在 StreamBuilder 中使用 AnimatedList

我正在使用 firebase 构建一个聊天应用程序，目前将每条消息作为文档存储在 firebase 的集合中。我使用 StreamBuilder 来获取最新消息并显示它们。我想在接收和发送新消息时添加动画。我尝试过使用 Animatedlist，但是，我不知道如何使其与 StreamBuilder 一起使用。据我了解，我必须致电插入项目每次添加新消息时都会起作用。有更聪明的方法吗？或者说这将如何实施？

这是我到目前为止所拥有的：

class Message {
  final String uid;
  final String message;
  final Timestamp timestamp;

  Message({this.uid, this.timestamp, this.message});
}

class MessagesWidget extends StatefulWidget {
  final String receiver;
  MessagesWidget({@required this.receiver});

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

class _MessagesWidgetState extends State<MessagesWidget>{
  final GlobalKey<AnimatedListState> _listKey = GlobalKey<AnimatedListState>();

  Tween<Offset> _offset = Tween(begin: Offset(1,0), end: Offset(0,0));

  @override
  Widget build(BuildContext context) {
    final user = Provider.of<User>(context);
    return Container(
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.center,
        mainAxisSize: MainAxisSize.max,
        mainAxisAlignment: MainAxisAlignment.end,
        children: <Widget>[
          Expanded(
            child: StreamBuilder<List<Message>>(
                stream: DatabaseService(uid: user.uid).getMessages(widget.receiver),
                builder: (context, snapshot) {
                  switch (snapshot.connectionState) {
                    case ConnectionState.waiting:
                      return Loading();
                    default:
                      final messages = snapshot.data;
                      return messages.isEmpty
                          ? SayHi(userID: widget.receiver,)
                          : AnimatedList(
                              key: _listKey,
                              physics: BouncingScrollPhysics(),
                              reverse: true,
                              initialItemCount: messages.length,
                              itemBuilder: (context, index, animation) {
                                final message = messages[index];
                                return SlideTransition(
                                    position: animation.drive(_offset),
                                    child: MessageWidget(
                                    message: message,
                                    userID: widget.receiver,
                                    isCurrentUser: message.uid == user.uid,
                                  ),
                                );
                              },
                            );
                  }
                }),
          ),
          SizedBox(
            height: 10,
          ),
          NewMessage(
            receiver: widget.receiver,
          )
        ],
      ),
    );
  }
}```

您可以更新您的小部件State对此如下：

class _MessagesWidgetState extends State<MessagesWidget> {
  final GlobalKey<AnimatedListState> _listKey = GlobalKey<AnimatedListState>();

  Tween<Offset> _offset = Tween(begin: Offset(1, 0), end: Offset(0, 0));

  Stream<List<Message>> stream;

  List<Message> currentMessageList = [];

  User user;

  @override
  void initState() {
    super.initState();

    user = Provider.of<User>(context, listen: false);

    stream = DatabaseService(uid: user.uid).getMessages(widget.receiver);

    stream.listen((newMessages) {
      final List<Message> messageList = newMessages;

      if (_listKey.currentState != null &&
          _listKey.currentState.widget.initialItemCount < messageList.length) {
        List<Message> updateList =
            messageList.where((e) => !currentMessageList.contains(e)).toList();

        for (var update in updateList) {
          final int updateIndex = messageList.indexOf(update);
          _listKey.currentState.insertItem(updateIndex);
        }
      }

      currentMessageList = messageList;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.center,
        mainAxisSize: MainAxisSize.max,
        mainAxisAlignment: MainAxisAlignment.end,
        children: <Widget>[
          Expanded(
            child: StreamBuilder<List<Message>>(
                stream: stream,
                builder: (context, snapshot) {
                  switch (snapshot.connectionState) {
                    case ConnectionState.waiting:
                      return Loading();
                    default:
                      final messages = snapshot.data;
                      return messages.isEmpty
                          ? SayHi(
                              userID: widget.receiver,
                            )
                          : AnimatedList(
                              key: _listKey,
                              physics: BouncingScrollPhysics(),
                              reverse: true,
                              initialItemCount: messages.length,
                              itemBuilder: (context, index, animation) {
                                final message = messages[index];
                                return SlideTransition(
                                  position: animation.drive(_offset),
                                  child: MessageWidget(
                                    message: message,
                                    userID: widget.receiver,
                                    isCurrentUser: message.uid == user.uid,
                                  ),
                                );
                              },
                            );
                  }
                }),
          ),
          SizedBox(
            height: 10,
          ),
          NewMessage(
            receiver: widget.receiver,
          )
        ],
      ),
    );
  }
}

另外，更新您的Message类如下代码：

// Using the equatable package, remember to add it to your pubspec.yaml file
import 'package:equatable/equatable.dart';

class Message extends Equatable{
  final String uid;
  final String message;
  final Timestamp timestamp;

  Message({this.uid, this.timestamp, this.message});

  @override
  List<Object> get props => [uid, message, timestamp];
}

解释：

The State上面的代码执行以下操作：

1. 它将当前消息存储在列表中currentMessageList在构建方法之外
2. 它监听流以获取新消息，并将新列表与中的前一个列表进行比较currentMessageList.
3. 它获取两个列表之间的差异并循环更新AnimatedList特定索引处的小部件updateIndex.

The Message上面的代码执行以下操作：

• 它覆盖了==操作符和对象hashcode允许检查这一行：List<Message> updateList = messageList.where((e) => !currentMessageList.contains(e)).toList();按预期工作。 [如果不重写这些吸气剂，检查将失败，因为两个不同的Message具有相同值的对象不会是等效的]。
• 它使用可等同的封装以避免样板文件。