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Have FocusManager
respond to app lifecycle state changes (#142930)
fixes #87061 It doesn't matter whether I'm using Google Chrome, VS Code, Discord, or a Terminal window: any time a text cursor is blinking, it means that the characters I type will show up there. And this isn't limited to text fields: if I repeatedly press `Tab` to navigate through a website, there's a visual indicator that goes away if I click away from the window, and it comes back if I click or `Alt+Tab` back into it. <details open> <summary>Example (Chrome):</summary> ![focus node](https://github.com/flutter/flutter/assets/10457200/bef42cd9-28e5-4214-b071-b7ef56b26609) </details> <details open> <summary>This PR adds the same functionality to Flutter apps:</summary> ![Flutter demo](https://github.com/flutter/flutter/assets/10457200/6eb34c44-5fb0-4b27-aa10-6606a1eb187e) </details>
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@ -1446,6 +1446,17 @@ enum FocusHighlightStrategy {
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alwaysTraditional,
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}
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// By extending the WidgetsBindingObserver class,
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// we can add a listener object to FocusManager as a private member.
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class _AppLifecycleListener extends WidgetsBindingObserver {
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_AppLifecycleListener(this.onLifecycleStateChanged);
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final void Function(AppLifecycleState) onLifecycleStateChanged;
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@override
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void didChangeAppLifecycleState(AppLifecycleState state) => onLifecycleStateChanged(state);
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}
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/// Manages the focus tree.
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///
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/// The focus tree is a separate, sparser, tree from the widget tree that
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@ -1508,6 +1519,8 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
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if (kFlutterMemoryAllocationsEnabled) {
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ChangeNotifier.maybeDispatchObjectCreation(this);
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}
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_appLifecycleListener = _AppLifecycleListener(_appLifecycleChange);
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WidgetsBinding.instance.addObserver(_appLifecycleListener);
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rootScope._manager = this;
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}
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@ -1524,6 +1537,7 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
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@override
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void dispose() {
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WidgetsBinding.instance.removeObserver(_appLifecycleListener);
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_highlightManager.dispose();
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rootScope.dispose();
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super.dispose();
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@ -1682,6 +1696,34 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
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// update.
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final Set<FocusNode> _dirtyNodes = <FocusNode>{};
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// Allows FocusManager to respond to app lifecycle state changes,
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// temporarily suspending the primaryFocus when the app is inactive.
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late final _AppLifecycleListener _appLifecycleListener;
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// Stores the node that was focused before the app lifecycle changed.
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// Will be restored as the primary focus once app is resumed.
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FocusNode? _suspendedNode;
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void _appLifecycleChange(AppLifecycleState state) {
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if (state == AppLifecycleState.resumed) {
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if (_primaryFocus != rootScope) {
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assert(_focusDebug(() => 'focus changed while app was paused, ignoring $_suspendedNode'));
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_suspendedNode = null;
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}
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else if (_suspendedNode != null) {
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assert(_focusDebug(() => 'marking node $_suspendedNode to be focused'));
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_markedForFocus = _suspendedNode;
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_suspendedNode = null;
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applyFocusChangesIfNeeded();
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}
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} else if (_primaryFocus != rootScope) {
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assert(_focusDebug(() => 'suspending $_primaryFocus'));
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_markedForFocus = rootScope;
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_suspendedNode = _primaryFocus;
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applyFocusChangesIfNeeded();
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}
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}
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// The node that has requested to have the primary focus, but hasn't been
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// given it yet.
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FocusNode? _markedForFocus;
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@ -1693,6 +1735,9 @@ class FocusManager with DiagnosticableTreeMixin, ChangeNotifier {
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if (_primaryFocus == node) {
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_primaryFocus = null;
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}
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if (_suspendedNode == node) {
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_suspendedNode = null;
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}
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_dirtyNodes.remove(node);
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}
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@ -354,6 +354,63 @@ void main() {
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logs.clear();
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// ignore: deprecated_member_use
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}, variant: KeySimulatorTransitModeVariant.all());
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testWidgets('FocusManager responds to app lifecycle changes.', (WidgetTester tester) async {
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Future<void> setAppLifecycleState(AppLifecycleState state) async {
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final ByteData? message = const StringCodec().encodeMessage(state.toString());
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await TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
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.handlePlatformMessage('flutter/lifecycle', message, (_) {});
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}
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final BuildContext context = await setupWidget(tester);
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final FocusScopeNode scope = FocusScopeNode(debugLabel: 'Scope');
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addTearDown(scope.dispose);
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final FocusAttachment scopeAttachment = scope.attach(context);
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final FocusNode focusNode = FocusNode(debugLabel: 'Focus Node');
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addTearDown(focusNode.dispose);
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final FocusAttachment focusNodeAttachment = focusNode.attach(context);
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scopeAttachment.reparent(parent: tester.binding.focusManager.rootScope);
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focusNodeAttachment.reparent(parent: scope);
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focusNode.requestFocus();
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await tester.pump();
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expect(focusNode.hasPrimaryFocus, isTrue);
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await setAppLifecycleState(AppLifecycleState.paused);
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expect(focusNode.hasPrimaryFocus, isFalse);
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await setAppLifecycleState(AppLifecycleState.resumed);
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expect(focusNode.hasPrimaryFocus, isTrue);
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});
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testWidgets('Node is removed completely even if app is paused.', (WidgetTester tester) async {
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Future<void> setAppLifecycleState(AppLifecycleState state) async {
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final ByteData? message = const StringCodec().encodeMessage(state.toString());
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await TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
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.handlePlatformMessage('flutter/lifecycle', message, (_) {});
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}
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final BuildContext context = await setupWidget(tester);
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final FocusScopeNode scope = FocusScopeNode(debugLabel: 'Scope');
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addTearDown(scope.dispose);
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final FocusAttachment scopeAttachment = scope.attach(context);
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final FocusNode focusNode = FocusNode(debugLabel: 'Focus Node');
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addTearDown(focusNode.dispose);
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final FocusAttachment focusNodeAttachment = focusNode.attach(context);
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scopeAttachment.reparent(parent: tester.binding.focusManager.rootScope);
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focusNodeAttachment.reparent(parent: scope);
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focusNode.requestFocus();
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await tester.pump();
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expect(focusNode.hasPrimaryFocus, isTrue);
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await setAppLifecycleState(AppLifecycleState.paused);
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expect(focusNode.hasPrimaryFocus, isFalse);
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focusNodeAttachment.detach();
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expect(focusNode.hasPrimaryFocus, isFalse);
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await setAppLifecycleState(AppLifecycleState.resumed);
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expect(focusNode.hasPrimaryFocus, isFalse);
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});
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});
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group(FocusScopeNode, () {
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