packages/flutter/lib/src/widgets/animated_list.dart - mirrors/flutter - Git at Google

 // Copyright 2017 The Chromium 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 'package:collection/collection.dart' show binarySearch;

 import 'package:flutter/animation.dart';

 import 'basic.dart';
 import 'framework.dart';
 import 'scroll_controller.dart';
 import 'scroll_physics.dart';
 import 'scroll_view.dart';
 import 'ticker_provider.dart';

 /// Signature for the builder callback used by [AnimatedList].
 typedef Widget AnimatedListItemBuilder(BuildContext context, int index, Animation<double> animation);

 /// Signature for the builder callback used by [AnimatedListState.removeItem].
 typedef Widget AnimatedListRemovedItemBuilder(BuildContext context, Animation<double> animation);

 // The default insert/remove animation duration.
 const Duration _kDuration = const Duration(milliseconds: 300);

 // Incoming and outgoing AnimatedList 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;
   final AnimatedListRemovedItemBuilder removedItemBuilder;
   int itemIndex;

   @override
   int compareTo(_ActiveItem other) => itemIndex - other.itemIndex;
 }

 /// A scrolling container that animates items when they are inserted or removed.
 ///
 /// This widget's [AnimatedListState] can be used to dynamically insert or remove
 /// items. To refer to the [AnimatedListState] either provide a [GlobalKey] or
 /// use the static [of] method from an item's input callback.
 ///
 /// This widget is similar to one created by [ListView.builder].
 class AnimatedList extends StatefulWidget {
   /// Creates a scrolling container that animates items when they are inserted or removed.
   const AnimatedList({
     Key key,
     @required this.itemBuilder,
     this.initialItemCount: 0,
     this.scrollDirection: Axis.vertical,
     this.reverse: false,
     this.controller,
     this.primary,
     this.physics,
     this.shrinkWrap: false,
     this.padding,
   }) : assert(itemBuilder != null),
        assert(initialItemCount != null && initialItemCount >= 0),
        super(key: key);

   /// Called, as needed, to build list item widgets.
   ///
   /// List items are only built when they're scrolled into view.
   ///
   /// The [AnimatedListItemBuilder] index parameter indicates the item's
   /// position in the list. The value of the index parameter will be between 0
   /// and [initialItemCount] plus the total number of items that have been
   /// inserted with [AnimatedListState.insertItem] and less the total number of
   /// items that have been removed with [AnimatedListState.removeItem].
   ///
   /// Implementations of this callback should assume that
   /// [AnimatedListState.removeItem] removes an item immediately.
   final AnimatedListItemBuilder itemBuilder;

   /// The number of items the list will start with.
   ///
   /// The appearance of the initial items is not animated. They
   /// are created, as needed, by [itemBuilder] with an animation parameter
   /// of [kAlwaysCompleteAnimation].
   final int initialItemCount;

   /// The axis along which the scroll view scrolls.
   ///
   /// Defaults to [Axis.vertical].
   final Axis scrollDirection;

   /// 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;

   /// An object that can be used to control the position to which this scroll
   /// view is scrolled.
   ///
   /// Must be null if [primary] is true.
   ///
   /// A [ScrollController] serves several purposes. It can be used to control
   /// the initial scroll position (see [ScrollController.initialScrollOffset]).
   /// It can be used to control whether the scroll view should automatically
   /// save and restore its scroll position in the [PageStorage] (see
   /// [ScrollController.keepScrollOffset]). It can be used to read the current
   /// scroll position (see [ScrollController.offset]), or change it (see
   /// [ScrollController.animateTo]).
   final ScrollController controller;

   /// Whether this is the primary scroll view associated with the parent
   /// [PrimaryScrollController].
   ///
   /// On iOS, this identifies the scroll view that will scroll to top in
   /// response to a tap in the status bar.
   ///
   /// Defaults to true when [scrollDirection] is [Axis.vertical] and
   /// [controller] is null.
   final bool primary;

   /// How the scroll view should respond to user input.
   ///
   /// For example, determines how the scroll view continues to animate after the
   /// user stops dragging the scroll view.
   ///
   /// Defaults to matching platform conventions.
   final ScrollPhysics physics;

