Flutter MQTT通信插件mqtt_client的使用

简介

mqtt_client 是一个用于Dart的MQTT客户端库，支持服务器和浏览器端。它实现了MQTT v3（3.1 和 3.1.1）协议，并且可以处理订阅、发布消息的所有QOS级别、保持连接活跃以及同步连接。该客户端旨在尽可能多地处理MQTT协议的工作，使得用户能够专注于发布和订阅操作，而无需深入了解MQTT协议的具体细节。

此客户端已成功与多个主要云提供商的IoT/MQTT平台（如Google IOT Core、Amazon AWS、Microsoft Azure和IBM）以及公共可用的代理（如Mosquitto）和专有代理一起使用。

安装

iOS

如果你在Flutter环境中使用此客户端于Android或iOS设备上，则需要设置以下设备权限：

在 ios/Runner/Info.plist 文件中添加如下键值：

<key>NSLocalNetworkUsageDescription</key>
<string>Looking for local tcp Bonjour service</string>
<key>NSBonjourServices</key>
<array>
  <string>_mqtt._tcp</string>
</array>

Android

在 android/app/src/main/AndroidManifest.xml 文件中添加如下权限：

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

示例代码

下面是一个简单的示例，展示了如何配置一个基本的服务器端MQTT客户端。此示例基于官方提供的 mqtt_server_client.dart 示例改编而来。

完整的示例Demo

添加依赖

首先，在你的pubspec.yaml文件中添加mqtt_client依赖：

dependencies:
  mqtt_client: ^9.0.0

创建MQTT客户端并连接到代理

创建一个新的Dart文件，例如main.dart，并在其中编写以下代码：

import 'package:flutter/material.dart';
import 'package:mqtt_client/mqtt_client.dart';
import 'package:mqtt_client/mqtt_server_client.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'MQTT Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MqttHomePage(),
    );
  }
}

class MqttHomePage extends StatefulWidget {
  @override
  _MqttHomePageState createState() => _MqttHomePageState();
}

class _MqttHomePageState extends State<MqttHomePage> {
  final MqttServerClient client = MqttServerClient('broker.hivemq.com', '');
  String receivedMessage = '';

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

  void connect() async {
    client.port = 1883;

    /// 设置连接丢失后的回调函数
    client.onDisconnected = onDisconnected;
    
    /// 设置连接成功的回调函数
    client.onConnected = onConnected;
    
    /// 设置收到消息的回调函数
    client.onSubscribed = onSubscribed;

    final connMess = MqttConnectMessage()
        .withClientIdentifier('flutter_mqtt_client')
        .startClean(); // 清除会话
    print('EXAMPLE::Mosquitto client connecting....');
    client.connectionMessage = connMess;

    try {
      await client.connect();
    } catch (e) {
      print('EXAMPLE::client exception - $e');
      client.disconnect();
    }

    if (client.connectionStatus!.state == MqttConnectionState.connected) {
      print('EXAMPLE::Mosquitto client connected');
    } else {
      print(
          'EXAMPLE::ERROR Mosquitto client connection failed - disconnecting, status is ${client.connectionStatus}');
      client.disconnect();
    }
  }

  void onSubscribed(String topic) {
    print('EXAMPLE::Subscription confirmed for topic $topic');
  }

  void onDisconnected() {
    print('EXAMPLE::OnDisconnected client callback - Client disconnection');
    if (client.connectionStatus!.disconnectionOrigin ==
        MqttDisconnectionOrigin.solicited) {
      print('EXAMPLE::OnDisconnected callback is solicited, client state is');
    } else {
      print('EXAMPLE::OnDisconnected callback is unsolicited or none, client state is');
    }
  }

  void onConnected() {
    print('EXAMPLE::OnConnected client callback - Client connection was sucessful');

    /// 订阅主题
    const topic = 'flutter/test'; // 替换为你要订阅的主题
    client.subscribe(topic, MqttQos.atMostOnce);

    /// 发布消息
    final builder = MqttClientPayloadBuilder();
    builder.addString('Hello from Flutter MQTT client');

    client.publishMessage(topic, MqttQos.exactlyOnce, builder.payload!);

    /// 监听消息
    client.updates!.listen((List<MqttReceivedMessage<MqttMessage?>>? c) {
      final recMess = c![0].payload as MqttPublishMessage;
      final pt =
          MqttPublishPayload.bytesToStringAsString(recMess.payload.message);
      print(
          'EXAMPLE::Change notification:: topic is <${c[0].topic}>, payload is <-- $pt -->');
      setState(() {
        receivedMessage = 'Received message: $pt';
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('MQTT Demo'),
      ),
      body: Center(
        child: Text(receivedMessage),
      ),
    );
  }
}

这个例子展示了如何建立一个MQTT客户端，连接到一个MQTT代理（在这个例子中是HiveMQ的公共测试代理），订阅一个主题，并监听来自该主题的消息。同时，它也展示了如何向特定主题发布消息。

请根据实际需求修改代理地址、端口、主题名称等参数。

通过以上步骤，你应该能够在Flutter应用中成功地集成和使用mqtt_client插件进行MQTT通信了。