Adding itemExtent to ReorderableList and ReorderableListView (#81372)

packages/flutter/lib/src/material/reorderable_list.dart

@@ -76,6 +76,7 @@ class ReorderableListView extends StatefulWidget {
     Key? key,
     required List<Widget> children,
     required this.onReorder,
+    this.itemExtent,
     this.proxyDecorator,
     this.buildDefaultDragHandles = true,
     this.padding,
@@ -168,6 +169,7 @@ class ReorderableListView extends StatefulWidget {
     required this.itemBuilder,
     required this.itemCount,
     required this.onReorder,
+    this.itemExtent,
     this.proxyDecorator,
     this.buildDefaultDragHandles = true,
     this.padding,
@@ -323,6 +325,9 @@ class ReorderableListView extends StatefulWidget {
   /// Defaults to [Clip.hardEdge].
   final Clip clipBehavior;
 
+  /// {@macro flutter.widgets.list_view.itemExtent}
+  final double? itemExtent;
+
   @override
   _ReorderableListViewState createState() => _ReorderableListViewState();
 }
@@ -545,6 +550,7 @@ class _ReorderableListViewState extends State<ReorderableListView> {
           padding: listPadding,
           sliver: SliverReorderableList(
             itemBuilder: _itemBuilder,
+            itemExtent: widget.itemExtent,
             itemCount: widget.itemCount,
             onReorder: widget.onReorder,
             proxyDecorator: widget.proxyDecorator ?? _proxyDecorator,

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

@@ -112,6 +112,7 @@ class ReorderableList extends StatefulWidget {
     required this.itemBuilder,
     required this.itemCount,
     required this.onReorder,
+    this.itemExtent,
     this.proxyDecorator,
     this.padding,
     this.scrollDirection = Axis.vertical,
@@ -211,6 +212,9 @@ class ReorderableList extends StatefulWidget {
   /// Defaults to [Clip.hardEdge].
   final Clip clipBehavior;
 
+  /// {@macro flutter.widgets.list_view.itemExtent}
+  final double? itemExtent;
+
   /// The state from the closest instance of this class that encloses the given
   /// context.
   ///
@@ -337,6 +341,7 @@ class ReorderableListState extends State<ReorderableList> {
           padding: widget.padding ?? EdgeInsets.zero,
           sliver: SliverReorderableList(
             key: _sliverReorderableListKey,
+            itemExtent: widget.itemExtent,
             itemBuilder: widget.itemBuilder,
             itemCount: widget.itemCount,
             onReorder: widget.onReorder,
@@ -380,6 +385,7 @@ class SliverReorderableList extends StatefulWidget {
     required this.itemBuilder,
     required this.itemCount,
     required this.onReorder,
+    this.itemExtent,
     this.proxyDecorator,
   }) : assert(itemCount >= 0),
        super(key: key);
@@ -396,6 +402,9 @@ class SliverReorderableList extends StatefulWidget {
   /// {@macro flutter.widgets.reorderable_list.proxyDecorator}
   final ReorderItemProxyDecorator? proxyDecorator;
 
+  /// {@macro flutter.widgets.list_view.itemExtent}
+  final double? itemExtent;
+
   @override
   SliverReorderableListState createState() => SliverReorderableListState();
 
@@ -825,15 +834,19 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasOverlay(context));
-    return SliverList(
+    final SliverChildBuilderDelegate childrenDelegate = SliverChildBuilderDelegate(
+      _itemBuilder,
       // When dragging, the dragged item is still in the list but has been replaced
       // by a zero height SizedBox, so that the gap can move around. To make the
       // list extent stable we add a dummy entry to the end.
-      delegate: SliverChildBuilderDelegate(
+      childCount: widget.itemCount + (_dragInfo != null ? 1 : 0),
-        _itemBuilder,
-        childCount: widget.itemCount + (_dragInfo != null ? 1 : 0),
-      ),
     );
+    return widget.itemExtent != null
+      ? SliverFixedExtentList(
+          itemExtent: widget.itemExtent!,
+          delegate: childrenDelegate,
+        )
+      : SliverList(delegate: childrenDelegate);
   }
 }
 

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

@@ -1476,6 +1476,7 @@ class ListView extends BoxScrollView {
          clipBehavior: clipBehavior,
        );
 
