Flutter MQTT通信插件kinnow_mqtt的使用

当然，下面是一个关于如何在Flutter项目中使用kinnow_mqtt插件来实现MQTT通信的代码示例。这个示例将展示如何连接到MQTT代理、订阅主题和发布消息。

首先，确保你已经在pubspec.yaml文件中添加了kinnow_mqtt依赖：

dependencies:
  flutter:
    sdk: flutter
  kinnow_mqtt: ^latest_version  # 替换为最新版本号

然后运行flutter pub get来安装依赖。

接下来是示例代码：

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

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  late MqttClient _client;
  String _clientId = 'flutterClient';
  String _broker = 'mqtt.eclipse.org'; // 公共MQTT代理地址
  int _port = 1883; // MQTT默认端口
  String _topic = 'test/topic';
  String _message = 'Hello MQTT';

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

  Future<void> connect() async {
    _client = MqttClient(_broker, _port, _clientId);
    _client.keepAliveInterval = 60;
    _client.logging(on: true);

    final MqttConnectOptions connOpts = MqttConnectOptions.builder()
        .clientIdentifier(_clientId)
        .cleanSession(true)
        .keepAliveInterval(60)
        .automaticReconnect(true)
        .withWillTopic(_topic)
        .withWillMessage('Client has disconnected unexpectedly')
        .withWillQos(MqttQos.atLeastOnce);

    try {
      await _client.connect(connOpts).then((_) {
        print('Connected');
        _client.updates!.listen((List<MqttReceivedMessage<MqttMessage>> c) {
          final MqttPublishMessage receivedMessage =
              c[0].payload as MqttPublishMessage;
          final MqttMessage receivedPayload = receivedMessage.payload;
          final String pt =
              MqttPublishPayload.bytesToStringAsString(receivedPayload.message,
                  encoding: MqttPayloadFormat.UTF8);
          print('Message received: topic is <${c[0].topic}>, payload is <-- $pt -->');
        });

        subscribe();
      }).catchError((e) {
        print('Exception has been thrown: $e');
        _client.disconnect();
      });
    } catch (e) {
      print('Exception has been thrown: $e');
      _client.disconnect();
    }
  }

  Future<void> subscribe() async {
    try {
      await _client.subscribe(_topic, MqttQos.atLeastOnce);
      print('Subscribed to topic: $_topic');
    } catch (e) {
      print('Exception has been thrown: $e');
    }
  }

  Future<void> publish() async {
    MqttMessageBuilder builder = MqttMessageBuilder();
    builder.addString(_message);
    MqttMessage message = builder.toMessage();
    MqttPublishMessage pubMsg = MqttPublishMessage.builder()
        .topic(_topic)
        .qos(MqttQos.atLeastOnce)
        .payload(message);

    _client.publishMessage(pubMsg);
    print('Message published');
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter MQTT Demo'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              ElevatedButton(
                onPressed: () {
                  publish();
                },
                child: Text('Publish Message'),
              ),
            ],
          ),
        ),
      ),
    );
  }

  @override
  void dispose() {
    // 清理MQTT客户端连接
    if (_client.isConnected()) {
      _client.disconnect();
    }
    super.dispose();
  }
}

这个示例代码做了以下几件事情：

1. 连接到MQTT代理：使用MqttClient类连接到MQTT代理（在这个例子中是mqtt.eclipse.org）。
2. 订阅主题：在成功连接到MQTT代理后，订阅一个主题（test/topic）。
3. 发布消息：通过点击按钮发布消息到订阅的主题。
4. 监听消息：监听并打印从MQTT代理接收到的消息。

请注意，kinnow_mqtt插件的具体API可能会有所变化，因此建议查阅最新的kinnow_mqtt文档（如果可用）以获取最新的使用信息和API参考。

此外，由于公共MQTT代理可能有连接限制，如果你在生产环境中使用MQTT，建议搭建自己的MQTT代理服务器。