packages/flutter/lib/src/material/reorderable_list.dart - external/github.com/flutter/flutter - Git at Google

 // Copyright 2014 The Flutter Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 import 'dart:ui' show lerpDouble;

 import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';

 import 'debug.dart';
 import 'icons.dart';
 import 'material.dart';
 import 'material_localizations.dart';
 import 'theme.dart';

 /// A list whose items the user can interactively reorder by dragging.
 ///
 /// This class is appropriate for views with a small number of
 /// children because constructing the [List] requires doing work for every
 /// child that could possibly be displayed in the list view instead of just
 /// those children that are actually visible.
 ///
 /// All list items must have a key.
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=3fB1mxOsqJE}
 ///
 /// This sample shows by dragging the user can reorder the items of the list.
 /// The [onReorder] parameter is required and will be called when a child
 /// widget is dragged to a new position.
 ///
 /// {@tool dartpad --template=stateful_widget_scaffold}
 ///
 /// ```dart
 /// final List<int> _items = List<int>.generate(50, (int index) => index);
 ///
 /// Widget build(BuildContext context){
 ///   final ColorScheme colorScheme = Theme.of(context).colorScheme;
 ///   final oddItemColor = colorScheme.primary.withOpacity(0.05);
 ///   final evenItemColor = colorScheme.primary.withOpacity(0.15);
 ///
 ///   return ReorderableListView(
 ///     padding: const EdgeInsets.symmetric(horizontal: 40),
 ///     children: <Widget>[
 ///       for (int index = 0; index < _items.length; index++)
 ///         ListTile(
 ///           key: Key('$index'),
 ///           tileColor: _items[index].isOdd ? oddItemColor : evenItemColor,
 ///           title: Text('Item ${_items[index]}'),
 ///         ),
 ///     ],
 ///     onReorder: (int oldIndex, int newIndex) {
 ///       setState(() {
 ///         if (oldIndex < newIndex) {
 ///           newIndex -= 1;
 ///         }
 ///         final int item = _items.removeAt(oldIndex);
 ///         _items.insert(newIndex, item);
 ///       });
 ///     },
 ///   );
 /// }
 ///
 /// ```
 ///
 ///{@end-tool}
 class ReorderableListView extends StatefulWidget {
   /// Creates a reorderable list from a pre-built list of widgets.
   ///
   /// See also:
   ///
   ///   * [ReorderableListView.builder], which allows you to build a reorderable
   ///     list where the items are built as needed when scrolling the list.
   ReorderableListView({
     Key? key,
     required List<Widget> children,
     required this.onReorder,
     this.proxyDecorator,
     this.buildDefaultDragHandles = true,
     this.padding,
     this.header,
     this.scrollDirection = Axis.vertical,
     this.reverse = false,
     this.scrollController,
     this.primary,
     this.physics,
     this.shrinkWrap = false,
     this.anchor = 0.0,
     this.cacheExtent,
     this.dragStartBehavior = DragStartBehavior.start,
     this.keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
     this.restorationId,
     this.clipBehavior = Clip.hardEdge,
   }) : assert(scrollDirection != null),
        assert(onReorder != null),
        assert(children != null),
        assert(
          children.every((Widget w) => w.key != null),
          'All children of this widget must have a key.',
        ),
        assert(buildDefaultDragHandles != null),
        itemBuilder = ((BuildContext context, int index) => children[index]),
        itemCount = children.length,
        super(key: key);

   /// Creates a reorderable list from widget items that are created on demand.
   ///
   /// This constructor is appropriate for list views with a large number of
   /// children because the builder is called only for those children
   /// that are actually visible.
   ///
   /// The `itemBuilder` callback will be called only with indices greater than
   /// or equal to zero and less than `itemCount`.
   ///
   /// The `itemBuilder` should always return a non-null widget, and actually
   /// create the widget instances when called. Avoid using a builder that
   /// returns a previously-constructed widget; if the list view's children are
   /// created in advance, or all at once when the [ReorderableListView] itself
   /// is created, it is more efficient to use the [ReorderableListView]
   /// constructor. Even more efficient, however, is to create the instances
   /// on demand using this constructor's `itemBuilder` callback.
   ///
   /// See also:
   ///
   ///   * [ReorderableListView], which allows you to build a reorderable
   ///     list with all the items passed into the constructor.
   const ReorderableListView.builder({
     Key? key,
     required this.itemBuilder,
     required this.itemCount,
     required this.onReorder,
     this.proxyDecorator,
     this.buildDefaultDragHandles = true,
     this.padding,
     this.header,
     this.scrollDirection = Axis.vertical,
     this.reverse = false,
     this.scrollController,
     this.primary,
     this.physics,
     this.shrinkWrap = false,
     this.anchor = 0.0,
     this.cacheExtent,
     this.dragStartBehavior = DragStartBehavior.start,
     this.keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
     this.restorationId,
     this.clipBehavior = Clip.hardEdge,
   }) : assert(scrollDirection != null),
        assert(itemCount >= 0),
        assert(onReorder != null),
        assert(buildDefaultDragHandles != null),
        super(key: key);

   /// {@macro flutter.widgets.reorderable_list.itemBuilder}
   final IndexedWidgetBuilder itemBuilder;

