Question

An AnimatedList that displays a list of cards that stay in sync with an app-specific ListModel. When an item is added to or removed from the model, the corresponding card animates in or out of view.

Tap an item to select it, tap it again to unselect. Tap ‘+’ to insert at the selected item, ‘-‘ to remove the selected item. The tap handlers add or remove items from a ListModel<E>, a simple encapsulation of List<E> that keeps the AnimatedList in sync. The list model has a GlobalKey for its animated list. It uses the key to call the insertItem and removeItem methods defined by AnimatedListState.

Try this app out by creating a new project with flutter create and replacing the contents of lib/main.dart with the following code.

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

class AnimatedListSample extends StatefulWidget {
  @override
  _AnimatedListSampleState createState() => _AnimatedListSampleState();
}

class _AnimatedListSampleState extends State<AnimatedListSample> {
  final GlobalKey<AnimatedListState> _listKey = GlobalKey<AnimatedListState>();
  ListModel<int> _list;
  int _selectedItem;
  int _nextItem; // The next item inserted when the user presses the '+' button.

  @override
  void initState() {
    super.initState();
    _list = ListModel<int>(
      listKey: _listKey,
      initialItems: <int>[0, 1, 2],
      removedItemBuilder: _buildRemovedItem,
    );
    _nextItem = 3;
  }

  // Used to build list items that haven't been removed.
  Widget _buildItem(
      BuildContext context, int index, Animation<double> animation) {
    return CardItem(
      animation: animation,
      item: _list[index],
      selected: _selectedItem == _list[index],
      onTap: () {
        setState(() {
          _selectedItem = _selectedItem == _list[index] ? null : _list[index];
        });
      },
    );
  }

  // Used to build an item after it has been removed from the list. This method is
  // needed because a removed item remains  visible until its animation has
  // completed (even though it's gone as far this ListModel is concerned).
  // The widget will be used by the [AnimatedListState.removeItem] method's
  // [AnimatedListRemovedItemBuilder] parameter.
  Widget _buildRemovedItem(
      int item, BuildContext context, Animation<double> animation) {
    return CardItem(
      animation: animation,
      item: item,
      selected: false,
      // No gesture detector here: we don't want removed items to be interactive.
    );
  }

  // Insert the "next item" into the list model.
  void _insert() {
    final int index =
        _selectedItem == null ? _list.length : _list.indexOf(_selectedItem);
    _list.insert(index, _nextItem++);
  }

  // Remove the selected item from the list model.
  void _remove() {
    if (_selectedItem != null) {
      _list.removeAt(_list.indexOf(_selectedItem));
      setState(() {
        _selectedItem = null;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('AnimatedList'),
          actions: <Widget>[
            IconButton(
              icon: const Icon(Icons.add_circle),
              onPressed: _insert,
              tooltip: 'insert a new item',
            ),
            IconButton(
              icon: const Icon(Icons.remove_circle),
              onPressed: _remove,
              tooltip: 'remove the selected item',
            ),
          ],
        ),
        body: Padding(
          padding: const EdgeInsets.all(16.0),
          child: AnimatedList(
            key: _listKey,
            initialItemCount: _list.length,
            itemBuilder: _buildItem,
          ),
        ),
      ),
    );
  }
}

/// Keeps a Dart List in sync with an AnimatedList.
///
/// The [insert] and [removeAt] methods apply to both the internal list and the
/// animated list that belongs to [listKey].
///
/// This class only exposes as much of the Dart List API as is needed by the
/// sample app. More list methods are easily added, however methods that mutate the
/// list must make the same changes to the animated list in terms of
/// [AnimatedListState.insertItem] and [AnimatedList.removeItem].
class ListModel<E> {
  ListModel({
    @required this.listKey,
    @required this.removedItemBuilder,
    Iterable<E> initialItems,
  })  : assert(listKey != null),
        assert(removedItemBuilder != null),
        _items = List<E>.from(initialItems ?? <E>[]);

  final GlobalKey<AnimatedListState> listKey;
  final dynamic removedItemBuilder;
  final List<E> _items;

  AnimatedListState get _animatedList => listKey.currentState;

  void insert(int index, E item) {
    _items.insert(index, item);
    _animatedList.insertItem(index);
  }

  E removeAt(int index) {
    final E removedItem = _items.removeAt(index);
    if (removedItem != null) {
      _animatedList.removeItem(index,
          (BuildContext context, Animation<double> animation) {
        return removedItemBuilder(removedItem, context, animation);
      });
    }
    return removedItem;
  }

  int get length => _items.length;

  E operator [](int index) => _items[index];

  int indexOf(E item) => _items.indexOf(item);
}

/// Displays its integer item as 'item N' on a Card whose color is based on
/// the item's value. The text is displayed in bright green if selected is true.
/// This widget's height is based on the animation parameter, it varies
/// from 0 to 128 as the animation varies from 0.0 to 1.0.
class CardItem extends StatelessWidget {
  const CardItem(
      {Key key,
      @required this.animation,
      this.onTap,
      @required this.item,
      this.selected: false})
      : assert(animation != null),
        assert(item != null && item >= 0),
        assert(selected != null),
        super(key: key);

  final Animation<double> animation;
  final VoidCallback onTap;
  final int item;
  final bool selected;

  @override
  Widget build(BuildContext context) {
    TextStyle textStyle = Theme.of(context).textTheme.display1;
    if (selected)
      textStyle = textStyle.copyWith(color: Colors.lightGreenAccent[400]);
    return Padding(
      padding: const EdgeInsets.all(2.0),
      child: SizeTransition(
        axis: Axis.vertical,
        sizeFactor: animation,
        child: GestureDetector(
          behavior: HitTestBehavior.opaque,
          onTap: onTap,
          child: SizedBox(
            height: 128.0,
            child: Card(
              color: Colors.primaries[item % Colors.primaries.length],
              child: Center(
                child: Text('Item $item', style: textStyle),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

void main() {
  runApp(AnimatedListSample());
}

See also

• The Components-Lists: Controls section of the Material Design specification:
• The source code in examples/catalog/lib/animated_list.dart.