Fix stuck predictive back platform channel calls (#133368)

Fix a Google test flakiness increase.

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

@@ -1346,12 +1346,18 @@ class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {
   /// the platform with [NavigationNotification.canHandlePop] and stops
   /// bubbling.
   bool _defaultOnNavigationNotification(NavigationNotification notification) {
-    // Don't do anything with navigation notifications if there is no engine
-    // attached.
-    if (_appLifecycleState != AppLifecycleState.detached) {
-      SystemNavigator.setFrameworkHandlesBack(notification.canHandlePop);
+    switch (_appLifecycleState) {
+      case null:
+      case AppLifecycleState.detached:
+      case AppLifecycleState.inactive:
+        // Avoid updating the engine when the app isn't ready.
+        return true;
+      case AppLifecycleState.resumed:
+      case AppLifecycleState.hidden:
+      case AppLifecycleState.paused:
+        SystemNavigator.setFrameworkHandlesBack(notification.canHandlePop);
+        return true;
     }
-    return true;
   }
 
   @override
@@ -1366,6 +1372,7 @@ class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {
     _updateRouting();
     _locale = _resolveLocales(WidgetsBinding.instance.platformDispatcher.locales, widget.supportedLocales);
     WidgetsBinding.instance.addObserver(this);
+    _appLifecycleState = WidgetsBinding.instance.lifecycleState;
   }
 
   @override

packages/flutter/test/cupertino/tab_scaffold_test.dart

@@ -1219,11 +1219,7 @@ void main() {
 
   group('Android Predictive Back', () {
     bool? lastFrameworkHandlesBack;
-    setUp(() {
-      // Initialize to false. Because this uses a static boolean internally, it
-      // is not reset between tests or calls to pumpWidget. Explicitly setting
-      // it to false before each test makes them behave deterministically.
-      SystemNavigator.setFrameworkHandlesBack(false);
+    setUp(() async {
       lastFrameworkHandlesBack = null;
       TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
         .setMockMethodCallHandler(SystemChannels.platform, (MethodCall methodCall) async {
@@ -1233,12 +1229,17 @@ void main() {
           }
           return;
         });
+      await TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .handlePlatformMessage(
+            'flutter/lifecycle',
+            const StringCodec().encodeMessage(AppLifecycleState.resumed.toString()),
+            (ByteData? data) {},
+          );
     });
 
     tearDown(() {
       TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
           .setMockMethodCallHandler(SystemChannels.platform, null);
-      SystemNavigator.setFrameworkHandlesBack(true);
     });
 
     testWidgets('System back navigation inside of tabs', (WidgetTester tester) async {

