帮我用 flutter 写一个聊天页面

好的，以下是一个简单的聊天页面示例代码：

import 'package:flutter/material.dart';

class ChatPage extends StatefulWidget {
  @override
  _ChatPageState createState() => _ChatPageState();
}

class _ChatPageState extends State<ChatPage> {
  final TextEditingController _textController = TextEditingController();
  final List<ChatMessage> _messages = [];

  void _handleSubmitted(String text) {
    _textController.clear();
    ChatMessage message = ChatMessage(
      text: text,
      isMe: true,
    );
    setState(() {
      _messages.insert(0, message);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Chat'),
      ),
      body: Column(
        children: <Widget>[
          Flexible(
            child: ListView.builder(
              padding: EdgeInsets.all(8.0),
              reverse: true,
              itemCount: _messages.length,
              itemBuilder: (_, int index) => _messages[index],
            ),
          ),
          Divider(height: 1.0),
          Container(
            decoration: BoxDecoration(color: Theme.of(context).cardColor),
            child: _buildTextComposer(),
          ),
        ],
      ),
    );
  }

  Widget _buildTextComposer() {
    return IconTheme(
      data: IconThemeData(color: Theme.of(context).accentColor),
      child: Container(
        margin: EdgeInsets.symmetric(horizontal: 8.0),
        child: Row(
          children: <Widget>[
            Flexible(
              child: TextField(
                controller: _textController,
                onSubmitted: _handleSubmitted,
                decoration:
                    InputDecoration.collapsed(hintText: 'Send a message'),
              ),
            ),
            Container(
              margin: EdgeInsets.symmetric(horizontal: 4.0),
              child: IconButton(
                icon: Icon(Icons.send),
                onPressed: () => _handleSubmitted(_textController.text),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class ChatMessage extends StatelessWidget {
  ChatMessage({this.text, this.isMe});

  final String text;
  final bool isMe;

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.symmetric(vertical: 10.0),
      child: Row(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          Container(
            margin: EdgeInsets.only(right: 16.0),
            child: CircleAvatar(child: Text('User')),
          ),
          Expanded(
            child: Column(
              crossAxisAlignment:
                  isMe ? CrossAxisAlignment.end : CrossAxisAlignment.start,
              children: <Widget>[
                Text('User', style: Theme.of(context).textTheme.subtitle1),
                Container(
                  margin: EdgeInsets.only(top: 5.0),
                  child: Text(text),
                ),
              ],
            ),
          ),
        ],
      ),
    );
  }
}

在这个示例中，我们使用了 `TextField` 和 `ListView` 来实现聊天界面。当用户在文本框中输入消息并按下发送按钮时，将创建一个新的 `ChatMessage` 对象并添加到 `_messages` 列表中。`ListView` 将会自动更新以显示新消息。最后，我们使用 `CircleAvatar` 和 `Text` 来显示用户的头像和名称。

你可以将这个代码复制到你的 Flutter 项目中并运行，然后你就可以在应用中查看一个简单的聊天界面了。