Enable ChangeNotifier clients to dispatch event of object creation in constructor. (#133060)

packages/flutter/lib/src/foundation/change_notifier.dart

@@ -207,6 +207,39 @@ mixin class ChangeNotifier implements Listenable {
   @protected
   bool get hasListeners => _count > 0;
 
+  /// Dispatches event of object creation to [MemoryAllocations.instance].
+  ///
+  /// If the event was already dispatched or [kFlutterMemoryAllocationsEnabled]
+  /// is false, the method is noop.
+  ///
+  /// Tools like leak_tracker use the event of object creation to help
+  /// developers identify the owner of the object, for troubleshooting purposes,
+  /// by taking stack trace at the moment of the event.
+  ///
+  /// But, as [ChangeNotifier] is mixin, it does not have its own constructor. So, it
+  /// communicates object creation in first `addListener`, that results
+  /// in the stack trace pointing to `addListener`, not to constructor.
+  ///
+  /// To make debugging easier, invoke [ChangeNotifier.maybeDispatchObjectCreation]
+  /// in constructor of the class. It will help
+  /// to identify the owner.
+  ///
+  /// Make sure to invoke it with condition `if (kFlutterMemoryAllocationsEnabled) ...`
+  /// so that the method is tree-shaken away when the flag is false.
+  @protected
+  void maybeDispatchObjectCreation() {
+    // Tree shaker does not include this method and the class MemoryAllocations
+    // if kFlutterMemoryAllocationsEnabled is false.
+    if (kFlutterMemoryAllocationsEnabled && !_creationDispatched) {
+      MemoryAllocations.instance.dispatchObjectCreated(
+        library: _flutterFoundationLibrary,
+        className: '$ChangeNotifier',
+        object: this,
+      );
+      _creationDispatched = true;
+    }
+  }
+
   /// Register a closure to be called when the object changes.
   ///
   /// If the given closure is already registered, an additional instance is
@@ -236,14 +269,11 @@ mixin class ChangeNotifier implements Listenable {
   @override
   void addListener(VoidCallback listener) {
     assert(ChangeNotifier.debugAssertNotDisposed(this));
-    if (kFlutterMemoryAllocationsEnabled && !_creationDispatched) {
+
-      MemoryAllocations.instance.dispatchObjectCreated(
+    if (kFlutterMemoryAllocationsEnabled) {
-        library: _flutterFoundationLibrary,
+      maybeDispatchObjectCreation();
-        className: '$ChangeNotifier',
-        object: this,
-      );
-      _creationDispatched = true;
     }
+
     if (_count == _listeners.length) {
       if (_count == 0) {
         _listeners = List<VoidCallback?>.filled(1, null);
@@ -505,13 +535,8 @@ class ValueNotifier<T> extends ChangeNotifier implements ValueListenable<T> {
   /// Creates a [ChangeNotifier] that wraps this value.
   ValueNotifier(this._value) {
     if (kFlutterMemoryAllocationsEnabled) {
-      MemoryAllocations.instance.dispatchObjectCreated(
+      maybeDispatchObjectCreation();
-        library: _flutterFoundationLibrary,
-        className: '$ValueNotifier',
-        object: this,
-      );
     }
-    _creationDispatched = true;
   }
 
   /// The current value stored in this notifier.

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

@@ -1196,6 +1196,13 @@ class ShortcutRegistryEntry {
 /// widgets that are not descendants of the registry can listen to it (e.g. in
 /// overlays).
 class ShortcutRegistry with ChangeNotifier {
+  /// Creates an instance of [ShortcutRegistry].
+  ShortcutRegistry() {
+    if (kFlutterMemoryAllocationsEnabled) {
+      maybeDispatchObjectCreation();
+    }
+  }
+
   bool _notificationScheduled = false;
   bool _disposed = false;
 

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

@@ -482,4 +482,8 @@ class _DefaultSnapshotPainter implements SnapshotPainter {
 
   @override
   bool shouldRepaint(covariant _DefaultSnapshotPainter oldPainter) => false;
+
+  @override
+  @protected
+  void maybeDispatchObjectCreation() { }
 }

packages/flutter/test/widgets/shortcuts_test.dart

@@ -1852,6 +1852,22 @@ void main() {
       }, throwsAssertionError);
       token.dispose();
     });
+
+    testWidgets('dispatches object creation in constructor', (WidgetTester tester) async {
+      final MemoryAllocations ma = MemoryAllocations.instance;
+      assert(!ma.hasListeners);
+      int eventCount = 0;
+      void listener(ObjectEvent event) => eventCount++;
+      ma.addListener(listener);
+
+      final ShortcutRegistry registry = ShortcutRegistry();
+
+      expect(eventCount, 1);
+
+      registry.dispose();
+      ma.removeListener(listener);
+      assert(!ma.hasListeners);
+    });
   });
 }