Cancel .task work when the view disappears

AndroidSwiftUICore/Sources/AndroidSwiftUICore/Modifiers/InteractionModifiers.swift

@@ -56,15 +56,36 @@ public extension View {
 
 // MARK: - task
 
+/// Running `.task` closures, keyed by the view's stable identity path. Keying by
+/// path (not the generation-scoped callback id) lets the cancel fired on disappear
+/// stop the right Task no matter how many times the view re-evaluated in between.
+enum _TaskRegistry {
+    nonisolated(unsafe) static var running: [String: Task<Void, Never>] = [:]
+
+    static func start(path: String, action: @escaping @Sendable () async -> Void) {
+        running[path]?.cancel()                       // replace a stale run at this path
+        running[path] = Task { await action() }
+    }
+
+    static func cancel(path: String) {
+        running[path]?.cancel()
+        running[path] = nil
+    }
+}
+
 public struct _TaskModifier: RenderModifier, _CallbackModifier {
     let action: @Sendable () async -> Void
     public var _modifierNode: ModifierNode { ModifierNode(kind: "task") }
     public func _callbackNode(in context: ResolveContext) -> ModifierNode {
-        // The interpreter fires this once on appear; the closure runs as a Task.
-        // (v1 does not cancel it on disappear.)
+        // Two callbacks: the interpreter fires `start` on appear and `cancel` on
+        // disappear. The launched Task is stored by identity path, so the closure
+        // it runs is cooperatively cancelled (Task.isCancelled / Task.sleep throws)
+        // when the view leaves the tree.
         let action = self.action
-        let id = context.callbacks.register(.void { Task { await action() } })
-        return ModifierNode(kind: "task", args: ["action": .int(Int(id))])
+        let path = context.path
+        let start = context.callbacks.register(.void { _TaskRegistry.start(path: path, action: action) })
+        let cancel = context.callbacks.register(.void { _TaskRegistry.cancel(path: path) })
+        return ModifierNode(kind: "task", args: ["start": .int(Int(start)), "cancel": .int(Int(cancel))])
     }
 }
 

AndroidSwiftUICore/Tests/AndroidSwiftUICoreTests/EvaluatorTests.swift

@@ -194,6 +194,22 @@ struct ModifierTests {
         #expect(node.modifiers.contains { $0.kind == "onDisappear" })
     }
 
+    @Test("task emits start and cancel callback ids that drive the same Task")
+    func taskStartAndCancel() async {
+        let host = ViewHost(Text("x").task { try? await Task.sleep(nanoseconds: 10_000_000_000) })
+        let node = host.evaluate()
+        let task = node.modifiers.first { $0.kind == "task" }
+        guard case .int(let start)? = task?.args["start"],
+              case .int(let cancel)? = task?.args["cancel"] else {
+            Issue.record("missing start/cancel ids"); return
+        }
+        #expect(start != cancel)
+        host.callbacks.invokeVoid(Int64(start))          // launches the Task
+        #expect(!_TaskRegistry.running.isEmpty)
+        host.callbacks.invokeVoid(Int64(cancel))         // cancels and clears it
+        #expect(_TaskRegistry.running.isEmpty)
+    }
+
     @Test("onChange emits a token describing the observed value")
     func onChange() {
         let node = ViewHost(Text("x").onChange(of: 42) {}).evaluate()

Demo/App.swiftpm/Sources/InteractionPlaygrounds.swift

@@ -10,6 +10,8 @@ struct InteractionPlayground: View {
     @State private var slider = 0.0
     @State private var sliderChanges = 0
     @State private var blocked = true
+    @State private var ticking = false
+    @State private var ticks = 0
     var body: some View {
         ScrollView {
             VStack(alignment: .leading, spacing: 0) {
@@ -35,6 +37,26 @@ struct InteractionPlayground: View {
                     }
                     .onChange(of: slider) { sliderChanges += 1 }
                 }
+                Example(".task (cancels on disappear)") {
+                    VStack(alignment: .leading, spacing: 8) {
+                        Text("Ticks: \(ticks)")
+                        Button(ticking ? "Hide (cancels task)" : "Show") { ticking.toggle() }
+                        if ticking {
+                            Text("Ticking every 0.5s while visible")
+                                .foregroundColor(.white)
+                                .padding()
+                                .background(Color.green)
+                                .cornerRadius(8)
+                                .task {
+                                    while true {
+                                        do { try await Task.sleep(nanoseconds: 500_000_000) }
+                                        catch { break }   // cancelled on disappear → stop
+                                        ticks += 1
+                                    }
+                                }
+                        }
+                    }
+                }
                 Example("disabled") {
                     VStack(alignment: .leading, spacing: 8) {
                         Button(blocked ? "Disabled button" : "Enabled button") { taps += 1 }

Demo/swiftui/src/commonMain/kotlin/com/pureswift/swiftui/Render.kt

@@ -764,7 +764,9 @@ private fun zStackAlignment(node: ViewNode): Alignment {
 private fun RenderEffects(node: ViewNode) {
     val onAppear = node.modifiers.firstOrNull { it.kind == "onAppear" }?.args?.long("action")
     val onDisappear = node.modifiers.firstOrNull { it.kind == "onDisappear" }?.args?.long("action")
-    val task = node.modifiers.firstOrNull { it.kind == "task" }?.args?.long("action")
+    val taskMod = node.modifiers.firstOrNull { it.kind == "task" }
+    val taskStart = taskMod?.args?.long("start")
+    val taskCancel = taskMod?.args?.long("cancel")
     val onChange = node.modifiers.firstOrNull { it.kind == "onChange" }
 
     if (onAppear != null || onDisappear != null) {
@@ -773,8 +775,16 @@ private fun RenderEffects(node: ViewNode) {
             onDispose { onDisappear?.let { SwiftBridge.sink.invokeVoid(it) } }
         }
     }
-    if (task != null) {
-        LaunchedEffect(Unit) { SwiftBridge.sink.invokeVoid(task) }
+    if (taskStart != null && taskCancel != null) {
+        // The cancel id changes each evaluation; onDispose can fire generations
+        // after appear, so hand it the freshest id (kept in a remembered holder)
+        // — still resolvable, and it cancels by stable path regardless.
+        val cancelHolder = remember { longArrayOf(taskCancel) }
+        cancelHolder[0] = taskCancel
+        DisposableEffect(Unit) {
+            SwiftBridge.sink.invokeVoid(taskStart)
+            onDispose { SwiftBridge.sink.invokeVoid(cancelHolder[0]) }
+        }
     }
     if (onChange != null) {
         val token = onChange.args.string("token")