   /// {@macro flutter.widgets.reorderable_list.itemCount}
   final int itemCount;

   /// {@macro flutter.widgets.reorderable_list.onReorder}
   final ReorderCallback onReorder;

   /// {@macro flutter.widgets.reorderable_list.proxyDecorator}
   final ReorderItemProxyDecorator? proxyDecorator;

   /// If true: on desktop platforms, a drag handle is stacked over the
   /// center of each item's trailing edge; on mobile platforms, a long
   /// press anywhere on the item starts a drag.
   ///
   /// The default desktop drag handle is just an [Icons.drag_handle]
   /// wrapped by a [ReorderableDragStartListener]. On mobile
   /// platforms, the entire item is wrapped with a
   /// [ReorderableDelayedDragStartListener].
   ///
   /// To change the appearance or the layout of the drag handles, make
   /// this parameter false and wrap each list item, or a widget within
   /// each list item, with [ReorderableDragStartListener] or
   /// [ReorderableDelayedDragStartListener], or a custom subclass
   /// of [ReorderableDragStartListener].
   ///
   /// The following sample specifies `buildDefaultDragHandles: false`, and
   /// uses a [Card] at the leading edge of each item for the item's drag handle.
   ///
   /// {@tool dartpad --template=stateful_widget_scaffold}
   ///
   /// ```dart
   /// final List<int> _items = List<int>.generate(50, (int index) => index);
   ///
   /// Widget build(BuildContext context){
   ///   final ColorScheme colorScheme = Theme.of(context).colorScheme;
   ///   final oddItemColor = colorScheme.primary.withOpacity(0.05);
   ///   final evenItemColor = colorScheme.primary.withOpacity(0.15);
   ///
   ///   return ReorderableListView(
   ///     buildDefaultDragHandles: false,
   ///     children: <Widget>[
   ///       for (int index = 0; index < _items.length; index++)
   ///         Container(
   ///           key: Key('$index'),
   ///           color: _items[index].isOdd ? oddItemColor : evenItemColor,
   ///           child: Row(
   ///             children: <Widget>[
   ///               Container(
   ///                 width: 64,
   ///                 height: 64,
   ///                 padding: const EdgeInsets.all(8),
   ///                 child: ReorderableDragStartListener(
   ///                   index: index,
   ///                   child: Card(
   ///                     color: colorScheme.primary,
   ///                     elevation: 2,
   ///                   ),
   ///                 ),
   ///               ),
   ///               Text('Item ${_items[index]}'),
   ///             ],
   ///           ),
   ///         ),
   ///     ],
   ///     onReorder: (oldIndex, newIndex) {
   ///       setState(() {
   ///         if (oldIndex < newIndex) {
   ///           newIndex -= 1;
   ///         }
   ///         final int item = _items.removeAt(oldIndex);
   ///         _items.insert(newIndex, item);
   ///       });
   ///     },
   ///   );
   /// }
   /// ```
   ///{@end-tool}
   final bool buildDefaultDragHandles;

   /// {@macro flutter.widgets.reorderable_list.padding}
   final EdgeInsets? padding;

   /// A non-reorderable header item to show before the items of the list.
   ///
   /// If null, no header will appear before the list.
   final Widget? header;

   /// {@macro flutter.widgets.scroll_view.scrollDirection}
   final Axis scrollDirection;

   /// {@macro flutter.widgets.scroll_view.reverse}
   final bool reverse;

   /// {@macro flutter.widgets.scroll_view.controller}
   final ScrollController? scrollController;

   /// {@macro flutter.widgets.scroll_view.primary}

   /// Defaults to true when [scrollDirection] is [Axis.vertical] and
   /// [scrollController] is null.
   final bool? primary;

   /// {@macro flutter.widgets.scroll_view.physics}
   final ScrollPhysics? physics;

   /// {@macro flutter.widgets.scroll_view.shrinkWrap}
   final bool shrinkWrap;

   /// {@macro flutter.widgets.scroll_view.anchor}
   final double anchor;

   /// {@macro flutter.rendering.RenderViewportBase.cacheExtent}
   final double? cacheExtent;

   /// {@macro flutter.widgets.scrollable.dragStartBehavior}
   final DragStartBehavior dragStartBehavior;

   /// {@macro flutter.widgets.scroll_view.keyboardDismissBehavior}
   ///
   /// The default is [ScrollViewKeyboardDismissBehavior.manual]
   final ScrollViewKeyboardDismissBehavior keyboardDismissBehavior;

   /// {@macro flutter.widgets.scrollable.restorationId}
   final String? restorationId;

   /// {@macro flutter.material.Material.clipBehavior}
   ///
   /// Defaults to [Clip.hardEdge].
   final Clip clipBehavior;

   @override
   _ReorderableListViewState createState() => _ReorderableListViewState();
 }

 // This top-level state manages an Overlay that contains the list and
 // also any items being dragged on top fo the list.
 //
 // The Overlay doesn't properly keep state by building new overlay entries,
 // and so we cache a single OverlayEntry for use as the list layer.
 // That overlay entry then builds a _ReorderableListContent which may
 // insert items being dragged into the Overlay above itself.
 class _ReorderableListViewState extends State<ReorderableListView> {
   // This entry contains the scrolling list itself.
   late OverlayEntry _listOverlayEntry;

