Fix TabBarView desynchronized after animation interruption (#132748)

Author: bleroux

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

@@ -63,6 +63,8 @@ abstract class ScrollActivity {
   ScrollActivityDelegate get delegate => _delegate;
   ScrollActivityDelegate _delegate;
 
+  bool _isDisposed = false;
+
   /// Updates the activity's link to the [ScrollActivityDelegate].
   ///
   /// This should only be called when an activity is being moved from a defunct
@@ -134,7 +136,9 @@ abstract class ScrollActivity {
 
   /// Called when the scroll view stops performing this activity.
   @mustCallSuper
-  void dispose() { }
+  void dispose() {
+    _isDisposed = true;
+  }
 
   @override
   String toString() => describeIdentity(this);
@@ -535,7 +539,7 @@ class BallisticScrollActivity extends ScrollActivity {
     )
       ..addListener(_tick)
       ..animateWith(simulation)
-       .whenComplete(_end); // won't trigger if we dispose _controller first
+       .whenComplete(_end); // won't trigger if we dispose _controller before it completes.
   }
 
   late AnimationController _controller;
@@ -569,7 +573,11 @@ class BallisticScrollActivity extends ScrollActivity {
   }
 
   void _end() {
-    delegate.goBallistic(0.0);
+    // Check if the activity was disposed before going ballistic because _end might be called
+    // if _controller is disposed just after completion.
+    if (!_isDisposed) {
+      delegate.goBallistic(0.0);
+    }
   }
 
   @override
@@ -628,7 +636,7 @@ class DrivenScrollActivity extends ScrollActivity {
     )
       ..addListener(_tick)
       ..animateTo(to, duration: duration, curve: curve)
-       .whenComplete(_end); // won't trigger if we dispose _controller first
+       .whenComplete(_end); // won't trigger if we dispose _controller before it completes.
   }
 
   late final Completer<void> _completer;
@@ -648,7 +656,11 @@ class DrivenScrollActivity extends ScrollActivity {
   }
 
   void _end() {
-    delegate.goBallistic(velocity);
+    // Check if the activity was disposed before going ballistic because _end might be called
+    // if _controller is disposed just after completion.
+    if (!_isDisposed) {
+      delegate.goBallistic(velocity);
+    }
   }
 
   @override

packages/flutter/test/material/tabs_test.dart

@@ -2224,6 +2224,57 @@ void main() {
     expect(tabController.index, 0);
   });
 
+  testWidgets('On going TabBarView animation can be interrupted by a new animation', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/132293.
+
+    final List<String> tabs = <String>['A', 'B', 'C'];
+    final TabController tabController = TabController(
+      vsync: const TestVSync(),
+      length: tabs.length,
+    );
+    await tester.pumpWidget(boilerplate(
+      child: Column(
+        children: <Widget>[
+          TabBar(
+            tabs: tabs.map<Widget>((String tab) => Tab(text: tab)).toList(),
+            controller: tabController,
+          ),
+          SizedBox(
+            width: 400.0,
+            height: 400.0,
+            child: TabBarView(
+              controller: tabController,
+              children: const <Widget>[
+                Center(child: Text('0')),
+                Center(child: Text('1')),
+                Center(child: Text('2')),
+              ],
+            ),
+          ),
+        ],
+      ),
+    ));
+
+    // First page is visible.
+    expect(tabController.index, 0);
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('1'), findsNothing);
+
+    // Animate to the second page.
+    tabController.animateTo(1);
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 300));
+
+    // Animate back to the first page before the previous animation ends.
+    tabController.animateTo(0);
+    await tester.pumpAndSettle();
+
+    // First page should be visible.
+    expect(tabController.index, 0);
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('1'), findsNothing);
+  });
+
   testWidgets('Can switch to non-neighboring tab in nested TabBarView without crashing', (WidgetTester tester) async {
     // This is a regression test for https://github.com/flutter/flutter/issues/18756
     final TabController mainTabController = _tabController(length: 4, vsync: const TestVSync());