Reduce severity of memory leak when BuildContext/Element is retained (#79957)

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

@@ -3085,8 +3085,21 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
 
   /// The configuration for this element.
   @override
-  Widget get widget => _widget;
-  Widget _widget;
+  Widget get widget => _widget!;
+  Widget? _widget;
+
+  /// Returns true if the Element is defunct.
+  ///
+  /// This getter always returns false in profile and release builds.
+  /// See the lifecycle documentation for [Element] for additional information.
+  bool get debugIsDefunct {
+    bool isDefunct = false;
+    assert(() {
+      isDefunct = _lifecycleState == _ElementLifecycle.defunct;
+      return true;
+    }());
+    return isDefunct;
+  }
 
   /// The object that manages the lifecycle of this element.
   @override
@@ -3801,10 +3814,14 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
     assert(depth != null);
     assert(owner != null);
     // Use the private property to avoid a CastError during hot reload.
-    final Key? key = _widget.key;
+    final Key? key = _widget!.key;
     if (key is GlobalKey) {
       owner!._unregisterGlobalKey(key, this);
     }
+    // Release resources to reduce the severity of memory leaks caused by
+    // defunct, but accidentally retained Elements.
+    _widget = null;
+    _dependencies = null;
     _lifecycleState = _ElementLifecycle.defunct;
   }
 
@@ -4116,7 +4133,7 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
 
   /// A short, textual description of this element.
   @override
-  String toStringShort() => widget.toStringShort();
+  String toStringShort() => _widget?.toStringShort() ?? '${describeIdentity(this)}(DEFUNCT)';
 
   @override
   DiagnosticsNode toDiagnosticsNode({ String? name, DiagnosticsTreeStyle? style }) {
@@ -4134,9 +4151,9 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
     if (_lifecycleState != _ElementLifecycle.initial) {
       properties.add(ObjectFlagProperty<int>('depth', depth, ifNull: 'no depth'));
     }
-    properties.add(ObjectFlagProperty<Widget>('widget', widget, ifNull: 'no widget'));
-    properties.add(DiagnosticsProperty<Key>('key', widget.key, showName: false, defaultValue: null, level: DiagnosticLevel.hidden));
-    widget.debugFillProperties(properties);
+    properties.add(ObjectFlagProperty<Widget>('widget', _widget, ifNull: 'no widget'));
+    properties.add(DiagnosticsProperty<Key>('key', _widget?.key, showName: false, defaultValue: null, level: DiagnosticLevel.hidden));
+    _widget?.debugFillProperties(properties);
     properties.add(FlagProperty('dirty', value: dirty, ifTrue: 'dirty'));
     if (_dependencies != null && _dependencies!.isNotEmpty) {
       final List<DiagnosticsNode> diagnosticsDependencies = _dependencies!
@@ -4296,7 +4313,9 @@ class _ElementDiagnosticableTreeNode extends DiagnosticableTreeNode {
   Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
     final Map<String, Object?> json = super.toJsonMap(delegate);
     final Element element = value as Element;
+    if (!element.debugIsDefunct) {
       json['widgetRuntimeType'] = element.widget.runtimeType.toString();
+    }
     json['stateful'] = stateful;
     return json;
   }
@@ -4660,7 +4679,7 @@ class StatelessElement extends ComponentElement {
 class StatefulElement extends ComponentElement {
   /// Creates an element that uses the given widget as its configuration.
   StatefulElement(StatefulWidget widget)
-      : state = widget.createState(),
+      : _state = widget.createState(),
         super(widget) {
     assert(() {
       if (!state._debugTypesAreRight(widget)) {
@@ -4695,7 +4714,8 @@ class StatefulElement extends ComponentElement {
   /// There is a one-to-one relationship between [State] objects and the
   /// [StatefulElement] objects that hold them. The [State] objects are created
   /// by [StatefulElement] in [mount].
-  final State<StatefulWidget> state;
+  State<StatefulWidget> get state => _state!;
+  State<StatefulWidget>? _state;
 
   @override
   void reassemble() {
@@ -4810,6 +4830,9 @@ class StatefulElement extends ComponentElement {
       ]);
     }());
     state._element = null;
+    // Release resources to reduce the severity of memory leaks caused by
+    // defunct, but accidentally retained Elements.
+    _state = null;
   }
 
   @override
@@ -4895,7 +4918,7 @@ class StatefulElement extends ComponentElement {
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
-    properties.add(DiagnosticsProperty<State<StatefulWidget>>('state', state, defaultValue: null));
+    properties.add(DiagnosticsProperty<State<StatefulWidget>>('state', _state, defaultValue: null));
   }
 }
 
