Record focus in route entry to move a11y focus to the last focused item (#135771)

issue: #97747 
engine pr:flutter/engine#47114

Author: hannah-hyj

packages/flutter/lib/src/services/binding.dart

@@ -43,6 +43,7 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
     _initKeyboard();
     initLicenses();
     SystemChannels.system.setMessageHandler((dynamic message) => handleSystemMessage(message as Object));
+    SystemChannels.accessibility.setMessageHandler((dynamic message) => _handleAccessibilityMessage(message as Object));
     SystemChannels.lifecycle.setMessageHandler(_handleLifecycleMessage);
     SystemChannels.platform.setMethodCallHandler(_handlePlatformMessage);
     TextInput.ensureInitialized();
@@ -353,6 +354,21 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
     return false;
   }
 
+
+  /// Listenable that notifies when the accessibility focus on the system have changed.
+  final ValueNotifier<int?> accessibilityFocus = ValueNotifier<int?>(null);
+
+  Future<void> _handleAccessibilityMessage(Object accessibilityMessage) async {
+    final Map<String, dynamic> message =
+        (accessibilityMessage as Map<Object?, Object?>).cast<String, dynamic>();
+    final String type = message['type'] as String;
+    switch (type) {
+      case 'didGainFocus':
+       accessibilityFocus.value = message['nodeId'] as int;
+    }
+    return;
+  }
+
   Future<dynamic> _handlePlatformMessage(MethodCall methodCall) async {
     final String method = methodCall.method;
     assert(method == 'SystemChrome.systemUIChange' || method == 'System.requestAppExit');

packages/flutter/lib/src/widgets/navigator.dart

@@ -27,6 +27,9 @@ import 'restoration_properties.dart';
 import 'routes.dart';
 import 'ticker_provider.dart';
 
+// Duration for delay before refocusing in android so that the focus won't be interrupted.
+const Duration _kAndroidRefocusingDelayDuration = Duration(milliseconds: 300);
+
 // Examples can assume:
 // typedef MyAppHome = Placeholder;
 // typedef MyHomePage = Placeholder;
@@ -372,6 +375,8 @@ abstract class Route<T> {
   Future<T?> get popped => _popCompleter.future;
   final Completer<T?> _popCompleter = Completer<T?>();
 
+  final Completer<T?> _disposeCompleter = Completer<T?>();
+
   /// A request was made to pop this route. If the route can handle it
   /// internally (e.g. because it has its own stack of internal state) then
   /// return false, otherwise return true (by returning the value of calling
@@ -511,6 +516,7 @@ abstract class Route<T> {
   void dispose() {
     _navigator = null;
     _restorationScopeId.dispose();
+    _disposeCompleter.complete();
     if (kFlutterMemoryAllocationsEnabled) {
       MemoryAllocations.instance.dispatchObjectDisposed(object: this);
     }
@@ -2940,6 +2946,7 @@ class _RouteEntry extends RouteTransitionRecord {
   Route<dynamic>? lastAnnouncedPreviousRoute = notAnnounced; // last argument to Route.didChangePrevious
   WeakReference<Route<dynamic>> lastAnnouncedPoppedNextRoute = WeakReference<Route<dynamic>>(notAnnounced); // last argument to Route.didPopNext
   Route<dynamic>? lastAnnouncedNextRoute = notAnnounced; // last argument to Route.didChangeNext
+  int? lastFocusNode; // The last focused semantic node for the route entry.
 
   /// Restoration ID to be used for the encapsulating route when restoration is
   /// enabled for it or null if restoration cannot be enabled for it.
@@ -3028,6 +3035,24 @@ class _RouteEntry extends RouteTransitionRecord {
   void handleDidPopNext(Route<dynamic> poppedRoute) {
     route.didPopNext(poppedRoute);
     lastAnnouncedPoppedNextRoute = WeakReference<Route<dynamic>>(poppedRoute);
+    if (lastFocusNode != null) {
+      // Move focus back to the last focused node.
+      poppedRoute._disposeCompleter.future.then((dynamic result) async {
+        switch (defaultTargetPlatform) {
+          case TargetPlatform.android:
+            // In the Android platform, we have to wait for the system refocus to complete before
+            // sending the refocus message. Otherwise, the refocus message will be ignored.
+            // TODO(hangyujin): update this logic if Android provide a better way to do so.
+            final int? reFocusNode = lastFocusNode;
+            await Future<void>.delayed(_kAndroidRefocusingDelayDuration);
+            SystemChannels.accessibility.send(const FocusSemanticEvent().toMap(nodeId: reFocusNode));
+          case TargetPlatform.iOS:
+            SystemChannels.accessibility.send(const FocusSemanticEvent().toMap(nodeId: lastFocusNode));
+          case _:
+            break ;
+        }
+      });
+    }
   }
 
