Fix(AnimatedScrollView): exclude outgoing items in removeAllItems (#176452)

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Fix: #176362

Issue provided reproducible code related to exception being thrown when
removing all items in an `AnimatedList` when some of the item were
already being removed but had a long animation duration.

In this PR: The `removeAllItems` method now correctly calculates the
range of items to remove, excluding those already undergoing a removal
animation. This prevents an assert from triggering when `removeAllItems`
is called while other items are still being removed.

<details open>
<summary> Video example after fix </summary>


https://github.com/user-attachments/assets/c7351702-b829-4e35-9978-b95f9f3ae8cd


</details>


<details open>
<summary> Updated reproducible code </summary>

```
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(debugShowCheckedModeBanner: false, home: AnimatedListExample());
  }
}

class AnimatedListExample extends StatefulWidget {
  const AnimatedListExample({super.key});

  @override
  State<AnimatedListExample> createState() => _AnimatedListExampleState();
}

class _AnimatedListExampleState extends State<AnimatedListExample> {
  final GlobalKey<AnimatedListState> _listKey = GlobalKey<AnimatedListState>();
  final List<int> _items = List<int>.generate(10, (int index) => index);

  void _removeItem(int index) {
    final int removedItem = _items[index];
    _listKey.currentState?.removeItem(
      index,
      (BuildContext context, Animation<double> animation) => SizeTransition(
        sizeFactor: animation,
        child: _buildItem(removedItem, animation, isRemoved: true),
      ),
      // 👇 Long delay so you can press "Remove All" while it’s still animating
      duration: const Duration(seconds: 10),
    );
    _items.removeAt(index);
  }

  void _addItem() {
    final int addingItem = _items.length;
    _listKey.currentState?.insertItem(addingItem, duration: const Duration(seconds: 3));
    _items.add(addingItem);
  }

  void _removeAll() {
    _listKey.currentState?.removeAllItems((BuildContext context, Animation<double> animation) {
      return SizeTransition(
        sizeFactor: animation,
        child: Container(
          color: Colors.red[100],
          child: const ListTile(title: Text('Removing...')),
        ),
      );
    }, duration: const Duration(seconds: 2));
    _items.clear();
  }

  Widget _buildItem(int item, Animation<double> animation, {bool isRemoved = false}) {
    return SizeTransition(
      sizeFactor: animation,
      child: Card(
        margin: const EdgeInsets.symmetric(vertical: 4, horizontal: 8),
        child: ListTile(
          title: Text('Item $item'),
          trailing: !isRemoved
              ? IconButton(
                  icon: const Icon(Icons.delete, color: Colors.red),
                  onPressed: () {
                    final int index = _items.indexOf(item);
                    if (index != -1) {
                      _removeItem(index);
                    }
                  },
                )
              : null,
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('AnimatedList Example'),
        actions: <Widget>[
          TextButton(onPressed: _addItem, child: const Text('Add')),
          TextButton(
            onPressed: _items.isNotEmpty ? _removeAll : null,
            child: const Text('Clear list'),
          ),
        ],
      ),
      body: AnimatedList(
        key: _listKey,
        initialItemCount: _items.length,
        itemBuilder: (BuildContext context, int index, Animation<double> animation) {
          return _buildItem(_items[index], animation);
        },
      ),
    );
  }
}
```

</details>


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---------

Co-authored-by: chunhtai <47866232+chunhtai@users.noreply.github.com>

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

@@ -920,7 +920,9 @@ const Duration _kDuration = Duration(milliseconds: 300);
 // Incoming and outgoing animated items.
 class _ActiveItem implements Comparable<_ActiveItem> {
   _ActiveItem.incoming(this.controller, this.itemIndex) : removedItemBuilder = null;
+
   _ActiveItem.outgoing(this.controller, this.itemIndex, this.removedItemBuilder);
+
   _ActiveItem.index(this.itemIndex) : controller = null, removedItemBuilder = null;
 