   @override
   void initState() {
     super.initState();
     _listOverlayEntry = OverlayEntry(
       opaque: true,
       builder: (BuildContext context) {
         return _ReorderableListContent(
           itemBuilder: widget.itemBuilder,
           itemCount: widget.itemCount,
           onReorder: widget.onReorder,
           proxyDecorator: widget.proxyDecorator,
           buildDefaultDragHandles: widget.buildDefaultDragHandles,
           padding: widget.padding,
           header: widget.header,
           scrollDirection: widget.scrollDirection,
           reverse: widget.reverse,
           scrollController: widget.scrollController,
           primary: widget.primary,
           physics: widget.physics,
           shrinkWrap: widget.shrinkWrap,
           anchor: widget.anchor,
           cacheExtent: widget.cacheExtent,
           dragStartBehavior: widget.dragStartBehavior,
           keyboardDismissBehavior: widget.keyboardDismissBehavior,
           restorationId: widget.restorationId,
           clipBehavior: widget.clipBehavior,
         );
       },
     );
   }

   @override
   void didUpdateWidget(ReorderableListView oldWidget) {
     super.didUpdateWidget(oldWidget);
     // As this depends on pretty much everything, it
     // is ok to mark this as dirty unconditionally.
     _listOverlayEntry.markNeedsBuild();
   }

   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterialLocalizations(context));
     return Overlay(
       initialEntries: <OverlayEntry>[
         _listOverlayEntry
       ],
     );
   }
 }

 class _ReorderableListContent extends StatefulWidget {
   const _ReorderableListContent({
     required this.itemBuilder,
     required this.itemCount,
     required this.onReorder,
     required this.proxyDecorator,
     required this.buildDefaultDragHandles,
     required this.padding,
     required this.header,
     required this.scrollDirection,
     required this.reverse,
     required this.scrollController,
     required this.primary,
     required this.physics,
     required this.shrinkWrap,
     required this.anchor,
     required this.cacheExtent,
     required this.dragStartBehavior,
     required this.keyboardDismissBehavior,
     required this.restorationId,
     required this.clipBehavior,
   });

   final IndexedWidgetBuilder itemBuilder;
   final int itemCount;
   final ReorderCallback onReorder;
   final ReorderItemProxyDecorator? proxyDecorator;
   final bool buildDefaultDragHandles;
   final EdgeInsets? padding;
   final Widget? header;
   final Axis scrollDirection;
   final bool reverse;
   final ScrollController? scrollController;
   final bool? primary;
   final ScrollPhysics? physics;
   final bool shrinkWrap;
   final double anchor;
   final double? cacheExtent;
   final DragStartBehavior dragStartBehavior;
   final ScrollViewKeyboardDismissBehavior keyboardDismissBehavior;
   final String? restorationId;
   final Clip clipBehavior;

   @override
   _ReorderableListContentState createState() => _ReorderableListContentState();
 }

 class _ReorderableListContentState extends State<_ReorderableListContent> {
   Widget _wrapWithSemantics(Widget child, int index) {
     void reorder(int startIndex, int endIndex) {
       if (startIndex != endIndex)
         widget.onReorder(startIndex, endIndex);
     }

     // First, determine which semantics actions apply.
     final Map<CustomSemanticsAction, VoidCallback> semanticsActions = <CustomSemanticsAction, VoidCallback>{};

     // Create the appropriate semantics actions.
     void moveToStart() => reorder(index, 0);
     void moveToEnd() => reorder(index, widget.itemCount);
     void moveBefore() => reorder(index, index - 1);
     // To move after, we go to index+2 because we are moving it to the space
     // before index+2, which is after the space at index+1.
     void moveAfter() => reorder(index, index + 2);

     final MaterialLocalizations localizations = MaterialLocalizations.of(context);

     // If the item can move to before its current position in the list.
     if (index > 0) {
       semanticsActions[CustomSemanticsAction(label: localizations.reorderItemToStart)] = moveToStart;
       String reorderItemBefore = localizations.reorderItemUp;
       if (widget.scrollDirection == Axis.horizontal) {
         reorderItemBefore = Directionality.of(context) == TextDirection.ltr
             ? localizations.reorderItemLeft
             : localizations.reorderItemRight;
       }
       semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] = moveBefore;
     }

     // If the item can move to after its current position in the list.
     if (index < widget.itemCount - 1) {
       String reorderItemAfter = localizations.reorderItemDown;
       if (widget.scrollDirection == Axis.horizontal) {
         reorderItemAfter = Directionality.of(context) == TextDirection.ltr
             ? localizations.reorderItemRight
             : localizations.reorderItemLeft;
       }
       semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] = moveAfter;
       semanticsActions[CustomSemanticsAction(label: localizations.reorderItemToEnd)] = moveToEnd;
     }

     // We pass toWrap with a GlobalKey into the item so that when it
     // gets dragged, the accessibility framework can preserve the selected
     // state of the dragging item.
     //
     // We also apply the relevant custom accessibility actions for moving the item
     // up, down, to the start, and to the end of the list.
     return MergeSemantics(
       child: Semantics(
         customSemanticsActions: semanticsActions,
         child: child,
       ),
     );
   }