@@ -5705,13 +5728,14 @@ abstract class RenderObjectElement extends Element {
 
   @override
   void unmount() {
+    final RenderObjectWidget oldWidget = widget;
     super.unmount();
     assert(
       !renderObject.attached,
       'A RenderObject was still attached when attempting to unmount its '
       'RenderObjectElement: $renderObject',
     );
-    widget.didUnmountRenderObject(renderObject);
+    oldWidget.didUnmountRenderObject(renderObject);
   }
 
   void _updateParentData(ParentDataWidget<ParentData> parentDataWidget) {

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

@@ -1205,6 +1205,7 @@ mixin WidgetInspectorService {
   ///
   /// Use this method only for testing to ensure that object references from one
   /// test case do not impact other test cases.
+  @visibleForTesting
   @protected
   void disposeAllGroups() {
     _groups.clear();
@@ -1213,6 +1214,19 @@ mixin WidgetInspectorService {
     _nextId = 0;
   }
 
+  /// Reset all InspectorService state.
+  ///
+  /// Use this method only for testing to write hermetic tests for
+  /// WidgetInspectorService.
+  @visibleForTesting
+  @protected
+  @mustCallSuper
+  void resetAllState() {
+    disposeAllGroups();
+    selection.clear();
+    setPubRootDirectories(<String>[]);
+  }
+
   /// Free all references to objects in a group.
   ///
   /// Objects and their associated ids in the group may be kept alive by
@@ -2413,7 +2427,8 @@ class InspectorSelection {
   /// Setting [candidates] or calling [clear] resets the selection.
   ///
   /// Returns null if the selection is invalid.
-  RenderObject? get current => _current;
+  RenderObject? get current => active ? _current : null;
+
   RenderObject? _current;
   set current(RenderObject? value) {
     if (_current != value) {
@@ -2427,7 +2442,10 @@ class InspectorSelection {
   /// Setting [candidates] or calling [clear] resets the selection.
   ///
   /// Returns null if the selection is invalid.
-  Element? get currentElement => _currentElement;
+  Element? get currentElement {
+    return _currentElement?.debugIsDefunct ?? true ? null : _currentElement;
+  }
+
   Element? _currentElement;
   set currentElement(Element? element) {
     if (currentElement != element) {
@@ -3039,7 +3057,7 @@ String? _describeCreationLocation(Object object) {
 ///
 /// Currently creation locations are only available for [Widget] and [Element].
 _Location? _getCreationLocation(Object? object) {
-  final Object? candidate =  object is Element ? object.widget : object;
+  final Object? candidate =  object is Element && !object.debugIsDefunct ? object.widget : object;
   return candidate is _HasCreationLocation ? candidate._location : null;
 }
 

packages/flutter/test/widgets/framework_test.dart

@@ -1572,6 +1572,22 @@ void main() {
     await tester.pumpWidget(Container());
     expect(tester.binding.buildOwner!.globalKeyCount, initialCount + 0);
   });
+
+  testWidgets('Widget and State properties are nulled out when unmounted', (WidgetTester tester) async {
+    await tester.pumpWidget(const _StatefulLeaf());
+    final StatefulElement element = tester.element<StatefulElement>(find.byType(_StatefulLeaf));
+    expect(element.state, isA<State<_StatefulLeaf>>());
+    expect(element.widget, isA<_StatefulLeaf>());
+    // Replace the widget tree to unmount the element.
+    await tester.pumpWidget(Container());
+    // Accessing state/widget now throws a CastError because they have been
+    // nulled out to reduce severity of memory leaks when an Element (e.g. in
+    // the form of a BuildContext) is retained past its useful life. See also
+    // https://github.com/flutter/flutter/issues/79605 for examples why this may
+    // occur.
+    expect(() => element.state, throwsA(isA<TypeError>()));
+    expect(() => element.widget, throwsA(isA<TypeError>()));
+  }, skip: kIsWeb);
 }
 
 class _WidgetWithNoVisitChildren extends StatelessWidget {

packages/flutter/test/widgets/widget_inspector_test.dart

@@ -233,6 +233,10 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
     final TestWidgetInspectorService service = TestWidgetInspectorService();
     WidgetInspectorService.instance = service;
 
+    tearDown(() {
+      service.resetAllState();
+    });
+
     testWidgets('WidgetInspector smoke test', (WidgetTester tester) async {
       // This is a smoke test to verify that adding the inspector doesn't crash.
       await tester.pumpWidget(
@@ -759,6 +763,61 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
       expect(service.selection.currentElement, equals(elementA));
     });
 