   final AnimationController? controller;
@@ -1436,10 +1438,14 @@ abstract class _SliverAnimatedMultiBoxAdaptorState<T extends _SliverAnimatedMult
   /// items will still appear for `duration` and during that time
   /// `builder` must construct its widget as needed.
   ///
-  /// This method's semantics are the same as Dart's [List.clear] method: it
-  /// removes all the items in the list.
+  /// This method removes all items from the list. Items that are in the
+  /// process of being inserted will also be removed. Items that are already in
+  /// the process of being removed will be excluded.
   void removeAllItems(AnimatedRemovedItemBuilder builder, {Duration duration = _kDuration}) {
-    for (int i = _itemsCount - 1; i >= 0; i--) {
+    assert(_itemsCount >= 0);
+    assert(_itemsCount - _outgoingItems.length >= 0);
+    final int visibleItemCount = _itemsCount - _outgoingItems.length;
+    for (int i = visibleItemCount - 1; i >= 0; i--) {
       removeItem(i, builder, duration: duration);
     }
   }

packages/flutter/test/widgets/animated_list_test.dart

@@ -107,6 +107,73 @@ void main() {
     expect(find.text('removing item'), findsNothing);
   });
 
+  testWidgets('AnimatedList should safely execute removeAllItems during long removal of one item', (
+    WidgetTester tester,
+  ) async {
+    Widget builder(BuildContext context, int index, Animation<double> animation) {
+      return SizedBox(height: 100.0, child: Center(child: Text('item $index')));
+    }
+
+    final GlobalKey<AnimatedListState> listKey = GlobalKey<AnimatedListState>();
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: AnimatedList(key: listKey, initialItemCount: 2, itemBuilder: builder),
+      ),
+    );
+
+    // Check that one AnimatedList with 2 items (item 0, item 1).
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is SliverAnimatedList &&
+            widget.initialItemCount == 2 &&
+            widget.itemBuilder == builder;
+      }),
+      findsOneWidget,
+    );
+    expect(find.byType(Text), findsExactly(2));
+    expect(find.text('item 0'), findsOne);
+    expect(find.text('item 1'), findsOne);
+
+    // Insert 1 item and check state (item 0, item 1, item 2).
+    listKey.currentState!.insertItem(0, duration: const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 50));
+    expect(find.byType(Text), findsExactly(3));
+    expect(find.text('item 0'), findsOne);
+    expect(find.text('item 1'), findsOne);
+    expect(find.text('item 2'), findsOne);
+
+    // Removing item 2 and check state (item 0, item 1, removing item 2).
+    listKey.currentState!.removeItem(2, (BuildContext context, Animation<double> animation) {
+      return const SizedBox(height: 100.0, child: Center(child: Text('removing item 2')));
+    }, duration: const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 50));
+    expect(find.byType(Text), findsExactly(3));
+    expect(find.text('item 0'), findsOne);
+    expect(find.text('item 1'), findsOne);
+    expect(find.text('removing item 2'), findsOne);
+    expect(find.text('item 2'), findsNothing);
+
+    // Call removeAllItems and check state (removing all items, removing all items, removing item 2).
+    listKey.currentState!.removeAllItems((BuildContext context, Animation<double> animation) {
+      return const SizedBox(height: 100.0, child: Center(child: Text('removing all items')));
+    }, duration: const Duration(milliseconds: 100));
+    await tester.pump(const Duration(milliseconds: 50));
+    expect(find.byType(Text), findsExactly(3));
+    expect(find.text('removing all items'), findsExactly(2));
+    expect(find.text('removing item 2'), findsWidgets);
+    expect(find.text('item 0'), findsNothing);
+    expect(find.text('item 1'), findsNothing);
+    expect(find.text('item 2'), findsNothing);
+
+    // After animation is done completed, list should be empty.
+    await tester.pumpAndSettle();
+    expect(find.byType(Text), findsNothing);
+    expect(find.text('removing one item'), findsNothing);
+    expect(find.text('removing all items'), findsNothing);
+  });
+
   group('SliverAnimatedList', () {
     testWidgets('initialItemCount', (WidgetTester tester) async {
       final Map<int, Animation<double>> animations = <int, Animation<double>>{};