   /// Whether the extent of the scroll view in the [scrollDirection] should be
   /// determined by the contents being viewed.
   ///
   /// If the scroll view does not shrink wrap, then the scroll view will expand
   /// to the maximum allowed size in the [scrollDirection]. If the scroll view
   /// has unbounded constraints in the [scrollDirection], then [shrinkWrap] must
   /// be true.
   ///
   /// Shrink wrapping the content of the scroll view is significantly more
   /// expensive than expanding to the maximum allowed size because the content
   /// can expand and contract during scrolling, which means the size of the
   /// scroll view needs to be recomputed whenever the scroll position changes.
   ///
   /// Defaults to false.
   final bool shrinkWrap;

   /// The amount of space by which to inset the children.
   final EdgeInsetsGeometry padding;

   /// The state from the closest instance of this class that encloses the given context.
   ///
   /// This method is typically used by [AnimatedList] item widgets that insert or
   /// remove items in response to user input.
   ///
   /// ```dart
   /// AnimatedListState animatedList = AnimatedList.of(context);
   /// ```
   static AnimatedListState of(BuildContext context, { bool nullOk: false }) {
     assert(context != null);
     assert(nullOk != null);
     final AnimatedListState result = context.ancestorStateOfType(const TypeMatcher<AnimatedListState>());
     if (nullOk || result != null)
       return result;
     throw new FlutterError(
       'AnimatedList.of() called with a context that does not contain a AnimatedList.\n'
       'No AnimatedList ancestor could be found starting from the context that was passed to AnimatedList.of(). '
       'This can happen when the context provided is from the same StatefulWidget that '
       'built the AnimatedList. Please see the AnimatedList documentation for examples '
       'of how to refer to an AnimatedListState object: '
       '  https://docs.flutter.io/flutter/widgets/AnimatedListState-class.html \n'
       'The context used was:\n'
       '  $context'
     );
   }

   @override
   AnimatedListState createState() => new AnimatedListState();
 }

 /// The state for a scrolling container that animates items when they are
 /// inserted or removed.
 ///
 /// When an item is inserted with [insertItem] an animation begins running. The
 /// animation is passed to [AnimatedList.itemBuilder] whenever the item's widget
 /// is needed.
 ///
 /// When an item is removed with [removeItem] its animation is reversed.
 /// The removed item's animation is passed to the [removeItem] builder
 /// parameter.
 ///
 /// An app that needs to insert or remove items in response to an event
 /// can refer to the [AnimatedList]'s state with a global key:
 ///
 /// ```dart
 /// GlobalKey<AnimatedListState> listKey = new GlobalKey<AnimatedListState>();
 /// ...
 /// new AnimatedList(key: listKey, ...);
 /// ...
 /// listKey.currentState.insert(123);
 /// ```
 ///
 /// [AnimatedList] item input handlers can also refer to their [AnimatedListState]
 /// with the static [AnimatedList.of] method.
 class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixin {
   final List<_ActiveItem> _incomingItems = <_ActiveItem>[];
   final List<_ActiveItem> _outgoingItems = <_ActiveItem>[];
   int _itemsCount = 0;

   @override
   void initState() {
     super.initState();
     _itemsCount = widget.initialItemCount;
   }

   @override
   void dispose() {
     for (_ActiveItem item in _incomingItems)
       item.controller.dispose();
     for (_ActiveItem item in _outgoingItems)
       item.controller.dispose();
     super.dispose();
   }

   _ActiveItem _removeActiveItemAt(List<_ActiveItem> items, int itemIndex) {
     final int i = binarySearch(items, new _ActiveItem.index(itemIndex));
     return i == -1 ? null : items.removeAt(i);
   }

   _ActiveItem _activeItemAt(List<_ActiveItem> items, int itemIndex) {
     final int i = binarySearch(items, new _ActiveItem.index(itemIndex));
     return i == -1 ? null : items[i];
   }

   // The insertItem() and removeItem() index parameters are defined as if the
   // removeItem() operation removed the corresponding list entry immediately.
   // The entry is only actually removed from the ListView when the remove animation
   // finishes. The entry is added to _outgoingItems when removeItem is called
   // and removed from _outgoingItems when the remove animation finishes.

   int _indexToItemIndex(int index) {
     int itemIndex = index;
     for (_ActiveItem item in _outgoingItems) {
       if (item.itemIndex <= itemIndex)
         itemIndex += 1;
       else
         break;
     }
     return itemIndex;
   }

   int _itemIndexToIndex(int itemIndex) {
     int index = itemIndex;
     for (_ActiveItem item in _outgoingItems) {
       assert(item.itemIndex != itemIndex);
       if (item.itemIndex < itemIndex)
         index -= 1;
       else
         break;
     }
     return index;
   }