+  /// {@template flutter.widgets.list_view.itemExtent}
   /// If non-null, forces the children to have the given extent in the scroll
   /// direction.
   ///
@@ -1489,6 +1490,7 @@ class ListView extends BoxScrollView {
   ///  * [SliverFixedExtentList], the sliver used internally when this property
   ///    is provided. It constrains its box children to have a specific given
   ///    extent along the main axis.
+  /// {@endtemplate}
   ///  * The [prototypeItem] property, which allows forcing the children's
   ///    extent to be the same as the given widget.
   final double? itemExtent;

packages/flutter/test/material/reorderable_list_test.dart

@@ -1505,6 +1505,51 @@ void main() {
     expect(exception.toString(), contains('No Overlay widget found'));
     expect(exception.toString(), contains('ReorderableListView widgets require an Overlay widget ancestor'));
   });
+
+  testWidgets('if itemExtent is non-null, children have same extent in the scroll direction', (WidgetTester tester) async {
+    final List<int> numbers = <int>[0,1,2];
+
+    await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: StatefulBuilder(
+              builder: (BuildContext context, StateSetter setState) {
+                return ReorderableListView.builder(
+                  itemBuilder: (BuildContext context, int index) {
+                    return SizedBox(
+                        key: ValueKey<int>(numbers[index]),
+                        // children with different heights
+                        height: 20 + numbers[index] * 10,
+                        child: ReorderableDragStartListener(
+                          index: index,
+                          child: Text(numbers[index].toString()),
+                        )
+                    );
+                  },
+                  itemCount: numbers.length,
+                  itemExtent: 30,
+                  onReorder: (int fromIndex, int toIndex) {
+                    if (fromIndex < toIndex) {
+                      toIndex--;
+                    }
+                    final int value = numbers.removeAt(fromIndex);
+                    numbers.insert(toIndex, value);
+                  },
+                );
+              },
+            ),
+          ),
+        )
+    );
+
+    final double item0Height = tester.getSize(find.text('0').hitTestable()).height;
+    final double item1Height = tester.getSize(find.text('1').hitTestable()).height;
+    final double item2Height = tester.getSize(find.text('2').hitTestable()).height;
+
+    expect(item0Height, 30.0);
+    expect(item1Height, 30.0);
+    expect(item2Height, 30.0);
+  });
 }
 
 Future<void> longPressDrag(WidgetTester tester, Offset start, Offset end) async {

packages/flutter/test/widgets/reorderable_list_test.dart

@@ -267,6 +267,51 @@ void main() {
     await tester.pumpAndSettle();
     expect(getItemFadeTransition(), findsNothing);
   });
+
+  testWidgets('if itemExtent is non-null, children have same extent in the scroll direction', (WidgetTester tester) async {
+    final List<int> numbers = <int>[0,1,2];
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: StatefulBuilder(
+            builder: (BuildContext context, StateSetter setState) {
+              return ReorderableList(
+                itemBuilder: (BuildContext context, int index) {
+                  return SizedBox(
+                    key: ValueKey<int>(numbers[index]),
+                    // children with different heights
+                    height: 20 + numbers[index] * 10,
+                    child: ReorderableDragStartListener(
+                      index: index,
+                      child: Text(numbers[index].toString()),
+                    )
+                  );
+                },
+                itemCount: numbers.length,
+                itemExtent: 30,
+                onReorder: (int fromIndex, int toIndex) {
+                  if (fromIndex < toIndex) {
+                    toIndex--;
+                  }
+                  final int value = numbers.removeAt(fromIndex);
+                  numbers.insert(toIndex, value);
+                },
+              );
+            },
+          ),
+        ),
+      )
+    );
+
+    final double item0Height = tester.getSize(find.text('0').hitTestable()).height;
+    final double item1Height = tester.getSize(find.text('1').hitTestable()).height;
+    final double item2Height = tester.getSize(find.text('2').hitTestable()).height;
+
+    expect(item0Height, 30.0);
+    expect(item1Height, 30.0);
+    expect(item2Height, 30.0);
+  });
 }
 
 class TestList extends StatefulWidget {

packages/flutter/test/widgets/scroll_view_test.dart

@@ -1334,4 +1334,42 @@ void main() {
       equals(const Rect.fromLTRB(0.0, 0.0, 800.0, 50.0)),
     );
   });
+
+  testWidgets('if itemExtent is non-null, children have same extent in the scroll direction', (WidgetTester tester) async {
+    final List<int> numbers = <int>[0,1,2];
+
+    await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: StatefulBuilder(
+              builder: (BuildContext context, StateSetter setState) {
+                return ListView.builder(
+                  itemBuilder: (BuildContext context, int index) {
+                    return SizedBox(
+                        key: ValueKey<int>(numbers[index]),
+                        // children with different heights
+                        height: 20 + numbers[index] * 10,
+                        child: ReorderableDragStartListener(
+                          index: index,
+                          child: Text(numbers[index].toString()),
+                        )
+                    );
+                  },
+                  itemCount: numbers.length,
+                  itemExtent: 30,
+                );
+              },
+            ),
+          ),
+        )
+    );
+
+    final double item0Height = tester.getSize(find.text('0').hitTestable()).height;
+    final double item1Height = tester.getSize(find.text('1').hitTestable()).height;
+    final double item2Height = tester.getSize(find.text('2').hitTestable()).height;
+
+    expect(item0Height, 30.0);
+    expect(item1Height, 30.0);
+    expect(item2Height, 30.0);
+  });
 }