Add support for reverse to ReorderableListView (#26203)

examples/flutter_gallery/lib/demo/material/reorderable_list_demo.dart

@@ -41,6 +41,7 @@ class _ListDemoState extends State<ReorderableListDemo> {
 
   PersistentBottomSheetController<void> _bottomSheet;
   _ReorderableListType _itemType = _ReorderableListType.threeLine;
+  bool _reverse = false;
   bool _reverseSort = false;
   final List<_ListItem> _items = <String>[
     'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N',
@@ -51,7 +52,21 @@ class _ListDemoState extends State<ReorderableListDemo> {
       _itemType = type;
     });
     // Rebuild the bottom sheet to reflect the selected list view.
-    _bottomSheet?.setState(() { });
+    _bottomSheet?.setState(() {
+      // Trigger a rebuild.
+    });
+    // Close the bottom sheet to give the user a clear view of the list.
+    _bottomSheet?.close();
+  }
+
+  void changeReverse(bool newValue) {
+    setState(() {
+      _reverse = newValue;
+    });
+    // Rebuild the bottom sheet to reflect the selected list view.
+    _bottomSheet?.setState(() {
+      // Trigger a rebuild.
+    });
     // Close the bottom sheet to give the user a clear view of the list.
     _bottomSheet?.close();
   }
@@ -67,6 +82,12 @@ class _ListDemoState extends State<ReorderableListDemo> {
             shrinkWrap: true,
             primary: false,
             children: <Widget>[
+              CheckboxListTile(
+                dense: true,
+                title: const Text('Reverse'),
+                value: _reverse,
+                onChanged: changeReverse,
+              ),
               RadioListTile<_ReorderableListType>(
                 dense: true,
                 title: const Text('Horizontal Avatars'),
@@ -189,6 +210,7 @@ class _ListDemoState extends State<ReorderableListDemo> {
                   child: Text('Header of the list', style: Theme.of(context).textTheme.headline))
               : null,
           onReorder: _onReorder,
+          reverse: _reverse,
           scrollDirection: _itemType == _ReorderableListType.horizontalAvatar ? Axis.horizontal : Axis.vertical,
           padding: const EdgeInsets.symmetric(vertical: 8.0),
           children: _items.map<Widget>(buildListTile).toList(),

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

@@ -59,6 +59,7 @@ class ReorderableListView extends StatefulWidget {
     @required this.onReorder,
     this.scrollDirection = Axis.vertical,
     this.padding,
+    this.reverse = false,
   }): assert(scrollDirection != null),
       assert(onReorder != null),
       assert(children != null),
@@ -83,6 +84,20 @@ class ReorderableListView extends StatefulWidget {
   /// The amount of space by which to inset the [children].
   final EdgeInsets padding;
 
+  /// Whether the scroll view scrolls in the reading direction.
+  ///
+  /// For example, if the reading direction is left-to-right and
+  /// [scrollDirection] is [Axis.horizontal], then the scroll view scrolls from
+  /// left to right when [reverse] is false and from right to left when
+  /// [reverse] is true.
+  ///
+  /// Similarly, if [scrollDirection] is [Axis.vertical], then the scroll view
+  /// scrolls from top to bottom when [reverse] is false and from bottom to top
+  /// when [reverse] is true.
+  ///
+  /// Defaults to false.
+  final bool reverse;
+
   /// Called when a list child is dropped into a new position to shuffle the
   /// underlying list.
   ///
@@ -122,6 +137,7 @@ class _ReorderableListViewState extends State<ReorderableListView> {
           scrollDirection: widget.scrollDirection,
           onReorder: widget.onReorder,
           padding: widget.padding,
+          reverse: widget.reverse,
         );
       },
     );
@@ -146,6 +162,7 @@ class _ReorderableListContent extends StatefulWidget {
     @required this.scrollDirection,
     @required this.padding,
     @required this.onReorder,
+    @required this.reverse,
   });
 
   final Widget header;
@@ -153,6 +170,7 @@ class _ReorderableListContent extends StatefulWidget {
   final Axis scrollDirection;
   final EdgeInsets padding;
   final ReorderCallback onReorder;
+  final bool reverse;
 
   @override
   _ReorderableListContentState createState() => _ReorderableListContentState();
@@ -544,16 +562,25 @@ class _ReorderableListContentState extends State<_ReorderableListContent> with T
             );
             break;
         }
-        wrappedChildren.add(_wrap(
+        if (widget.reverse) {
-          finalDropArea,
+          wrappedChildren.insert(0, _wrap(
-          widget.children.length,
+            finalDropArea,
-          constraints),
+            widget.children.length,
-        );
+            constraints),
+          );
+        } else {
+          wrappedChildren.add(_wrap(
+            finalDropArea,
+            widget.children.length,
+            constraints),
+          );
+        }
         return SingleChildScrollView(
           scrollDirection: widget.scrollDirection,
           child: _buildContainerForScrollDirection(children: wrappedChildren),
           padding: widget.padding,
           controller: _scrollController,
+          reverse: widget.reverse,
         );
     });
   }

packages/flutter/test/material/reorderable_list_test.dart

@@ -900,6 +900,30 @@ void main() {
 
     });
 
+    testWidgets('ReorderableListView can be reversed', (WidgetTester tester) async {
+      final Widget reorderableListView = ReorderableListView(
+        children: const <Widget>[
+          SizedBox(
+            key: Key('A'),
+            child: Text('A'),
+          ),
+          SizedBox(
+            key: Key('B'),
+            child: Text('B'),
+          ),
+          SizedBox(
+            key: Key('C'),
+            child: Text('C'),
+          )
+        ],
+        reverse: true,
+        onReorder: (int oldIndex, int newIndex) {},
+      );
+      await tester.pumpWidget(MaterialApp(
+        home: reorderableListView,
+      ));
+      expect(tester.getCenter(find.text('A')), greaterThan(tester.getCenter(find.text('B'))));
+    });
     // TODO(djshuckerow): figure out how to write a test for scrolling the list.
   });
 }