wave)
+                    quest.npcs.filteredCell {
+                        it.sectionInitialized and
+                                (it.areaId == area.id) and
+                                ((wave == null) or (it.wave eq wave))
                     }
                 } else {
                     emptyListCell()
@@ -45,8 +48,8 @@ class QuestEditorMeshManager(
         ) { quest, area ->
             loadObjectMeshes(
                 if (quest != null && area != null) {
-                    quest.objects.filtered {
-                        it.sectionInitialized.value && it.areaId == area.id
+                    quest.objects.filteredCell {
+                        it.sectionInitialized and (it.areaId == area.id)
                     }
                 } else {
                     emptyListCell()

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

@@ -6,7 +6,6 @@ import kotlinx.coroutines.launch
 import world.phantasmal.core.disposable.Disposable
 import world.phantasmal.core.disposable.DisposableSupervisedScope
 import world.phantasmal.observable.cell.list.ListCell
-import world.phantasmal.observable.cell.list.ListChange
 import world.phantasmal.observable.cell.list.ListChangeEvent
 import world.phantasmal.psolib.Episode
 import world.phantasmal.web.questEditor.loading.AreaAssetLoader
@@ -75,19 +74,15 @@ abstract class QuestMeshManager protected constructor(
 
     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)
-            }
+            change.removed.forEach(npcMeshManager::remove)
+            change.inserted.forEach(npcMeshManager::add)
         }
     }
 
     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)
-            }
+            change.removed.forEach(objectMeshManager::remove)
+            change.inserted.forEach(objectMeshManager::add)
         }
     }
 }

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

@@ -39,7 +39,16 @@ class QuestRenderer(
             ),
         )
 
-        observeNow(questEditorStore.currentQuest) { inputManager.resetCamera() }
-        observeNow(questEditorStore.currentAreaVariant) { inputManager.resetCamera() }
+        var prevQuest = questEditorStore.currentQuest.value
+        var prevAreaVariant = questEditorStore.currentAreaVariant.value
+
+        observeNow(questEditorStore.currentQuest, questEditorStore.currentAreaVariant) { q, av ->
+            if (q !== prevQuest || av !== prevAreaVariant) {
+                inputManager.resetCamera()
+
+                prevQuest = q
+                prevAreaVariant = av
+            }
+        }
     }
 }

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

@@ -209,14 +209,14 @@ class IdleState(
         val entity: QuestEntityModel<*, *>,
 
         /**
-         * Vector that points from the grabbing point (somewhere on the model's surface) to the entity's
-         * origin.
+         * Vector that points from the grabbing point (somewhere on the model's surface) to the
+         * entity's origin.
          */
         val grabOffset: Vector3,
 
         /**
-         * Vector that points from the grabbing point to the terrain point directly under the entity's
-         * origin.
+         * Vector that points from the grabbing point to the terrain point directly under the
+         * entity's origin.
          */
         val dragAdjust: Vector3,
     )

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

@@ -158,8 +158,8 @@ class QuestEditorStore(
             updateQuestEntitySections(quest)
 
             // Ensure all entities have their section initialized.
-            quest.npcs.value.forEach { it.setSectionInitialized() }
-            quest.objects.value.forEach { it.setSectionInitialized() }
+            quest.npcs.value.forEach(QuestNpcModel::setSectionInitialized)
+            quest.objects.value.forEach(QuestObjectModel::setSectionInitialized)
         }
     }
 

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

@@ -1,11 +1,13 @@
 package world.phantasmal.web.questEditor.controllers
 
-import world.phantasmal.observable.cell.observeNow
 import world.phantasmal.web.questEditor.models.QuestEventActionModel
 import world.phantasmal.web.questEditor.models.QuestEventModel
 import world.phantasmal.web.test.WebTestSuite
 import world.phantasmal.web.test.createQuestModel
-import kotlin.test.*
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
 
 class EventsControllerTests : WebTestSuite {
     @Test
@@ -113,38 +115,27 @@ class EventsControllerTests : WebTestSuite {
             (ctrl.events[0].actions[0] as QuestEventActionModel.TriggerEvent).eventId
         )
 
-        // We test the observed value instead of the cell's value property.
-        var canGoToEventValue: Boolean? = null
-
-        disposer.add(canGoToEvent.observeNow {
-            assertNull(canGoToEventValue)
-            canGoToEventValue = it
-        })
-
-        assertEquals(true, canGoToEventValue)
+        assertEquals(true, canGoToEvent.value)
 
         // Let event 100 point to nonexistent event 102.
-        canGoToEventValue = null
         ctrl.setActionEventId(
             ctrl.events[0],
             ctrl.events[0].actions[0] as QuestEventActionModel.TriggerEvent,
             102,
         )
 
-        assertEquals(false, canGoToEventValue)
+        assertEquals(false, canGoToEvent.value)
 
         // Add event 102.
-        canGoToEventValue = null
         ctrl.selectEvent(null) // Deselect so the next event will be added at the end of the list.
         ctrl.addEvent()
         ctrl.setId(ctrl.events.value.last(), 102)
 
-        assertEquals(true, canGoToEventValue)
+        assertEquals(true, canGoToEvent.value)
 
         // Remove event 102.
-        canGoToEventValue = null
         ctrl.removeEvent(ctrl.events.value.last())
 
-        assertEquals(false, canGoToEventValue)
+        assertEquals(false, canGoToEvent.value)
     }
 }

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

@@ -14,7 +14,6 @@ import world.phantasmal.core.unsafe.UnsafeMap
 import world.phantasmal.core.unsafe.getOrPut
 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
 import world.phantasmal.webui.externals.fontawesome.IconDefinition
 import world.phantasmal.webui.externals.fontawesome.freeBrandsSvgIcons.faGithub
@@ -288,28 +287,26 @@ private fun <T> bindChildrenTo(
 
     return list.observeListChange { event: ListChangeEvent<T> ->
         for (change in event.changes) {
-            if (change is ListChange.Structural) {
-                if (change.allRemoved) {
-                    parent.innerHTML = ""
-                } else {
-                    repeat(change.removed.size) {
-                        parent.removeChild(parent.childNodes[change.index].unsafeCast<Node>())
-                    }
+            if (change.allRemoved) {
+                parent.innerHTML = ""
+            } else {
+                repeat(change.removed.size) {
+                    parent.removeChild(parent.childNodes[change.index].unsafeCast<Node>())
                 }
+            }
 
-                childrenRemoved(change.index, change.removed.size)
+            childrenRemoved(change.index, change.removed.size)
 
-                val frag = document.createDocumentFragment()
+            val frag = document.createDocumentFragment()
 
-                change.inserted.forEachIndexed { i, value ->
-                    frag.appendChild(frag.createChild(value, change.index + i))
-                }
+            change.inserted.forEachIndexed { i, value ->
+                frag.appendChild(frag.createChild(value, change.index + i))
+            }
 
-                if (change.index >= parent.childNodes.length) {
-                    parent.appendChild(frag)
-                } else {
-                    parent.insertBefore(frag, parent.childNodes[change.index])
-                }
+            if (change.index >= parent.childNodes.length) {
+                parent.appendChild(frag)
+            } else {
+                parent.insertBefore(frag, parent.childNodes[change.index])
             }
         }
 

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

@@ -72,7 +72,7 @@ class HTMLElementSizeCell(element: HTMLElement? = null) : AbstractCell<Size>() {
         if (newValue != _value) {
             emitMightChange()
             _value = newValue
-            emitDependencyChanged(ChangeEvent(newValue))
+            emitDependencyChangedEvent(ChangeEvent(newValue))
         }
     }
 }