[web] Implement AppLifecycleState.detached as documented (flutter/engine#53506)

Currently, we are transitioning to the `AppLifecycleState.detached` incorrectly. This is causing the framework to stop pumping frames when the app is still active and visible.

This PR re-implements the transition to `AppLifecycleState.detached` as documented [here](https://api.flutter.dev/flutter/dart-ui/AppLifecycleState.html#detached) (based on whether the app has any views or not).

Fixes https://github.com/flutter/flutter/issues/150636
Fixes https://github.com/flutter/flutter/issues/149417

engine/src/flutter/lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -78,7 +78,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     _addFontSizeObserver();
     _addLocaleChangedListener();
     registerHotRestartListener(dispose);
-    AppLifecycleState.instance.addListener(_setAppLifecycleState);
+    _appLifecycleState.addListener(_setAppLifecycleState);
     _viewFocusBinding.init();
     domDocument.body?.prepend(accessibilityPlaceholder);
     _onViewDisposedListener = viewManager.onViewDisposed.listen((_) {
@@ -122,7 +122,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     _disconnectFontSizeObserver();
     _removeLocaleChangedListener();
     HighContrastSupport.instance.removeListener(_updateHighContrast);
-    AppLifecycleState.instance.removeListener(_setAppLifecycleState);
+    _appLifecycleState.removeListener(_setAppLifecycleState);
     _viewFocusBinding.dispose();
     accessibilityPlaceholder.remove();
     _onViewDisposedListener.cancel();
@@ -155,6 +155,9 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
 
   late final FlutterViewManager viewManager = FlutterViewManager(this);
 
+  late final AppLifecycleState _appLifecycleState =
+      AppLifecycleState.create(viewManager);
+
   /// The current list of windows.
   @override
   Iterable<EngineFlutterView> get views => viewManager.views;

engine/src/flutter/lib/web_ui/lib/src/engine/platform_dispatcher/app_lifecycle_state.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
 import 'package:meta/meta.dart';
 import 'package:ui/ui.dart' as ui;
 
@@ -12,7 +14,9 @@ typedef AppLifecycleStateListener = void Function(ui.AppLifecycleState state);
 
 /// Determines the [ui.AppLifecycleState].
 abstract class AppLifecycleState {
-  static final AppLifecycleState instance = _BrowserAppLifecycleState();
+  static AppLifecycleState create(FlutterViewManager viewManager) {
+    return _BrowserAppLifecycleState(viewManager);
+  }
 
   ui.AppLifecycleState get appLifecycleState => _appLifecycleState;
   ui.AppLifecycleState _appLifecycleState = ui.AppLifecycleState.resumed;
@@ -56,28 +60,36 @@ abstract class AppLifecycleState {
 /// browser events.
 ///
 /// This class listens to:
-/// - 'beforeunload' on [DomWindow] to detect detachment,
 /// - 'visibilitychange' on [DomHTMLDocument] to observe visibility changes,
 /// - 'focus' and 'blur' on [DomWindow] to track application focus shifts.
 class _BrowserAppLifecycleState extends AppLifecycleState {
+  _BrowserAppLifecycleState(this._viewManager);
+
+  final FlutterViewManager _viewManager;
+  final List<StreamSubscription<void>> _subscriptions = <StreamSubscription<void>>[];
+
   @override
   void activate() {
     domWindow.addEventListener('focus', _focusListener);
     domWindow.addEventListener('blur', _blurListener);
-    // TODO(web): Register 'beforeunload' only if lifecycle listeners exist, to improve efficiency: https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event#usage_notes
-    domWindow.addEventListener('beforeunload', _beforeUnloadListener);
     domDocument.addEventListener('visibilitychange', _visibilityChangeListener);
+    _subscriptions
+      ..add(_viewManager.onViewCreated.listen(_onViewCountChanged))
+      ..add(_viewManager.onViewDisposed.listen(_onViewCountChanged));
   }
 
   @override
   void deactivate() {
     domWindow.removeEventListener('focus', _focusListener);
     domWindow.removeEventListener('blur', _blurListener);
-    domWindow.removeEventListener('beforeunload', _beforeUnloadListener);
     domDocument.removeEventListener(
       'visibilitychange',
       _visibilityChangeListener,
     );
+    for (final StreamSubscription<void> subscription in _subscriptions) {
+      subscription.cancel();
+    }
+    _subscriptions.clear();
   }
 
   late final DomEventListener _focusListener =
@@ -90,11 +102,6 @@ class _BrowserAppLifecycleState extends AppLifecycleState {
     onAppLifecycleStateChange(ui.AppLifecycleState.inactive);
   });
 
-  late final DomEventListener _beforeUnloadListener =
-      createDomEventListener((DomEvent event) {
-    onAppLifecycleStateChange(ui.AppLifecycleState.detached);
-  });
-
   late final DomEventListener _visibilityChangeListener =
       createDomEventListener((DomEvent event) {
     if (domDocument.visibilityState == 'visible') {
@@ -103,4 +110,12 @@ class _BrowserAppLifecycleState extends AppLifecycleState {
       onAppLifecycleStateChange(ui.AppLifecycleState.hidden);
     }
   });
+
+  void _onViewCountChanged(_) {
+    if (_viewManager.views.isEmpty) {
+      onAppLifecycleStateChange(ui.AppLifecycleState.detached);
+    } else {
+      onAppLifecycleStateChange(ui.AppLifecycleState.resumed);
+    }
+  }
 }

engine/src/flutter/lib/web_ui/test/engine/platform_dispatcher/app_lifecycle_state_test.dart

@@ -0,0 +1,54 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart' as ui;
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+void testMain() {
+  group(AppLifecycleState, () {
+    test('listens to changes in view manager', () {
+      final FlutterViewManager viewManager = FlutterViewManager(EnginePlatformDispatcher.instance);
+      final AppLifecycleState state = AppLifecycleState.create(viewManager);
+
+      ui.AppLifecycleState? currentState;
+      void listener(ui.AppLifecycleState newState) {
+        currentState = newState;
+      }
+
+      state.addListener(listener);
+
+      final view1 = EngineFlutterView(EnginePlatformDispatcher.instance, createDomHTMLDivElement());
+      viewManager.registerView(view1);
+      expect(currentState, ui.AppLifecycleState.resumed);
+      currentState = null;
+
+      final view2 = EngineFlutterView(EnginePlatformDispatcher.instance, createDomHTMLDivElement());
+      viewManager.registerView(view2);
+      // The listener should not be called again. The view manager is still not empty.
+      expect(currentState, isNull);
+
+      viewManager.disposeAndUnregisterView(view1.viewId);
+      // The listener should not be called again. The view manager is still not empty.
+      expect(currentState, isNull);
+
+      viewManager.disposeAndUnregisterView(view2.viewId);
+      expect(currentState, ui.AppLifecycleState.detached);
+      currentState = null;
+
+      final view3 = EngineFlutterView(EnginePlatformDispatcher.instance, createDomHTMLDivElement());
+      viewManager.registerView(view3);
+      // The state should go back to `resumed` after a new view is registered.
+      expect(currentState, ui.AppLifecycleState.resumed);
+
+      viewManager.dispose();
+      state.removeListener(listener);
+    });
+  });
+}