当然，下面是一个关于如何使用Flutter的MQTT通信插件mqtt_client（注意：mqtt_helper并非一个广泛认知的官方或主流Flutter插件，而mqtt_client是较为常用的MQTT客户端库）的示例代码。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，确保在pubspec.yaml文件中添加mqtt_client依赖：

dependencies:
  flutter:
    sdk: flutter
  mqtt_client: ^x.y.z  # 替换为最新版本号

然后，运行flutter pub get来获取依赖。

接下来是一个完整的Flutter应用示例，展示如何使用mqtt_client：

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 Client Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  MqttClient? _client;
  String _connectionStatus = 'Disconnected';
  String _subscriptionMessage = '';

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

  void connectToMqttBroker() async {
    final String broker = 'mqtt.eclipse.org'; // MQTT代理地址
    final int port = 1883; // MQTT代理端口
    final String clientId = 'FlutterMqttClient-' + DateTime.now().millisecondsSinceEpoch.toString();

    _client = MqttServerClient(broker, clientId);
    _client!.port = port;
    _client!.keepAlivePeriod = 60;
    _client!.protocolVersion = MqttProtocolVersion.v311;

    final MqttConnectOptions connOpts = MqttConnectOptions.builder()
        .setClientId(clientId)
        .setCleanSession()
        .setKeepAliveInterval(20)
        .setWillTopic('willtopic') // 遗嘱主题
        .setWillMessage('My device is offline') // 遗嘱消息
        .setWillQos(MqttQos.atLeastOnce)
        .setAutomaticReconnect()
        .build();

    try {
      await _client!.connect(connOpts).timeout(Duration(seconds: 30), onTimeout: () {
        throw Exception('Connection timed out');
      });
      setState(() {
        _connectionStatus = 'Connected';
      });
      _client!.updates!.listen((List<MqttReceivedMessage<MqttMessage>>? c) async {
        final MqttPublishMessage? pubMsg = c![0].payload as MqttPublishMessage?;
        final String? pt = pubMsg!.payloadAsString;
        setState(() {
          _subscriptionMessage = pt ?? 'null';
        });
      });
      await _client!.subscribe('test/topic', MqttQos.atLeastOnce);
    } catch (e) {
      print(e);
      setState(() {
        _connectionStatus = 'Disconnected with error: ${e.message ?? ''}';
      });
    }
  }

  void publishMessage() async {
    if (_client!.isConnected()) {
      final MqttPublishMessage pubMsg = MqttPublishMessage.builder()
          .withTopic('test/topic')
          .withQos(MqttQos.atLeastOnce)
          .withPayload('Hello MQTT')
          .build();
      _client!.publishMessage(pubMsg);
    } else {
      print('Client is not connected');
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('MQTT Client Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('Connection Status: $_connectionStatus'),
            SizedBox(height: 20),
            Text('Subscription Message: $_subscriptionMessage'),
            SizedBox(height: 20),
            ElevatedButton(
              onPressed: publishMessage,
              child: Text('Publish Message'),
            ),
          ],
        ),
      ),
    );
  }

  @override
  void dispose() {
    _client?.disconnect();
    super.dispose();
  }
}

代码解释：

1. 依赖项：在pubspec.yaml文件中添加mqtt_client依赖。
2. 主应用：创建了一个简单的Flutter应用，包含一个文本显示连接状态、订阅消息和一个按钮用于发布消息。
3. 连接到MQTT代理：在initState方法中调用connectToMqttBroker函数，该函数负责连接到MQTT代理，设置连接选项，并订阅一个主题。
4. 消息监听：使用_client!.updates!.listen来监听传入的消息，并更新UI显示收到的消息。
5. 发布消息：publishMessage函数用于发布消息到订阅的主题。
6. 资源清理：在dispose方法中断开与MQTT代理的连接。

这个示例提供了一个基本的MQTT客户端功能，可以根据需要进行扩展和修改。