   Widget _itemBuilder(BuildContext context, int index) {
     final Widget item = widget.itemBuilder(context, index);

     // TODO(goderbauer): The semantics stuff should probably happen inside
     //   _ReorderableItem so the widget versions can have them as well.
     final Widget itemWithSemantics = _wrapWithSemantics(item, index);
     final Key itemGlobalKey = _ReorderableListViewChildGlobalKey(item.key!, this);

     if (widget.buildDefaultDragHandles) {
       switch (Theme.of(context).platform) {
         case TargetPlatform.fuchsia:
         case TargetPlatform.linux:
         case TargetPlatform.windows:
         case TargetPlatform.macOS:
           return Stack(
             key: itemGlobalKey,
             children: <Widget>[
               itemWithSemantics,
               Positioned.directional(
                 textDirection: Directionality.of(context),
                 top: 0,
                 bottom: 0,
                 end: 8,
                 child: Align(
                   alignment: AlignmentDirectional.centerEnd,
                   child: ReorderableDragStartListener(
                     index: index,
                     child: const Icon(Icons.drag_handle),
                   ),
                 ),
               ),
             ],
           );

         case TargetPlatform.iOS:
         case TargetPlatform.android:
           return ReorderableDelayedDragStartListener(
             key: itemGlobalKey,
             index: index,
             child: itemWithSemantics,
           );
       }
     }

     return KeyedSubtree(
       key: itemGlobalKey,
       child: itemWithSemantics,
     );
   }

   Widget _proxyDecorator(Widget child, int index, Animation<double> animation) {
     return AnimatedBuilder(
       animation: animation,
       builder: (BuildContext context, Widget? child) {
         final double animValue = Curves.easeInOut.transform(animation.value);
         final double elevation = lerpDouble(0, 6, animValue)!;
         return Material(
           child: child,
           elevation: elevation,
         );
       },
       child: child,
     );
   }

   @override
   Widget build(BuildContext context) {
     // If there is a header we can't just apply the padding to the list,
     // so we wrap the CustomScrollView in the padding for the top, left and right
     // and only add the padding from the bottom to the sliver list (or the equivalent
     // for other axis directions).
     final EdgeInsets padding = widget.padding ?? EdgeInsets.zero;
     late EdgeInsets outerPadding;
     late EdgeInsets listPadding;
     switch (widget.scrollDirection) {

       case Axis.horizontal:
         if (widget.reverse) {
           outerPadding = EdgeInsets.fromLTRB(0, padding.top, padding.right, padding.bottom);
           listPadding = EdgeInsets.fromLTRB(padding.left, 0, 0, 0);
         } else {
           outerPadding = EdgeInsets.fromLTRB(padding.left, padding.top, 0, padding.bottom);
           listPadding = EdgeInsets.fromLTRB(0, 0, padding.right, 0);
         }
         break;
       case Axis.vertical:
         if (widget.reverse) {
           outerPadding = EdgeInsets.fromLTRB(padding.left, 0, padding.right, padding.bottom);
           listPadding = EdgeInsets.fromLTRB(0, padding.top, 0, 0);
         } else {
           outerPadding = EdgeInsets.fromLTRB(padding.left, padding.top, padding.right, 0);
           listPadding = EdgeInsets.fromLTRB(0, 0, 0, padding.bottom);
         }
         break;
     }

     return Padding(
       padding: outerPadding,
       child: CustomScrollView(
         scrollDirection: widget.scrollDirection,
         reverse: widget.reverse,
         controller: widget.scrollController,
         primary: widget.primary,
         physics: widget.physics,
         shrinkWrap: widget.shrinkWrap,
         anchor: widget.anchor,
         cacheExtent: widget.cacheExtent,
         dragStartBehavior: widget.dragStartBehavior,
         keyboardDismissBehavior: widget.keyboardDismissBehavior,
         restorationId: widget.restorationId,
         clipBehavior: widget.clipBehavior,
         slivers: <Widget>[
           if (widget.header != null)
             SliverToBoxAdapter(child: widget.header!),
           SliverPadding(
             padding: listPadding,
             sliver: SliverReorderableList(
               itemBuilder: _itemBuilder,
               itemCount: widget.itemCount,
               onReorder: widget.onReorder,
               proxyDecorator: widget.proxyDecorator ?? _proxyDecorator,
             ),
           ),
         ],
       ),
     );
   }
 }

 // A global key that takes its identity from the object and uses a value of a
 // particular type to identify itself.
 //
 // The difference with GlobalObjectKey is that it uses [==] instead of [identical]
 // of the objects used to generate widgets.
 @optionalTypeArgs
 class _ReorderableListViewChildGlobalKey extends GlobalObjectKey {
   const _ReorderableListViewChildGlobalKey(this.subKey, this.state) : super(subKey);

   final Key subKey;
   final State state;

   @override
   bool operator ==(Object other) {
     if (other.runtimeType != runtimeType)
       return false;
     return other is _ReorderableListViewChildGlobalKey
         && other.subKey == subKey
         && other.state == state;
   }