   /// Process the to-be-popped route.
@@ -3576,9 +3601,16 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
       SystemNavigator.selectSingleEntryHistory();
     }
 
+    ServicesBinding.instance.accessibilityFocus.addListener(_recordLastFocus);
     _history.addListener(_handleHistoryChanged);
   }
 
+  // Record the last focused node in route entry.
+  void _recordLastFocus(){
+    final _RouteEntry? entry = _history.where(_RouteEntry.isPresentPredicate).lastOrNull;
+    entry?.lastFocusNode = ServicesBinding.instance.accessibilityFocus.value;
+  }
+
   // Use [_nextPagelessRestorationScopeId] to get the next id.
   final RestorableNum<int> _rawNextPagelessRestorationScopeId = RestorableNum<int>(0);
 
@@ -3871,6 +3903,7 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
     _rawNextPagelessRestorationScopeId.dispose();
     _serializableHistory.dispose();
     userGestureInProgressNotifier.dispose();
+    ServicesBinding.instance.accessibilityFocus.removeListener(_recordLastFocus);
     _history.removeListener(_handleHistoryChanged);
     _history.dispose();
     super.dispose();

packages/flutter/test/widgets/navigator_test.dart

@@ -4287,6 +4287,98 @@ void main() {
     expect(policy, isA<ReadingOrderTraversalPolicy>());
   });
 
+  testWidgetsWithLeakTracking(
+      'Send semantic event to move a11y focus to the last focused item when pop next page',
+      (WidgetTester tester) async {
+    dynamic semanticEvent;
+    tester.binding.defaultBinaryMessenger.setMockDecodedMessageHandler<dynamic>(
+        SystemChannels.accessibility, (dynamic message) async {
+      semanticEvent = message;
+    });
+    final Key openSheetKey = UniqueKey();
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(primarySwatch: Colors.blue),
+        initialRoute: '/',
+        routes: <String, WidgetBuilder>{
+          '/': (BuildContext context) => _LinksPage(
+            title: 'Home page',
+            buttons: <Widget>[
+              TextButton(
+                onPressed: () {
+                  Navigator.of(context).pushNamed('/one');
+                },
+                child: const Text('Go to one'),
+              ),
+            ],
+          ),
+          '/one': (BuildContext context) => Scaffold(
+            body: Column(
+              children: <Widget>[
+                const ListTile(title: Text('Title 1')),
+                const ListTile(title: Text('Title 2')),
+                const ListTile(title: Text('Title 3')),
+                ElevatedButton(
+                  key: openSheetKey,
+                  onPressed: () {
+                    showModalBottomSheet<void>(
+                      context: context,
+                      builder: (BuildContext context) {
+                        return Center(
+                          child: ElevatedButton(
+                            onPressed: () => Navigator.pop(context),
+                            child: const Text('Close Sheet'),
+                          ),
+                        );
+                      },
+                    );
+                  },
+                  child: const Text('Open Sheet'),
+                )
+              ],
+            ),
+          ),
+        },
+      ),
+    );
+
+    expect(find.text('Home page'), findsOneWidget);
+
+    await tester.tap(find.text('Go to one'));
+    await tester.pumpAndSettle();
+
+    // The focused node before opening the sheet.
+    final ByteData? fakeMessage =
+        SystemChannels.accessibility.codec.encodeMessage(<String, dynamic>{
+      'type': 'didGainFocus',
+      'nodeId': 5,
+    });
+    tester.binding.defaultBinaryMessenger.handlePlatformMessage(
+      SystemChannels.accessibility.name,
+      fakeMessage,
+      (ByteData? data) {},
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.text('Open Sheet'));
+    await tester.pumpAndSettle();
+    expect(find.text('Close Sheet'), findsOneWidget);
+    await tester.tap(find.text('Close Sheet'));
+    await tester.pumpAndSettle(const Duration(milliseconds: 500));
+
+    // The focused node before opening the sheet regains the focus;
+    expect(semanticEvent, <String, dynamic>{
+      'type': 'focus',
+      'nodeId': 5,
+      'data': <String, dynamic>{},
+    });
+
+    tester.binding.defaultBinaryMessenger.setMockDecodedMessageHandler<dynamic>(SystemChannels.accessibility, null);
+  },
+    variant:  const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS}),
+    skip: isBrowser, // [intended] only non-web supports move a11y focus back to last item.
+  );
+
   group('RouteSettings.toString', () {
     test('when name is not null, should have double quote', () {
       expect(const RouteSettings(name: '/home').toString(), 'RouteSettings("/home", null)');