Fix memory leak in ListWheelScrollView (#134732)

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

@@ -7,7 +7,6 @@ import 'dart:math' as math;
 
 import 'package:flutter/physics.dart';
 import 'package:flutter/rendering.dart';
-import 'package:flutter/scheduler.dart';
 
 import 'basic.dart';
 import 'framework.dart';
@@ -709,12 +708,14 @@ class ListWheelScrollView extends StatefulWidget {
 
 class _ListWheelScrollViewState extends State<ListWheelScrollView> {
   int _lastReportedItemIndex = 0;
-  ScrollController? scrollController;
+  ScrollController? _backupController;
+
+  ScrollController get _effectiveController =>
+    widget.controller ?? (_backupController ??= FixedExtentScrollController());
 
   @override
   void initState() {
     super.initState();
-    scrollController = widget.controller ?? FixedExtentScrollController();
     if (widget.controller is FixedExtentScrollController) {
       final FixedExtentScrollController controller = widget.controller! as FixedExtentScrollController;
       _lastReportedItemIndex = controller.initialItem;
@@ -722,15 +723,9 @@ class _ListWheelScrollViewState extends State<ListWheelScrollView> {
   }
 
   @override
-  void didUpdateWidget(ListWheelScrollView oldWidget) {
-    super.didUpdateWidget(oldWidget);
-    if (widget.controller != null && widget.controller != scrollController) {
-      final ScrollController? oldScrollController = scrollController;
-      SchedulerBinding.instance.addPostFrameCallback((_) {
-        oldScrollController!.dispose();
-      });
-      scrollController = widget.controller;
-    }
+  void dispose() {
+    _backupController?.dispose();
+    super.dispose();
   }
 
   bool _handleScrollNotification(ScrollNotification notification) {
@@ -754,7 +749,7 @@ class _ListWheelScrollViewState extends State<ListWheelScrollView> {
     return NotificationListener<ScrollNotification>(
       onNotification: _handleScrollNotification,
       child: _FixedExtentScrollable(
-        controller: scrollController,
+        controller: _effectiveController,
         physics: widget.physics,
         itemExtent: widget.itemExtent,
         restorationId: widget.restorationId,

packages/flutter/test/widgets/list_wheel_scroll_view_test.dart

@@ -11,6 +11,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 
 import '../rendering/rendering_tester.dart' show TestCallbackPainter, TestClipPaintingContext;
 
@@ -1285,7 +1286,7 @@ void main() {
       expect(controller.selectedItem, 10);
     });
 
-    testWidgets('controller hot swappable', (WidgetTester tester) async {
+    testWidgetsWithLeakTracking('controller hot swappable', (WidgetTester tester) async {
       await tester.pumpWidget(
         Directionality(
           textDirection: TextDirection.ltr,
@@ -1302,14 +1303,16 @@ void main() {
       await tester.drag(find.byType(ListWheelScrollView), const Offset(0.0, -500.0));
       await tester.pump();
 
-      final FixedExtentScrollController newController =
+      final FixedExtentScrollController controller1 =
           FixedExtentScrollController(initialItem: 30);
+      addTearDown(controller1.dispose);
 
+      // Attaching first controller.
       await tester.pumpWidget(
         Directionality(
           textDirection: TextDirection.ltr,
           child: ListWheelScrollView(
-            controller: newController,
+            controller: controller1,
             itemExtent: 100.0,
             children: List<Widget>.generate(100, (int index) {
               return const Placeholder();
@@ -1320,17 +1323,94 @@ void main() {
 
       // initialItem doesn't do anything since the scroll position was already
       // created.
-      expect(newController.selectedItem, 5);
+      expect(controller1.selectedItem, 5);
 
-      newController.jumpToItem(50);
-      expect(newController.selectedItem, 50);
-      expect(newController.position.pixels, 5000.0);
+      controller1.jumpToItem(50);
+      expect(controller1.selectedItem, 50);
+      expect(controller1.position.pixels, 5000.0);
+
+      final FixedExtentScrollController controller2 =
+          FixedExtentScrollController(initialItem: 33);
+      addTearDown(controller2.dispose);
+
+      // Attaching the second controller.
+      await tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.ltr,
+          child: ListWheelScrollView(
+            controller: controller2,
+            itemExtent: 100.0,
+            children: List<Widget>.generate(100, (int index) {
+              return const Placeholder();
+            }),
+          ),
+        ),
+      );
+
+      // First controller is now detached.
+      expect(controller1.hasClients, isFalse);
+      // initialItem doesn't do anything since the scroll position was already
+      // created.
+      expect(controller2.selectedItem, 50);
+
+      controller2.jumpToItem(40);
+      expect(controller2.selectedItem, 40);
+      expect(controller2.position.pixels, 4000.0);
+
+      // Now, use the internal controller.
+      await tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.ltr,
+          child: ListWheelScrollView(
+            itemExtent: 100.0,
+            children: List<Widget>.generate(100, (int index) {
+              return const Placeholder();
+            }),
+          ),
+        ),
+      );
+
+      // Both controllers are now detached.
+      expect(controller1.hasClients, isFalse);
+      expect(controller2.hasClients, isFalse);
+    });
+
+    testWidgetsWithLeakTracking('controller can be reused', (WidgetTester tester) async {
+      final FixedExtentScrollController controller =
+          FixedExtentScrollController(initialItem: 3);
+      addTearDown(controller.dispose);
+
+      await tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.ltr,
+          child: ListWheelScrollView(
+            controller: controller,
+            itemExtent: 100.0,
+            children: List<Widget>.generate(100, (int index) {
+              return const Placeholder();
+            }),
+          ),
+        ),
+      );
+
+      // selectedItem is equal to the initialItem.
+      expect(controller.selectedItem, 3);
+      expect(controller.position.pixels, 300.0);
+
+      controller.jumpToItem(10);
+      expect(controller.selectedItem, 10);
+      expect(controller.position.pixels, 1000.0);
+
+      await tester.pumpWidget(const Center());
+
+      // Controller is now detached.
+      expect(controller.hasClients, isFalse);
 
-      // Now remove the controller
       await tester.pumpWidget(
         Directionality(
           textDirection: TextDirection.ltr,
           child: ListWheelScrollView(
+            controller: controller,
             itemExtent: 100.0,
             children: List<Widget>.generate(100, (int index) {
               return const Placeholder();
@@ -1339,9 +1419,10 @@ void main() {
         ),
       );
 
-      // Internally, that same controller is still attached and still at the
-      // same place.
-      expect(newController.selectedItem, 50);
+      // Controller is now attached again.
+      expect(controller.hasClients, isTrue);
+      expect(controller.selectedItem, 3);
+      expect(controller.position.pixels, 300.0);
     });
   });