   @override
   int get hashCode => hashValues(subKey, state);
 }
	// Copyright 2014 The Flutter Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	import 'dart:ui' show lerpDouble;

	import 'package:flutter/gestures.dart';
	import 'package:flutter/rendering.dart';
	import 'package:flutter/widgets.dart';

	import 'debug.dart';
	import 'icons.dart';
	import 'material.dart';
	import 'material_localizations.dart';
	import 'theme.dart';

	/// A list whose items the user can interactively reorder by dragging.
	///
	/// This class is appropriate for views with a small number of
	/// children because constructing the [List] requires doing work for every
	/// child that could possibly be displayed in the list view instead of just
	/// those children that are actually visible.
	///
	/// All list items must have a key.
	///
	/// {@youtube 560 315 https://www.youtube.com/watch?v=3fB1mxOsqJE}
	///
	/// This sample shows by dragging the user can reorder the items of the list.
	/// The [onReorder] parameter is required and will be called when a child
	/// widget is dragged to a new position.
	///
	/// {@tool dartpad --template=stateful_widget_scaffold}
	///
	/// ```dart
	/// final List<int> _items = List<int>.generate(50, (int index) => index);
	///
	/// Widget build(BuildContext context){
	/// final ColorScheme colorScheme = Theme.of(context).colorScheme;
	/// final oddItemColor = colorScheme.primary.withOpacity(0.05);
	/// final evenItemColor = colorScheme.primary.withOpacity(0.15);
	///
	/// return ReorderableListView(
	/// padding: const EdgeInsets.symmetric(horizontal: 40),
	/// children: <Widget>[
	/// for (int index = 0; index < _items.length; index++)
	/// ListTile(
	/// key: Key('$index'),
	/// tileColor: _items[index].isOdd ? oddItemColor : evenItemColor,
	/// title: Text('Item ${_items[index]}'),
	/// ),
	/// ],
	/// onReorder: (int oldIndex, int newIndex) {
	/// setState(() {
	/// if (oldIndex < newIndex) {
	/// newIndex -= 1;
	/// }
	/// final int item = _items.removeAt(oldIndex);
	/// _items.insert(newIndex, item);
	/// });
	/// },
	/// );
	/// }
	///
	/// ```
	///
	///{@end-tool}
	class ReorderableListView extends StatefulWidget {
	/// Creates a reorderable list from a pre-built list of widgets.
	///
	/// See also:
	///
	/// * [ReorderableListView.builder], which allows you to build a reorderable
	/// list where the items are built as needed when scrolling the list.
	ReorderableListView({
	Key? key,
	required List<Widget> children,
	required this.onReorder,
	this.proxyDecorator,
	this.buildDefaultDragHandles = true,
	this.padding,
	this.header,
	this.scrollDirection = Axis.vertical,
	this.reverse = false,
	this.scrollController,
	this.primary,
	this.physics,
	this.shrinkWrap = false,
	this.anchor = 0.0,
	this.cacheExtent,
	this.dragStartBehavior = DragStartBehavior.start,
	this.keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
	this.restorationId,
	this.clipBehavior = Clip.hardEdge,
	}) : assert(scrollDirection != null),
	assert(onReorder != null),
	assert(children != null),
	assert(
	children.every((Widget w) => w.key != null),
	'All children of this widget must have a key.',
	),
	assert(buildDefaultDragHandles != null),
	itemBuilder = ((BuildContext context, int index) => children[index]),
	itemCount = children.length,
	super(key: key);

	/// Creates a reorderable list from widget items that are created on demand.
	///
	/// This constructor is appropriate for list views with a large number of
	/// children because the builder is called only for those children
	/// that are actually visible.
	///
	/// The `itemBuilder` callback will be called only with indices greater than
	/// or equal to zero and less than `itemCount`.
	///
	/// The `itemBuilder` should always return a non-null widget, and actually
	/// create the widget instances when called. Avoid using a builder that
	/// returns a previously-constructed widget; if the list view's children are
	/// created in advance, or all at once when the [ReorderableListView] itself
	/// is created, it is more efficient to use the [ReorderableListView]
	/// constructor. Even more efficient, however, is to create the instances
	/// on demand using this constructor's `itemBuilder` callback.
	///
	/// See also:
	///
	/// * [ReorderableListView], which allows you to build a reorderable
	/// list with all the items passed into the constructor.
	const ReorderableListView.builder({
	Key? key,
	required this.itemBuilder,
	required this.itemCount,
	required this.onReorder,
	this.proxyDecorator,
	this.buildDefaultDragHandles = true,
	this.padding,
	this.header,
	this.scrollDirection = Axis.vertical,
	this.reverse = false,
	this.scrollController,
	this.primary,
	this.physics,
	this.shrinkWrap = false,
	this.anchor = 0.0,
	this.cacheExtent,
	this.dragStartBehavior = DragStartBehavior.start,
	this.keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
	this.restorationId,
	this.clipBehavior = Clip.hardEdge,
	}) : assert(scrollDirection != null),
	assert(itemCount >= 0),
	assert(onReorder != null),
	assert(buildDefaultDragHandles != null),
	super(key: key);