   /// Insert an item at [index] and start an animation that will be passed
   /// to [AnimatedList.itemBuilder] when the item is visible.
   ///
   /// This method's semantics are the same as Dart's [List.insert] method:
   /// it increases the length of the list by one and shifts all items at or
   /// after [index] towards the end of the list.
   void insertItem(int index, { Duration duration: _kDuration }) {
     assert(index != null && index >= 0);
     assert(duration != null);

     final int itemIndex = _indexToItemIndex(index);
     assert(itemIndex >= 0 && itemIndex <= _itemsCount);

     // Increment the incoming and outgoing item indices to account
     // for the insertion.
     for (_ActiveItem item in _incomingItems) {
       if (item.itemIndex >= itemIndex)
         item.itemIndex += 1;
     }
     for (_ActiveItem item in _outgoingItems) {
       if (item.itemIndex >= itemIndex)
         item.itemIndex += 1;
     }

     final AnimationController controller = new AnimationController(duration: duration, vsync: this);
     final _ActiveItem incomingItem = new _ActiveItem.incoming(controller, itemIndex);
     setState(() {
       _incomingItems
         ..add(incomingItem)
         ..sort();
       _itemsCount += 1;
     });

     controller.forward().then<void>((Null value) {
       _removeActiveItemAt(_incomingItems, incomingItem.itemIndex).controller.dispose();
     });
   }

   /// Remove the item at [index] and start an animation that will be passed
   /// to [builder] when the item is visible.
   ///
   /// Items are removed immediately. After an item has been removed, its index
   /// will no longer be passed to the [AnimatedList.itemBuilder]. However the
   /// item will still appear in the list for [duration] and during that time
   /// [builder] must construct its widget as needed.
   ///
   /// This method's semantics are the same as Dart's [List.remove] method:
   /// it decreases the length of the list by one and shifts all items at or
   /// before [index] towards the beginning of the list.
   void removeItem(int index, AnimatedListRemovedItemBuilder builder, { Duration duration: _kDuration }) {
     assert(index != null && index >= 0);
     assert(builder != null);
     assert(duration != null);

     final int itemIndex = _indexToItemIndex(index);
     assert(itemIndex >= 0 && itemIndex < _itemsCount);
     assert(_activeItemAt(_outgoingItems, itemIndex) == null);

     final _ActiveItem incomingItem = _removeActiveItemAt(_incomingItems, itemIndex);
     final AnimationController controller = incomingItem?.controller
       ?? new AnimationController(duration: duration, value: 1.0, vsync: this);
     final _ActiveItem outgoingItem = new _ActiveItem.outgoing(controller, itemIndex, builder);
     setState(() {
       _outgoingItems
         ..add(outgoingItem)
         ..sort();
     });

     controller.reverse().then<void>((Null value) {
       _removeActiveItemAt(_outgoingItems, outgoingItem.itemIndex).controller.dispose();

       // Decrement the incoming and outgoing item indices to account
       // for the removal.
       for (_ActiveItem item in _incomingItems) {
         if (item.itemIndex > outgoingItem.itemIndex)
           item.itemIndex -= 1;
       }
       for (_ActiveItem item in _outgoingItems) {
         if (item.itemIndex > outgoingItem.itemIndex)
           item.itemIndex -= 1;
       }

       setState(() {
         _itemsCount -= 1;
       });
     });
   }

   Widget _itemBuilder(BuildContext context, int itemIndex) {
     final _ActiveItem outgoingItem = _activeItemAt(_outgoingItems, itemIndex);
     if (outgoingItem != null)
       return outgoingItem.removedItemBuilder(context, outgoingItem.controller.view);

     final _ActiveItem incomingItem = _activeItemAt(_incomingItems, itemIndex);
     final Animation<double> animation = incomingItem?.controller?.view ?? kAlwaysCompleteAnimation;
     return widget.itemBuilder(context, _itemIndexToIndex(itemIndex), animation);
   }