packages/flutter/test/widgets/app_test.dart

@@ -683,6 +683,90 @@ void main() {
     expect(copySpy.invoked, isTrue);
     expect(pasteSpy.invoked, isTrue);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS, TargetPlatform.macOS }));
+
+  group('Android Predictive Back', () {
+    Future<void> setAppLifeCycleState(AppLifecycleState state) async {
+      final ByteData? message = const StringCodec().encodeMessage(state.toString());
+      await TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .handlePlatformMessage('flutter/lifecycle', message, (ByteData? data) {});
+    }
+
+    final List<bool> frameworkHandlesBacks = <bool>[];
+    setUp(() async {
+      frameworkHandlesBacks.clear();
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(SystemChannels.platform, (MethodCall methodCall) async {
+          if (methodCall.method == 'SystemNavigator.setFrameworkHandlesBack') {
+            expect(methodCall.arguments, isA<bool>());
+            frameworkHandlesBacks.add(methodCall.arguments as bool);
+          }
+          return;
+        });
+    });
+
+    tearDown(() async {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockMethodCallHandler(SystemChannels.platform, null);
+      await setAppLifeCycleState(AppLifecycleState.resumed);
+    });
+
+    testWidgets('WidgetsApp calls setFrameworkHandlesBack only when app is ready', (WidgetTester tester) async {
+      // Start in the `resumed` state, where setFrameworkHandlesBack should be
+      // called like normal.
+      await setAppLifeCycleState(AppLifecycleState.resumed);
+
+      late BuildContext currentContext;
+      await tester.pumpWidget(
+        WidgetsApp(
+          color: const Color(0xFF123456),
+          builder: (BuildContext context, Widget? child) {
+            currentContext = context;
+            return const Placeholder();
+          },
+        ),
+      );
+
+      expect(frameworkHandlesBacks, isEmpty);
+
+      const NavigationNotification(canHandlePop: true).dispatch(currentContext);
+      await tester.pumpAndSettle();
+      expect(frameworkHandlesBacks, isNotEmpty);
+      expect(frameworkHandlesBacks.last, isTrue);
+
+      const NavigationNotification(canHandlePop: false).dispatch(currentContext);
+      await tester.pumpAndSettle();
+      expect(frameworkHandlesBacks.last, isFalse);
+
+      // Set the app state to inactive, where setFrameworkHandlesBack shouldn't
+      // be called.
+      await setAppLifeCycleState(AppLifecycleState.inactive);
+
+      final int finalCallsLength = frameworkHandlesBacks.length;
+      const NavigationNotification(canHandlePop: true).dispatch(currentContext);
+      await tester.pumpAndSettle();
+      expect(frameworkHandlesBacks, hasLength(finalCallsLength));
+
+      const NavigationNotification(canHandlePop: false).dispatch(currentContext);
+      await tester.pumpAndSettle();
+      expect(frameworkHandlesBacks, hasLength(finalCallsLength));
+
+      // Set the app state to detached, which also shouldn't call
+      // setFrameworkHandlesBack. Must go to paused, then detached.
+      await setAppLifeCycleState(AppLifecycleState.paused);
+      await setAppLifeCycleState(AppLifecycleState.detached);
+
+      const NavigationNotification(canHandlePop: true).dispatch(currentContext);
+      await tester.pumpAndSettle();
+      expect(frameworkHandlesBacks, hasLength(finalCallsLength));
+
+      const NavigationNotification(canHandlePop: false).dispatch(currentContext);
+      await tester.pumpAndSettle();
+      expect(frameworkHandlesBacks, hasLength(finalCallsLength));
+    },
+      skip: kIsWeb, // [intended] predictive back is only native Android.
+      variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.android })
+    );
+  });
 }
 
 typedef SimpleRouterDelegateBuilder = Widget Function(BuildContext, RouteInformation);

packages/flutter/test/widgets/navigator_test.dart

@@ -4159,11 +4159,7 @@ void main() {
 
   group('Android Predictive Back', () {
     bool? lastFrameworkHandlesBack;
-    setUp(() {
-      // Initialize to false. Because this uses a static boolean internally, it
-      // is not reset between tests or calls to pumpWidget. Explicitly setting
-      // it to false before each test makes them behave deterministically.
-      SystemNavigator.setFrameworkHandlesBack(false);
+    setUp(() async {
       lastFrameworkHandlesBack = null;
       TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
         .setMockMethodCallHandler(SystemChannels.platform, (MethodCall methodCall) async {
@@ -4173,12 +4169,17 @@ void main() {
           }
           return;
         });
+      await TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .handlePlatformMessage(
+            'flutter/lifecycle',
+            const StringCodec().encodeMessage(AppLifecycleState.resumed.toString()),
+            (ByteData? data) {},
+          );
     });
 
     tearDown(() {
       TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
           .setMockMethodCallHandler(SystemChannels.platform, null);
-      SystemNavigator.setFrameworkHandlesBack(true);
     });
 
     testWidgets('a single route is already defaulted to false', (WidgetTester tester) async {

packages/flutter/test/widgets/pop_scope_test.dart

@@ -11,11 +11,7 @@ import 'navigator_utils.dart';
 
 void main() {
   bool? lastFrameworkHandlesBack;
-  setUp(() {
-    // Initialize to false. Because this uses a static boolean internally, it
-    // is not reset between tests or calls to pumpWidget. Explicitly setting
-    // it to false before each test makes them behave deterministically.
-    SystemNavigator.setFrameworkHandlesBack(false);
+  setUp(() async {
     lastFrameworkHandlesBack = null;
     TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
       .setMockMethodCallHandler(SystemChannels.platform, (MethodCall methodCall) async {
@@ -25,12 +21,17 @@ void main() {
         }
         return;
       });
+    await TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .handlePlatformMessage(
+          'flutter/lifecycle',
+          const StringCodec().encodeMessage(AppLifecycleState.resumed.toString()),
+          (ByteData? data) {},
+        );
   });
 
   tearDown(() {
     TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
         .setMockMethodCallHandler(SystemChannels.platform, null);
-    SystemNavigator.setFrameworkHandlesBack(true);
   });
 
   testWidgets('toggling canPop on root route allows/prevents backs', (WidgetTester tester) async {