Added some observe extension methods, added a mutation test and removed old commented-out code.

cell/src/commonMain/kotlin/world/phantasmal/cell/Cells.kt

@@ -48,10 +48,46 @@ fun <T> mutableCell(getter: () -> T, setter: (T) -> Unit): MutableCell<T> =
 fun <T> Cell<T>.observe(observer: (T) -> Unit): Disposable =
     observeChange { observer(it.value) }
 
+fun <T1, T2> observe(
+    c1: Cell<T1>,
+    c2: Cell<T2>,
+    observer: (T1, T2) -> Unit,
+): Disposable =
+    CallbackObserver(c1, c2) { observer(c1.value, c2.value) }
+
+fun <T1, T2, T3> observe(
+    c1: Cell<T1>,
+    c2: Cell<T2>,
+    c3: Cell<T3>,
+    observer: (T1, T2, T3) -> Unit,
+): Disposable =
+    CallbackObserver(c1, c2, c3) { observer(c1.value, c2.value, c3.value) }
+
+fun <T1, T2, T3, T4> observe(
+    c1: Cell<T1>,
+    c2: Cell<T2>,
+    c3: Cell<T3>,
+    c4: Cell<T4>,
+    observer: (T1, T2, T3, T4) -> Unit,
+): Disposable =
+    CallbackObserver(c1, c2, c3, c4) { observer(c1.value, c2.value, c3.value, c4.value) }
+
+fun <T1, T2, T3, T4, T5> observe(
+    c1: Cell<T1>,
+    c2: Cell<T2>,
+    c3: Cell<T3>,
+    c4: Cell<T4>,
+    c5: Cell<T5>,
+    observer: (T1, T2, T3, T4, T5) -> Unit,
+): Disposable =
+    CallbackObserver(c1, c2, c3, c4, c5) {
+        observer(c1.value, c2.value, c3.value, c4.value, c5.value)
+    }
+
 fun <T> Cell<T>.observeNow(
     observer: (T) -> Unit,
 ): Disposable {
-    val disposable = observeChange { observer(it.value) }
+    val disposable = observe(observer)
     // Call observer after observeChange to avoid double recomputation in most cells.
     observer(value)
     return disposable
@@ -62,7 +98,7 @@ fun <T1, T2> observeNow(
     c2: Cell<T2>,
     observer: (T1, T2) -> Unit,
 ): Disposable {
-    val disposable = CallbackObserver(c1, c2) { observer(c1.value, c2.value) }
+    val disposable = observe(c1, c2, observer)
     // Call observer after observeChange to avoid double recomputation in most cells.
     observer(c1.value, c2.value)
     return disposable
@@ -74,7 +110,7 @@ fun <T1, T2, T3> observeNow(
     c3: Cell<T3>,
     observer: (T1, T2, T3) -> Unit,
 ): Disposable {
-    val disposable = CallbackObserver(c1, c2, c3) { observer(c1.value, c2.value, c3.value) }
+    val disposable = observe(c1, c2, c3, observer)
     // Call observer after observeChange to avoid double recomputation in most cells.
     observer(c1.value, c2.value, c3.value)
     return disposable
@@ -87,8 +123,7 @@ fun <T1, T2, T3, T4> observeNow(
     c4: Cell<T4>,
     observer: (T1, T2, T3, T4) -> Unit,
 ): Disposable {
-    val disposable =
-        CallbackObserver(c1, c2, c3, c4) { observer(c1.value, c2.value, c3.value, c4.value) }
+    val disposable = observe(c1, c2, c3, c4, observer)
     // Call observer after observeChange to avoid double recomputation in most cells.
     observer(c1.value, c2.value, c3.value, c4.value)
     return disposable
@@ -102,9 +137,7 @@ fun <T1, T2, T3, T4, T5> observeNow(
     c5: Cell<T5>,
     observer: (T1, T2, T3, T4, T5) -> Unit,
 ): Disposable {
-    val disposable = CallbackObserver(c1, c2, c3, c4, c5) {
-        observer(c1.value, c2.value, c3.value, c4.value, c5.value)
-    }
+    val disposable = observe(c1, c2, c3, c4, c5, observer)
     // Call observer after observeChange to avoid double recomputation in most cells.
     observer(c1.value, c2.value, c3.value, c4.value, c5.value)
     return disposable

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

@@ -2,12 +2,7 @@ package world.phantasmal.cell
 
 import world.phantasmal.cell.test.CellTestSuite
 import world.phantasmal.core.disposable.use
-import kotlin.test.Test
-import kotlin.test.assertEquals
-import kotlin.test.assertNotEquals
-import kotlin.test.assertNotNull
-import kotlin.test.assertNull
-import kotlin.test.assertTrue
+import kotlin.test.*
 
 /**
  * Test suite for all [Cell] implementations. There is a subclass of this suite for every [Cell]
@@ -243,7 +238,7 @@ interface CellTests : CellTestSuite {
             }
         )
 
-        // Repeat 3 times to check if temporary state is always reset.
+        // Repeat 3 times to check that temporary state is always reset.
         repeat(3) {
             changes = 0
 
