Add BuildContext.mounted (#111619)

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

@@ -2081,6 +2081,29 @@ typedef ElementVisitor = void Function(Element element);
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=rIaaH87z1-g}
 ///
+/// Avoid storing instances of [BuildContext]s because they may become invalid
+/// if the widget they are associated with is unmounted from the widget tree.
+/// {@template flutter.widgets.BuildContext.asynchronous_gap}
+/// If a [BuildContext] is used across an asynchronous gap (i.e. after performing
+/// an asynchronous operation), consider checking [mounted] to determine whether
+/// the context is still valid before interacting with it:
+///
+/// ```dart
+///   @override
+///   Widget build(BuildContext context) {
+///     return OutlinedButton(
+///       onPressed: () async {
+///         await Future<void>.delayed(const Duration(seconds: 1));
+///         if (context.mounted) {
+///           Navigator.of(context).pop();
+///         }
+///       },
+///       child: const Text('Delayed pop'),
+///     );
+///   }
+/// ```
+/// {@endtemplate}
+///
 /// [BuildContext] objects are actually [Element] objects. The [BuildContext]
 /// interface is used to discourage direct manipulation of [Element] objects.
 abstract class BuildContext {
@@ -2091,6 +2114,18 @@ abstract class BuildContext {
   /// managing the rendering pipeline for this context.
   BuildOwner? get owner;
 
+  /// Whether the [Widget] this context is associated with is currently
+  /// mounted in the widget tree.
+  ///
+  /// Accessing the properties of the [BuildContext] or calling any methods on
+  /// it is only valid while mounted is true. If mounted is false, assertions
+  /// will trigger.
+  ///
+  /// Once unmounted, a given [BuildContext] will never become mounted again.
+  ///
+  /// {@macro flutter.widgets.BuildContext.asynchronous_gap}
+  bool get mounted;
+
   /// Whether the [widget] is currently updating the widget or render tree.
   ///
   /// For [StatefulWidget]s and [StatelessWidget]s this flag is true while
@@ -3271,6 +3306,9 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
   Widget get widget => _widget!;
   Widget? _widget;
 
+  @override
+  bool get mounted => _widget != null;
+
   /// Returns true if the Element is defunct.
   ///
   /// This getter always returns false in profile and release builds.

packages/flutter/test/widgets/build_context_test.dart

@@ -0,0 +1,63 @@
+// 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/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('StatefulWidget BuildContext.mounted', (WidgetTester tester) async {
+    late BuildContext capturedContext;
+    await tester.pumpWidget(TestStatefulWidget(
+        onBuild: (BuildContext context) {
+          capturedContext = context;
+        }
+    ));
+    expect(capturedContext.mounted, isTrue);
+    await tester.pumpWidget(Container());
+    expect(capturedContext.mounted, isFalse);
+  });
+
+  testWidgets('StatelessWidget BuildContext.mounted', (WidgetTester tester) async {
+    late BuildContext capturedContext;
+    await tester.pumpWidget(TestStatelessWidget(
+      onBuild: (BuildContext context) {
+        capturedContext = context;
+      }
+    ));
+    expect(capturedContext.mounted, isTrue);
+    await tester.pumpWidget(Container());
+    expect(capturedContext.mounted, isFalse);
+  });
+}
+
+typedef BuildCallback = void Function(BuildContext);
+
+class TestStatelessWidget extends StatelessWidget {
+  const TestStatelessWidget({super.key, required this.onBuild});
+
+  final BuildCallback onBuild;
+
+  @override
+  Widget build(BuildContext context) {
+    onBuild(context);
+    return Container();
+  }
+}
+
+class TestStatefulWidget extends StatefulWidget {
+  const TestStatefulWidget({super.key, required this.onBuild});
+
+  final BuildCallback onBuild;
+
+  @override
+  State<TestStatefulWidget> createState() => _TestStatefulWidgetState();
+}
+
+class _TestStatefulWidgetState extends State<TestStatefulWidget> {
+  @override
+  Widget build(BuildContext context) {
+    widget.onBuild(context);
+    return Container();
+  }
+}