+    testWidgets('WidgetInspectorService defunct selection regression test', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.ltr,
+          child: Stack(
+            children: const <Widget>[
+              Text('a', textDirection: TextDirection.ltr),
+            ],
+          ),
+        ),
+      );
+      final Element elementA = find.text('a').evaluate().first;
+
+      service.setSelection(elementA);
+      expect(service.selection.currentElement, equals(elementA));
+      expect(service.selection.current, equals(elementA.renderObject));
+
+      await tester.pumpWidget(
+        const SizedBox(
+          child: Text('b', textDirection: TextDirection.ltr),
+        ),
+      );
+      // Selection is now empty as the element is defunct.
+      expect(service.selection.currentElement, equals(null));
+      expect(service.selection.current, equals(null));
+
+      // Verify that getting the debug creation location of the defunct element
+      // does not crash.
+      expect(debugIsLocalCreationLocation(elementA), isFalse);
+
+      // Verify that generating json for a defunct element does not crash.
+      expect(
+        elementA.toDiagnosticsNode().toJsonMap(
+          InspectorSerializationDelegate(
+            service: service,
+            summaryTree: false,
+            includeProperties: true,
+            addAdditionalPropertiesCallback: null,
+          ),
+        ),
+        isNotNull,
+      );
+
+      final Element elementB = find.text('b').evaluate().first;
+      service.setSelection(elementB);
+      expect(service.selection.currentElement, equals(elementB));
+      expect(service.selection.current, equals(elementB.renderObject));
+
+      // Set selection back to a defunct element.
+      service.setSelection(elementA);
+
+      expect(service.selection.currentElement, equals(null));
+      expect(service.selection.current, equals(null));
+    });
+
     testWidgets('WidgetInspectorService getParentChain', (WidgetTester tester) async {
       const String group = 'test-group';
 
@@ -934,6 +993,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
         ),
       );
       final Element elementA = find.text('a').evaluate().first;
+      service.setSelection(elementA, 'my-group');
       late String pubRootTest;
       if (widgetTracked) {
         final Map<String, Object?> jsonObject = json.decode(
@@ -1047,6 +1107,8 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
       activeDevToolsServerAddress = 'http://127.0.0.1:9100';
       connectedVmServiceUri = 'http://127.0.0.1:55269/798ay5al_FM=/';
 
+      setupDefaultPubRootDirectory(service);
+
       await tester.pumpWidget(
         Directionality(
           textDirection: TextDirection.ltr,
@@ -1079,6 +1141,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
     testWidgets('test transformDebugCreator will not add DevToolsDeepLinkProperty for non-overflow errors', (WidgetTester tester) async {
       activeDevToolsServerAddress = 'http://127.0.0.1:9100';
       connectedVmServiceUri = 'http://127.0.0.1:55269/798ay5al_FM=/';
+      setupDefaultPubRootDirectory(service);
 
       await tester.pumpWidget(
         Directionality(
@@ -1110,6 +1173,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
     testWidgets('test transformDebugCreator will not add DevToolsDeepLinkProperty if devtoolsServerAddress is unavailable', (WidgetTester tester) async {
       activeDevToolsServerAddress = null;
       connectedVmServiceUri = 'http://127.0.0.1:55269/798ay5al_FM=/';
+      setupDefaultPubRootDirectory(service);
 
       await tester.pumpWidget(
         Directionality(
@@ -2782,6 +2846,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
           ),
         ),
       );
+      service.setSelection(find.text('Hello, World!').evaluate().first, 'my-group');
 
       // Figure out the pubRootDirectory
       final Map<String, Object?> jsonObject = (await service.testExtension(
@@ -2855,11 +2920,14 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
               final Map<String, Object> additionalJson = <String, Object>{};
               final Object? value = node.value;
               if (value is Element) {
+                final RenderObject? renderObject = value.renderObject;
+                if (renderObject != null) {
                   additionalJson['renderObject'] =
-                  value.renderObject!.toDiagnosticsNode().toJsonMap(
+                      renderObject.toDiagnosticsNode().toJsonMap(
                         delegate.copyWith(subtreeDepth: 0),
                       );
                 }
+              }
               additionalJson['callbackExecuted'] = true;
               return additionalJson;
             },
@@ -2892,6 +2960,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
     });
 
     testWidgets('debugIsLocalCreationLocation test', (WidgetTester tester) async {
+      setupDefaultPubRootDirectory(service);
 
       final GlobalKey key = GlobalKey();
 
@@ -2953,6 +3022,23 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
       );
     });
   }
+
+  static void setupDefaultPubRootDirectory(TestWidgetInspectorService service) {
+    final Map<String, Object?> jsonObject = const SizedBox().toDiagnosticsNode().toJsonMap(InspectorSerializationDelegate(service: service));
+    final Map<String, Object?> creationLocation = jsonObject['creationLocation']! as Map<String, Object?>;
+    expect(creationLocation, isNotNull);
+    final String file = creationLocation['file']! as String;
+    expect(file, endsWith('widget_inspector_test.dart'));
+    final List<String> segments = Uri
+        .parse(file)
+        .pathSegments;
+    final String pubRootTest = '/' +
+        segments.take(segments.length - 2).join('/');
+
+    // Strip a couple subdirectories away to generate a plausible pub root
+    // directory.
+    service.setPubRootDirectories(<String>[pubRootTest]);
+  }
 }
 
 void addToKnownLocationsMap({

packages/flutter/test/widgets/widget_inspector_test_utils.dart

@@ -62,4 +62,11 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
       binding.buildOwner!.reassemble(binding.renderViewElement!);
     }
   }
+
+  @override
+  void resetAllState() {
+    super.resetAllState();
+    eventsDispatched.clear();
+    rebuildCount = 0;
+  }
 }