	/// {@macro flutter.widgets.reorderable_list.itemBuilder}
	final IndexedWidgetBuilder itemBuilder;

	/// {@macro flutter.widgets.reorderable_list.itemCount}
	final int itemCount;

	/// {@macro flutter.widgets.reorderable_list.onReorder}
	final ReorderCallback onReorder;

	/// {@macro flutter.widgets.reorderable_list.proxyDecorator}
	final ReorderItemProxyDecorator? proxyDecorator;

	/// If true: on desktop platforms, a drag handle is stacked over the
	/// center of each item's trailing edge; on mobile platforms, a long
	/// press anywhere on the item starts a drag.
	///
	/// The default desktop drag handle is just an [Icons.drag_handle]
	/// wrapped by a [ReorderableDragStartListener]. On mobile
	/// platforms, the entire item is wrapped with a
	/// [ReorderableDelayedDragStartListener].
	///
	/// To change the appearance or the layout of the drag handles, make
	/// this parameter false and wrap each list item, or a widget within
	/// each list item, with [ReorderableDragStartListener] or
	/// [ReorderableDelayedDragStartListener], or a custom subclass
	/// of [ReorderableDragStartListener].
	///
	/// The following sample specifies `buildDefaultDragHandles: false`, and
	/// uses a [Card] at the leading edge of each item for the item's drag handle.
	///
	/// {@tool dartpad --template=stateful_widget_scaffold}
	///
	/// ```dart
	/// final List<int> _items = List<int>.generate(50, (int index) => index);
	///
	/// Widget build(BuildContext context){
	/// final ColorScheme colorScheme = Theme.of(context).colorScheme;
	/// final oddItemColor = colorScheme.primary.withOpacity(0.05);
	/// final evenItemColor = colorScheme.primary.withOpacity(0.15);
	///
	/// return ReorderableListView(
	/// buildDefaultDragHandles: false,
	/// children: <Widget>[
	/// for (int index = 0; index < _items.length; index++)
	/// Container(
	/// key: Key('$index'),
	/// color: _items[index].isOdd ? oddItemColor : evenItemColor,
	/// child: Row(
	/// children: <Widget>[
	/// Container(
	/// width: 64,
	/// height: 64,
	/// padding: const EdgeInsets.all(8),
	/// child: ReorderableDragStartListener(
	/// index: index,
	/// child: Card(
	/// color: colorScheme.primary,
	/// elevation: 2,
	/// ),
	/// ),
	/// ),
	/// Text('Item ${_items[index]}'),
	/// ],
	/// ),
	/// ),
	/// ],
	/// onReorder: (oldIndex, newIndex) {
	/// setState(() {
	/// if (oldIndex < newIndex) {
	/// newIndex -= 1;
	/// }
	/// final int item = _items.removeAt(oldIndex);
	/// _items.insert(newIndex, item);
	/// });
	/// },
	/// );
	/// }
	/// ```
	///{@end-tool}
	final bool buildDefaultDragHandles;

	/// {@macro flutter.widgets.reorderable_list.padding}
	final EdgeInsets? padding;

	/// A non-reorderable header item to show before the items of the list.
	///
	/// If null, no header will appear before the list.
	final Widget? header;

	/// {@macro flutter.widgets.scroll_view.scrollDirection}
	final Axis scrollDirection;

	/// {@macro flutter.widgets.scroll_view.reverse}
	final bool reverse;

	/// {@macro flutter.widgets.scroll_view.controller}
	final ScrollController? scrollController;

	/// {@macro flutter.widgets.scroll_view.primary}

	/// Defaults to true when [scrollDirection] is [Axis.vertical] and
	/// [scrollController] is null.
	final bool? primary;

	/// {@macro flutter.widgets.scroll_view.physics}
	final ScrollPhysics? physics;

	/// {@macro flutter.widgets.scroll_view.shrinkWrap}
	final bool shrinkWrap;

	/// {@macro flutter.widgets.scroll_view.anchor}
	final double anchor;

	/// {@macro flutter.rendering.RenderViewportBase.cacheExtent}
	final double? cacheExtent;

	/// {@macro flutter.widgets.scrollable.dragStartBehavior}
	final DragStartBehavior dragStartBehavior;

	/// {@macro flutter.widgets.scroll_view.keyboardDismissBehavior}
	///
	/// The default is [ScrollViewKeyboardDismissBehavior.manual]
	final ScrollViewKeyboardDismissBehavior keyboardDismissBehavior;

	/// {@macro flutter.widgets.scrollable.restorationId}
	final String? restorationId;

	/// {@macro flutter.material.Material.clipBehavior}
	///
	/// Defaults to [Clip.hardEdge].
	final Clip clipBehavior;

	@override
	_ReorderableListViewState createState() => _ReorderableListViewState();
	}

	// This top-level state manages an Overlay that contains the list and
	// also any items being dragged on top fo the list.
	//
	// The Overlay doesn't properly keep state by building new overlay entries,
	// and so we cache a single OverlayEntry for use as the list layer.
	// That overlay entry then builds a _ReorderableListContent which may
	// insert items being dragged into the Overlay above itself.
	class _ReorderableListViewState extends State<ReorderableListView> {
	// This entry contains the scrolling list itself.
	late OverlayEntry _listOverlayEntry;