   @override
   Widget build(BuildContext context) {
     return new ListView.builder(
       itemBuilder: _itemBuilder,
       itemCount: _itemsCount,
       scrollDirection: widget.scrollDirection,
       reverse: widget.reverse,
       controller: widget.controller,
       primary: widget.primary,
       physics: widget.physics,
       shrinkWrap: widget.shrinkWrap,
       padding: widget.padding,
     );
   }
 }
	// Copyright 2017 The Chromium 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 'package:collection/collection.dart' show binarySearch;

	import 'package:flutter/animation.dart';

	import 'basic.dart';
	import 'framework.dart';
	import 'scroll_controller.dart';
	import 'scroll_physics.dart';
	import 'scroll_view.dart';
	import 'ticker_provider.dart';

	/// Signature for the builder callback used by [AnimatedList].
	typedef Widget AnimatedListItemBuilder(BuildContext context, int index, Animation<double> animation);

	/// Signature for the builder callback used by [AnimatedListState.removeItem].
	typedef Widget AnimatedListRemovedItemBuilder(BuildContext context, Animation<double> animation);

	// The default insert/remove animation duration.
	const Duration _kDuration = const Duration(milliseconds: 300);

	// Incoming and outgoing AnimatedList 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;
	final AnimatedListRemovedItemBuilder removedItemBuilder;
	int itemIndex;

	@override
	int compareTo(_ActiveItem other) => itemIndex - other.itemIndex;
	}

	/// A scrolling container that animates items when they are inserted or removed.
	///
	/// This widget's [AnimatedListState] can be used to dynamically insert or remove
	/// items. To refer to the [AnimatedListState] either provide a [GlobalKey] or
	/// use the static [of] method from an item's input callback.
	///
	/// This widget is similar to one created by [ListView.builder].
	class AnimatedList extends StatefulWidget {
	/// Creates a scrolling container that animates items when they are inserted or removed.
	const AnimatedList({
	Key key,
	@required this.itemBuilder,
	this.initialItemCount: 0,
	this.scrollDirection: Axis.vertical,
	this.reverse: false,
	this.controller,
	this.primary,
	this.physics,
	this.shrinkWrap: false,
	this.padding,
	}) : assert(itemBuilder != null),
	assert(initialItemCount != null && initialItemCount >= 0),
	super(key: key);

	/// Called, as needed, to build list item widgets.
	///
	/// List items are only built when they're scrolled into view.
	///
	/// The [AnimatedListItemBuilder] index parameter indicates the item's
	/// position in the list. The value of the index parameter will be between 0
	/// and [initialItemCount] plus the total number of items that have been
	/// inserted with [AnimatedListState.insertItem] and less the total number of
	/// items that have been removed with [AnimatedListState.removeItem].
	///
	/// Implementations of this callback should assume that
	/// [AnimatedListState.removeItem] removes an item immediately.
	final AnimatedListItemBuilder itemBuilder;

	/// The number of items the list will start with.
	///
	/// The appearance of the initial items is not animated. They
	/// are created, as needed, by [itemBuilder] with an animation parameter
	/// of [kAlwaysCompleteAnimation].
	final int initialItemCount;

	/// The axis along which the scroll view scrolls.
	///
	/// Defaults to [Axis.vertical].
	final Axis scrollDirection;

	/// 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;

	/// An object that can be used to control the position to which this scroll
	/// view is scrolled.
	///
	/// Must be null if [primary] is true.
	///
	/// A [ScrollController] serves several purposes. It can be used to control
	/// the initial scroll position (see [ScrollController.initialScrollOffset]).
	/// It can be used to control whether the scroll view should automatically
	/// save and restore its scroll position in the [PageStorage] (see
	/// [ScrollController.keepScrollOffset]). It can be used to read the current
	/// scroll position (see [ScrollController.offset]), or change it (see
	/// [ScrollController.animateTo]).
	final ScrollController controller;

	/// Whether this is the primary scroll view associated with the parent
	/// [PrimaryScrollController].
	///
	/// On iOS, this identifies the scroll view that will scroll to top in
	/// response to a tap in the status bar.
	///
	/// Defaults to true when [scrollDirection] is [Axis.vertical] and
	/// [controller] is null.
	final bool primary;

	/// How the scroll view should respond to user input.
	///
	/// For example, determines how the scroll view continues to animate after the
	/// user stops dragging the scroll view.
	///
	/// Defaults to matching platform conventions.
	final ScrollPhysics physics;

