Flutter MQTT通信（实现聊天功能）

import 'package:flutter/material.dart';
import 'package:mqtt_client/mqtt_client.dart';
 
class ChatPage extends StatefulWidget {
  @override
  _ChatPageState createState() => _ChatPageState();
}
 
class _ChatPageState extends State<ChatPage> {
  final MqttClient client = MqttClient('mqtt://your_broker_address');
  String currentMessage = '';
 
  @override
  void initState() {
    super.initState();
    client.onConnected = onConnected;
    client.onDisconnected = onDisconnected;
    client.onSubscribed = onSubscribed;
    client.onUnsubscribed = onUnsubscribed;
    client.onMessage = onMessage;
    connect();
  }
 
  void connect() async {
    final connMessage = MqttConnectMessage()
        .authenticateAs('yourUsername', 'yourPassword')
        .withClientIdentifier('yourClientId')
        .startClean();
    client.connectionMessage = connMessage;
 
    try {
      await client.connect();
    } catch (e) {
      print('Exception: $e');
      client.disconnect();
    }
  }
 
  void onConnected() {
    print('Connected');
    client.subscribe('chat_topic', MqttQos.atLeastOnce);
  }
 
  void onDisconnected() {
    print('Disconnected');
  }
 
  void onSubscribed(String topic) {
    print('Subscribed topic: $topic');
  }
 
  void onUnsubscribed(String topic) {
    print('Unsubscribed topic: $topic');
  }
 
  void onMessage(MqttReceivedMessage message) {
    print('Received message: ${message.payloadToString()}');
  }
 
  void publishMessage(String message) {
    client.publishMessage('chat_topic', MqttQos.atLeastOnce, message);
  }
 
  @override
  void dispose() {
    client.disconnect();
    super.dispose();
  }
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Chat'),
      ),
      body: Container(
        child: Column(
          children: <Widget>[
            Expanded(
              child: ListView.builder(
                reverse: true,
                itemCount: null, // TODO: calculate item count
                itemBuilder: (context, index) {
                  // TODO: build list item for each message
                  return ListTile(
                    title: Text('Message'),