	@override
	void initState() {
	super.initState();
	_listOverlayEntry = OverlayEntry(
	opaque: true,
	builder: (BuildContext context) {
	return _ReorderableListContent(
	itemBuilder: widget.itemBuilder,
	itemCount: widget.itemCount,
	onReorder: widget.onReorder,
	proxyDecorator: widget.proxyDecorator,
	buildDefaultDragHandles: widget.buildDefaultDragHandles,
	padding: widget.padding,
	header: widget.header,
	scrollDirection: widget.scrollDirection,
	reverse: widget.reverse,
	scrollController: widget.scrollController,
	primary: widget.primary,
	physics: widget.physics,
	shrinkWrap: widget.shrinkWrap,
	anchor: widget.anchor,
	cacheExtent: widget.cacheExtent,
	dragStartBehavior: widget.dragStartBehavior,
	keyboardDismissBehavior: widget.keyboardDismissBehavior,
	restorationId: widget.restorationId,
	clipBehavior: widget.clipBehavior,
	);
	},
	);
	}

	@override
	void didUpdateWidget(ReorderableListView oldWidget) {
	super.didUpdateWidget(oldWidget);
	// As this depends on pretty much everything, it
	// is ok to mark this as dirty unconditionally.
	_listOverlayEntry.markNeedsBuild();
	}

	@override
	Widget build(BuildContext context) {
	assert(debugCheckHasMaterialLocalizations(context));
	return Overlay(
	initialEntries: <OverlayEntry>[
	_listOverlayEntry
	],
	);
	}
	}

	class _ReorderableListContent extends StatefulWidget {
	const _ReorderableListContent({
	required this.itemBuilder,
	required this.itemCount,
	required this.onReorder,
	required this.proxyDecorator,
	required this.buildDefaultDragHandles,
	required this.padding,
	required this.header,
	required this.scrollDirection,
	required this.reverse,
	required this.scrollController,
	required this.primary,
	required this.physics,
	required this.shrinkWrap,
	required this.anchor,
	required this.cacheExtent,
	required this.dragStartBehavior,
	required this.keyboardDismissBehavior,
	required this.restorationId,
	required this.clipBehavior,
	});

	final IndexedWidgetBuilder itemBuilder;
	final int itemCount;
	final ReorderCallback onReorder;
	final ReorderItemProxyDecorator? proxyDecorator;
	final bool buildDefaultDragHandles;
	final EdgeInsets? padding;
	final Widget? header;
	final Axis scrollDirection;
	final bool reverse;
	final ScrollController? scrollController;
	final bool? primary;
	final ScrollPhysics? physics;
	final bool shrinkWrap;
	final double anchor;
	final double? cacheExtent;
	final DragStartBehavior dragStartBehavior;
	final ScrollViewKeyboardDismissBehavior keyboardDismissBehavior;
	final String? restorationId;
	final Clip clipBehavior;

	@override
	_ReorderableListContentState createState() => _ReorderableListContentState();
	}

	class _ReorderableListContentState extends State<_ReorderableListContent> {
	Widget _wrapWithSemantics(Widget child, int index) {
	void reorder(int startIndex, int endIndex) {
	if (startIndex != endIndex)
	widget.onReorder(startIndex, endIndex);
	}

	// First, determine which semantics actions apply.
	final Map<CustomSemanticsAction, VoidCallback> semanticsActions = <CustomSemanticsAction, VoidCallback>{};

	// Create the appropriate semantics actions.
	void moveToStart() => reorder(index, 0);
	void moveToEnd() => reorder(index, widget.itemCount);
	void moveBefore() => reorder(index, index - 1);
	// To move after, we go to index+2 because we are moving it to the space
	// before index+2, which is after the space at index+1.
	void moveAfter() => reorder(index, index + 2);

	final MaterialLocalizations localizations = MaterialLocalizations.of(context);

	// If the item can move to before its current position in the list.
	if (index > 0) {
	semanticsActions[CustomSemanticsAction(label: localizations.reorderItemToStart)] = moveToStart;
	String reorderItemBefore = localizations.reorderItemUp;
	if (widget.scrollDirection == Axis.horizontal) {
	reorderItemBefore = Directionality.of(context) == TextDirection.ltr
	? localizations.reorderItemLeft
	: localizations.reorderItemRight;
	}
	semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] = moveBefore;
	}

	// If the item can move to after its current position in the list.
	if (index < widget.itemCount - 1) {
	String reorderItemAfter = localizations.reorderItemDown;
	if (widget.scrollDirection == Axis.horizontal) {
	reorderItemAfter = Directionality.of(context) == TextDirection.ltr
	? localizations.reorderItemRight
	: localizations.reorderItemLeft;
	}
	semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] = moveAfter;
	semanticsActions[CustomSemanticsAction(label: localizations.reorderItemToEnd)] = moveToEnd;
	}

	// We pass toWrap with a GlobalKey into the item so that when it
	// gets dragged, the accessibility framework can preserve the selected
	// state of the dragging item.
	//
	// We also apply the relevant custom accessibility actions for moving the item
	// up, down, to the start, and to the end of the list.
	return MergeSemantics(
	child: Semantics(
	customSemanticsActions: semanticsActions,
	child: child,
	),
	);
	}

