Update WidgetsBindingsObserver example (#101512)

examples/api/lib/widgets/binding/widget_binding_observer.0.dart

+// Copyright 2014 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.
+
+// Flutter code sample for WidgetBindingsObserver
+
+import 'package:flutter/material.dart';
+
+void main() => runApp(const MyApp());
+
+class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(title: const Text('WidgetBindingsObserver Sample')),
+        body: const WidgetBindingsObserverSample(),
+      ),
+    );
+  }
+}
+
+class WidgetBindingsObserverSample extends StatefulWidget {
+  const WidgetBindingsObserverSample({Key? key}) : super(key: key);
+
+  @override
+  State<WidgetBindingsObserverSample> createState() => _WidgetBindingsObserverSampleState();
+}
+
+class _WidgetBindingsObserverSampleState extends State<WidgetBindingsObserverSample> with WidgetsBindingObserver {
+ final List<AppLifecycleState> _stateHistoryList = <AppLifecycleState>[];
+
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addObserver(this);
+    if(WidgetsBinding.instance.lifecycleState != null) {
+      _stateHistoryList.add(WidgetsBinding.instance.lifecycleState!);
+    }
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    setState(() { _stateHistoryList.add(state); });
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (_stateHistoryList.isNotEmpty) {
+      return ListView.builder(
+        key: const ValueKey<String>('stateHistoryList'),
+        itemCount: _stateHistoryList.length,
+        itemBuilder: (BuildContext context, int index) {
+            return Text('state is: ${_stateHistoryList[index]}');
+          },
+        );
+    }
+
+    return const Center(child: Text('There are no AppLifecycleStates to show.'));
+  }
+}

examples/api/test/widgets/binding/widget_binding_observer.0_test.dart

+// Copyright 2014 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:flutter/services.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_api_samples/widgets/binding/widget_binding_observer.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('App tracks lifecycle states', (WidgetTester tester) async {
+    Future<void> setAppLifeCycleState(AppLifecycleState state) async {
+      final ByteData? message =
+          const StringCodec().encodeMessage(state.toString());
+      await ServicesBinding.instance.defaultBinaryMessenger
+          .handlePlatformMessage('flutter/lifecycle', message, (_) {});
+    }
+
+    await tester.pumpWidget(
+     const example.MyApp(),
+    );
+
+    expect(find.text('There are no AppLifecycleStates to show.'), findsOneWidget);
+
+    await setAppLifeCycleState(AppLifecycleState.resumed);
+    await tester.pumpAndSettle();
+    expect(find.text('state is: AppLifecycleState.resumed'), findsOneWidget);
+
+    await setAppLifeCycleState(AppLifecycleState.inactive);
+    await tester.pumpAndSettle();
+    expect(find.text('state is: AppLifecycleState.inactive'), findsOneWidget);
+
+    await setAppLifeCycleState(AppLifecycleState.paused);
+    await tester.pumpAndSettle();
+    await setAppLifeCycleState(AppLifecycleState.resumed);
+    await tester.pumpAndSettle();
+    expect(find.text('state is: AppLifecycleState.paused'), findsOneWidget);
+
+    await setAppLifeCycleState(AppLifecycleState.detached);
+    await tester.pumpAndSettle();
+    await setAppLifeCycleState(AppLifecycleState.resumed);
+    await tester.pumpAndSettle();
+    expect(find.text('state is: AppLifecycleState.detached'), findsOneWidget);
+  });
+}

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

@@ -33,45 +33,12 @@ export 'dart:ui' show AppLifecycleState, Locale;
 /// handlers must be implemented (and the analyzer will list those that have
 /// been omitted).
 ///
-/// {@tool snippet}
-///
-/// This [StatefulWidget] implements the parts of the [State] and
+/// {@tool dartpad}
+/// This sample shows how to implement parts of the [State] and
 /// [WidgetsBindingObserver] protocols necessary to react to application
 /// lifecycle messages. See [didChangeAppLifecycleState].
 ///
-/// ```dart
-/// class AppLifecycleReactor extends StatefulWidget {
-///   const AppLifecycleReactor({ Key? key }) : super(key: key);
-///
-///   @override
-///   State<AppLifecycleReactor> createState() => _AppLifecycleReactorState();
-/// }
-///
-/// class _AppLifecycleReactorState extends State<AppLifecycleReactor> with WidgetsBindingObserver {
-///   @override
-///   void initState() {
-///     super.initState();
-///     WidgetsBinding.instance.addObserver(this);
-///   }
-///
-///   @override
-///   void dispose() {
-///     WidgetsBinding.instance.removeObserver(this);
-///     super.dispose();
-///   }
-///
-///   late AppLifecycleState _notification;
-///
-///   @override
-///   void didChangeAppLifecycleState(AppLifecycleState state) {
-///     setState(() { _notification = state; });
-///   }
-///
-///   @override
-///   Widget build(BuildContext context) {
-///     return Text('Last notification: $_notification');
-///   }
-/// }
+/// ** See code in examples/api/lib/widgets/binding/widget_binding_observer.0.dart **
 /// ```
 /// {@end-tool}
 ///