	/// Whether the extent of the scroll view in the [scrollDirection] should be
	/// determined by the contents being viewed.
	///
	/// If the scroll view does not shrink wrap, then the scroll view will expand
	/// to the maximum allowed size in the [scrollDirection]. If the scroll view
	/// has unbounded constraints in the [scrollDirection], then [shrinkWrap] must
	/// be true.
	///
	/// Shrink wrapping the content of the scroll view is significantly more
	/// expensive than expanding to the maximum allowed size because the content
	/// can expand and contract during scrolling, which means the size of the
	/// scroll view needs to be recomputed whenever the scroll position changes.
	///
	/// Defaults to false.
	final bool shrinkWrap;

	/// The amount of space by which to inset the children.
	final EdgeInsetsGeometry padding;

	/// The state from the closest instance of this class that encloses the given context.
	///
	/// This method is typically used by [AnimatedList] item widgets that insert or
	/// remove items in response to user input.
	///
	/// ```dart
	/// AnimatedListState animatedList = AnimatedList.of(context);
	/// ```
	static AnimatedListState of(BuildContext context, { bool nullOk: false }) {
	assert(context != null);
	assert(nullOk != null);
	final AnimatedListState result = context.ancestorStateOfType(const TypeMatcher<AnimatedListState>());
	if (nullOk \|\| result != null)
	return result;
	throw new FlutterError(
	'AnimatedList.of() called with a context that does not contain a AnimatedList.\n'
	'No AnimatedList ancestor could be found starting from the context that was passed to AnimatedList.of(). '
	'This can happen when the context provided is from the same StatefulWidget that '
	'built the AnimatedList. Please see the AnimatedList documentation for examples '
	'of how to refer to an AnimatedListState object: '
	' https://docs.flutter.io/flutter/widgets/AnimatedListState-class.html \n'
	'The context used was:\n'
	' $context'
	);
	}

	@override
	AnimatedListState createState() => new AnimatedListState();
	}

	/// The state for a scrolling container that animates items when they are
	/// inserted or removed.
	///
	/// When an item is inserted with [insertItem] an animation begins running. The
	/// animation is passed to [AnimatedList.itemBuilder] whenever the item's widget
	/// is needed.
	///
	/// When an item is removed with [removeItem] its animation is reversed.
	/// The removed item's animation is passed to the [removeItem] builder
	/// parameter.
	///
	/// An app that needs to insert or remove items in response to an event
	/// can refer to the [AnimatedList]'s state with a global key:
	///
	/// ```dart
	/// GlobalKey<AnimatedListState> listKey = new GlobalKey<AnimatedListState>();
	/// ...
	/// new AnimatedList(key: listKey, ...);
	/// ...
	/// listKey.currentState.insert(123);
	/// ```
	///
	/// [AnimatedList] item input handlers can also refer to their [AnimatedListState]
	/// with the static [AnimatedList.of] method.
	class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixin {
	final List<_ActiveItem> _incomingItems = <_ActiveItem>[];
	final List<_ActiveItem> _outgoingItems = <_ActiveItem>[];
	int _itemsCount = 0;

	@override
	void initState() {
	super.initState();
	_itemsCount = widget.initialItemCount;
	}

	@override
	void dispose() {
	for (_ActiveItem item in _incomingItems)
	item.controller.dispose();
	for (_ActiveItem item in _outgoingItems)
	item.controller.dispose();
	super.dispose();
	}

	_ActiveItem _removeActiveItemAt(List<_ActiveItem> items, int itemIndex) {
	final int i = binarySearch(items, new _ActiveItem.index(itemIndex));
	return i == -1 ? null : items.removeAt(i);
	}

	_ActiveItem _activeItemAt(List<_ActiveItem> items, int itemIndex) {
	final int i = binarySearch(items, new _ActiveItem.index(itemIndex));
	return i == -1 ? null : items[i];
	}

	// The insertItem() and removeItem() index parameters are defined as if the
	// removeItem() operation removed the corresponding list entry immediately.
	// The entry is only actually removed from the ListView when the remove animation
	// finishes. The entry is added to _outgoingItems when removeItem is called
	// and removed from _outgoingItems when the remove animation finishes.

	int _indexToItemIndex(int index) {
	int itemIndex = index;
	for (_ActiveItem item in _outgoingItems) {
	if (item.itemIndex <= itemIndex)
	itemIndex += 1;
	else
	break;
	}
	return itemIndex;
	}

	int _itemIndexToIndex(int itemIndex) {
	int index = itemIndex;
	for (_ActiveItem item in _outgoingItems) {
	assert(item.itemIndex != itemIndex);
	if (item.itemIndex < itemIndex)
	index -= 1;
	else
	break;
	}
	return index;
	}