	Widget _itemBuilder(BuildContext context, int index) {
	final Widget item = widget.itemBuilder(context, index);

	// TODO(goderbauer): The semantics stuff should probably happen inside
	// _ReorderableItem so the widget versions can have them as well.
	final Widget itemWithSemantics = _wrapWithSemantics(item, index);
	final Key itemGlobalKey = _ReorderableListViewChildGlobalKey(item.key!, this);

	if (widget.buildDefaultDragHandles) {
	switch (Theme.of(context).platform) {
	case TargetPlatform.fuchsia:
	case TargetPlatform.linux:
	case TargetPlatform.windows:
	case TargetPlatform.macOS:
	return Stack(
	key: itemGlobalKey,
	children: <Widget>[
	itemWithSemantics,
	Positioned.directional(
	textDirection: Directionality.of(context),
	top: 0,
	bottom: 0,
	end: 8,
	child: Align(
	alignment: AlignmentDirectional.centerEnd,
	child: ReorderableDragStartListener(
	index: index,
	child: const Icon(Icons.drag_handle),
	),
	),
	),
	],
	);

	case TargetPlatform.iOS:
	case TargetPlatform.android:
	return ReorderableDelayedDragStartListener(
	key: itemGlobalKey,
	index: index,
	child: itemWithSemantics,
	);
	}
	}

	return KeyedSubtree(
	key: itemGlobalKey,
	child: itemWithSemantics,
	);
	}

	Widget _proxyDecorator(Widget child, int index, Animation<double> animation) {
	return AnimatedBuilder(
	animation: animation,
	builder: (BuildContext context, Widget? child) {
	final double animValue = Curves.easeInOut.transform(animation.value);
	final double elevation = lerpDouble(0, 6, animValue)!;
	return Material(
	child: child,
	elevation: elevation,
	);
	},
	child: child,
	);
	}

	@override
	Widget build(BuildContext context) {
	// If there is a header we can't just apply the padding to the list,
	// so we wrap the CustomScrollView in the padding for the top, left and right
	// and only add the padding from the bottom to the sliver list (or the equivalent
	// for other axis directions).
	final EdgeInsets padding = widget.padding ?? EdgeInsets.zero;
	late EdgeInsets outerPadding;
	late EdgeInsets listPadding;
	switch (widget.scrollDirection) {

	case Axis.horizontal:
	if (widget.reverse) {
	outerPadding = EdgeInsets.fromLTRB(0, padding.top, padding.right, padding.bottom);
	listPadding = EdgeInsets.fromLTRB(padding.left, 0, 0, 0);
	} else {
	outerPadding = EdgeInsets.fromLTRB(padding.left, padding.top, 0, padding.bottom);
	listPadding = EdgeInsets.fromLTRB(0, 0, padding.right, 0);
	}
	break;
	case Axis.vertical:
	if (widget.reverse) {
	outerPadding = EdgeInsets.fromLTRB(padding.left, 0, padding.right, padding.bottom);
	listPadding = EdgeInsets.fromLTRB(0, padding.top, 0, 0);
	} else {
	outerPadding = EdgeInsets.fromLTRB(padding.left, padding.top, padding.right, 0);
	listPadding = EdgeInsets.fromLTRB(0, 0, 0, padding.bottom);
	}
	break;
	}

	return Padding(
	padding: outerPadding,
	child: CustomScrollView(
	scrollDirection: widget.scrollDirection,
	reverse: widget.reverse,
	controller: widget.scrollController,
	primary: widget.primary,
	physics: widget.physics,
	shrinkWrap: widget.shrinkWrap,
	anchor: widget.anchor,
	cacheExtent: widget.cacheExtent,
	dragStartBehavior: widget.dragStartBehavior,
	keyboardDismissBehavior: widget.keyboardDismissBehavior,
	restorationId: widget.restorationId,
	clipBehavior: widget.clipBehavior,
	slivers: <Widget>[
	if (widget.header != null)
	SliverToBoxAdapter(child: widget.header!),
	SliverPadding(
	padding: listPadding,
	sliver: SliverReorderableList(
	itemBuilder: _itemBuilder,
	itemCount: widget.itemCount,
	onReorder: widget.onReorder,
	proxyDecorator: widget.proxyDecorator ?? _proxyDecorator,
	),
	),
	],
	),
	);
	}
	}

	// A global key that takes its identity from the object and uses a value of a
	// particular type to identify itself.
	//
	// The difference with GlobalObjectKey is that it uses [==] instead of [identical]
	// of the objects used to generate widgets.
	@optionalTypeArgs
	class _ReorderableListViewChildGlobalKey extends GlobalObjectKey {
	const _ReorderableListViewChildGlobalKey(this.subKey, this.state) : super(subKey);

	final Key subKey;
	final State state;

	@override
	bool operator ==(Object other) {
	if (other.runtimeType != runtimeType)
	return false;
	return other is _ReorderableListViewChildGlobalKey
	&& other.subKey == subKey
	&& other.state == state;
	}

	@override
	int get hashCode => hashValues(subKey, state);
	}