@@ -261,6 +256,37 @@ interface CellTests : CellTestSuite {
         }
     }
 
+    /**
+     * When two dependencies of an observer are changed in a single mutation, the observer is called
+     * just once.
+     */
+    @Test
+    fun changing_two_dependencies_during_a_mutation_results_in_one_callback_call() = test {
+        val p1 = createProvider()
+        val p2 = createProvider()
+        var callbackCalled = 0
+
+        disposer.add(
+            observe(p1.cell, p2.cell) { _, _ -> callbackCalled++ }
+        )
+
+        // Repeat 3 times to check that temporary state is always reset.
+        repeat(3) {
+            callbackCalled = 0
+
+            mutate {
+                p1.emit()
+                p2.emit()
+
+                // Change should be deferred until this mutation finishes.
+                assertEquals(0, callbackCalled)
+            }
+
+            // All changes should result in a single callback call.
+            assertEquals(1, callbackCalled)
+        }
+    }
+
     @Test
     fun value_can_be_accessed_during_a_mutation() = test {
         val p = createProvider()

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

@@ -25,118 +25,6 @@ interface MutableCellTests<T : Any> : CellTests {
         assertEquals(newValue, observedValue)
     }
 
-    // TODO: Figure out change set bug and enable change sets again.
-    /**
-     * Modifying mutable cells in a change set doesn't result in calls to
-     * [Dependent.dependencyChanged] of their dependents until the change set is completed.
-     */
-//    @Test
-//    fun cell_changes_in_change_set_dont_immediately_produce_dependencyChanged_calls() = test {
-//        val dependencies = (1..5).map { createProvider() }
-//
-//        var dependencyMightChangeCount = 0
-//        var dependencyChangedCount = 0
-//
-//        val dependent = object : Dependent {
-//            override fun dependencyMightChange() {
-//                dependencyMightChangeCount++
-//            }
-//
-//            override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-//                dependencyChangedCount++
-//            }
-//        }
-//
-//        for (dependency in dependencies) {
-//            dependency.observable.addDependent(dependent)
-//        }
-//
-//        change {
-//            for (dependency in dependencies) {
-//                dependency.observable.value = dependency.createValue()
-//            }
-//
-//            // Calls to dependencyMightChange happen immediately.
-//            assertEquals(dependencies.size, dependencyMightChangeCount)
-//            // Calls to dependencyChanged happen later.
-//            assertEquals(0, dependencyChangedCount)
-//        }
-//
-//        assertEquals(dependencies.size, dependencyMightChangeCount)
-//        assertEquals(dependencies.size, dependencyChangedCount)
-//    }
-
-    // TODO: Figure out change set bug and enable change sets again.
-    /**
-     * Modifying a mutable cell multiple times in one change set results in a single call to
-     * [Dependent.dependencyInvalidated] and [Dependent.dependencyChanged].
-     */
-//    @Test
-//    fun multiple_changes_to_one_cell_in_change_set() = test {
-//        val dependency = createProvider()
-//
-//        var dependencyMightChangeCount = 0
-//        var dependencyChangedCount = 0
-//
-//        val dependent = object : Dependent {
-//            override fun dependencyMightChange() {
-//                dependencyMightChangeCount++
-//            }
-//
-//            override fun dependencyChanged(dependency: Dependency, event: ChangeEvent<*>?) {
-//                dependencyChangedCount++
-//            }
-//        }
-//
-//        dependency.observable.addDependent(dependent)
-//
-//        // Change the dependency multiple times in a transaction.
-//        change {
-//            repeat(5) {
-//                dependency.observable.value = dependency.createValue()
-//            }
-//
-//            // Calls to dependencyMightChange happen immediately.
-//            assertEquals(1, dependencyMightChangeCount)
-//            // Calls to dependencyChanged happen later.
-//            assertEquals(0, dependencyChangedCount)
-//        }
-//
-//        assertEquals(1, dependencyMightChangeCount)
-//        assertEquals(1, dependencyChangedCount)
-//    }
-
-    // TODO: Figure out change set bug and enable change sets again.
-    /**
-     * Modifying two mutable cells in a change set results in a single recomputation of their
-     * dependent.
-     */
-//    @Test
-//    fun modifying_two_cells_together_results_in_one_recomputation() = test {
-//        val dependency1 = createProvider()
-//        val dependency2 = createProvider()
-//
-//        var computeCount = 0
-//
-//        val dependent = DependentCell(dependency1.observable, dependency2.observable) {
-//            computeCount++
-//            Unit
-//        }
-//
-//        // Observe dependent to ensure it gets recomputed when its dependencies change.
-//        disposer.add(dependent.observe {})
-//
-//        // DependentCell's compute function is called once when we start observing.
-//        assertEquals(1, computeCount)
-//
-//        change {
-//            dependency1.observable.value = dependency1.createValue()
-//            dependency2.observable.value = dependency2.createValue()
-//        }
-//
-//        assertEquals(2, computeCount)
-//    }
-
     interface Provider<T : Any> : CellTests.Provider {
         override val cell: MutableCell<T>