	/// Insert an item at [index] and start an animation that will be passed
	/// to [AnimatedList.itemBuilder] when the item is visible.
	///
	/// This method's semantics are the same as Dart's [List.insert] method:
	/// it increases the length of the list by one and shifts all items at or
	/// after [index] towards the end of the list.
	void insertItem(int index, { Duration duration: _kDuration }) {
	assert(index != null && index >= 0);
	assert(duration != null);

	final int itemIndex = _indexToItemIndex(index);
	assert(itemIndex >= 0 && itemIndex <= _itemsCount);

	// Increment the incoming and outgoing item indices to account
	// for the insertion.
	for (_ActiveItem item in _incomingItems) {
	if (item.itemIndex >= itemIndex)
	item.itemIndex += 1;
	}
	for (_ActiveItem item in _outgoingItems) {
	if (item.itemIndex >= itemIndex)
	item.itemIndex += 1;
	}

	final AnimationController controller = new AnimationController(duration: duration, vsync: this);
	final _ActiveItem incomingItem = new _ActiveItem.incoming(controller, itemIndex);
	setState(() {
	_incomingItems
	..add(incomingItem)
	..sort();
	_itemsCount += 1;
	});

	controller.forward().then<void>((Null value) {
	_removeActiveItemAt(_incomingItems, incomingItem.itemIndex).controller.dispose();
	});
	}

	/// Remove the item at [index] and start an animation that will be passed
	/// to [builder] when the item is visible.
	///
	/// Items are removed immediately. After an item has been removed, its index
	/// will no longer be passed to the [AnimatedList.itemBuilder]. However the
	/// item will still appear in the list for [duration] and during that time
	/// [builder] must construct its widget as needed.
	///
	/// This method's semantics are the same as Dart's [List.remove] method:
	/// it decreases the length of the list by one and shifts all items at or
	/// before [index] towards the beginning of the list.
	void removeItem(int index, AnimatedListRemovedItemBuilder builder, { Duration duration: _kDuration }) {
	assert(index != null && index >= 0);
	assert(builder != null);
	assert(duration != null);

	final int itemIndex = _indexToItemIndex(index);
	assert(itemIndex >= 0 && itemIndex < _itemsCount);
	assert(_activeItemAt(_outgoingItems, itemIndex) == null);

	final _ActiveItem incomingItem = _removeActiveItemAt(_incomingItems, itemIndex);
	final AnimationController controller = incomingItem?.controller
	?? new AnimationController(duration: duration, value: 1.0, vsync: this);
	final _ActiveItem outgoingItem = new _ActiveItem.outgoing(controller, itemIndex, builder);
	setState(() {
	_outgoingItems
	..add(outgoingItem)
	..sort();
	});

	controller.reverse().then<void>((Null value) {
	_removeActiveItemAt(_outgoingItems, outgoingItem.itemIndex).controller.dispose();

	// Decrement the incoming and outgoing item indices to account
	// for the removal.
	for (_ActiveItem item in _incomingItems) {
	if (item.itemIndex > outgoingItem.itemIndex)
	item.itemIndex -= 1;
	}
	for (_ActiveItem item in _outgoingItems) {
	if (item.itemIndex > outgoingItem.itemIndex)
	item.itemIndex -= 1;
	}

	setState(() {
	_itemsCount -= 1;
	});
	});
	}

	Widget _itemBuilder(BuildContext context, int itemIndex) {
	final _ActiveItem outgoingItem = _activeItemAt(_outgoingItems, itemIndex);
	if (outgoingItem != null)
	return outgoingItem.removedItemBuilder(context, outgoingItem.controller.view);

	final _ActiveItem incomingItem = _activeItemAt(_incomingItems, itemIndex);
	final Animation<double> animation = incomingItem?.controller?.view ?? kAlwaysCompleteAnimation;
	return widget.itemBuilder(context, _itemIndexToIndex(itemIndex), animation);
	}

	@override
	Widget build(BuildContext context) {
	return new ListView.builder(
	itemBuilder: _itemBuilder,
	itemCount: _itemsCount,
	scrollDirection: widget.scrollDirection,
	reverse: widget.reverse,
	controller: widget.controller,
	primary: widget.primary,
	physics: widget.physics,
	shrinkWrap: widget.shrinkWrap,
	padding: widget.padding,
	);
	}
	}