How To Use MQTT in Dart

This article mainly introduces how to use the mqtt_client library in the Dart project to realize the connection, subscription, message sending and receiving between the client and MQTT Broker.

Zhiwei Yu

Dec. 02, 22 · Tutorial

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Dart is a client-optimized language for developing fast apps on any platform. Its goal is to offer the most productive programming language for multi-platform development, paired with a flexible execution runtime platform for app frameworks.

MQTT is a lightweight IoT messaging protocol based on publish/subscribe model, which can provide real-time and reliable messaging services for connected devices with minimal code and bandwidth. It is widely used in industries, such as IoT, mobile Internet, smart hardware, Internet of vehicles, and power and energy.

This article mainly introduces how to use the mqtt_client library in the Dart project to realize the connection, subscription, message sending, and receiving between the client and MQTT Broker.

Preparation

The examples in this article are based on the macOS environment.

Get the SDK

Please refer to Get the SDK

      Shell 
    
    $ brew tap dart-lang/dart
$ brew install dart
$ dart --version
Dart SDK version: 2.13.0 (stable) (Wed May 12 12:45:49 2021 +0200) on "macos_x64"

Initializing the Project

      Shell 
    
    $ dart create -t console-full mqtt_demo
$ cd mqtt_demo

The directory structure is as follows.

      Plain Text 
    
 
 
    ├── CHANGELOG.md
├── README.md
├── analysis_options.yaml
├── bin
│   └── mqtt_demo.dart
├── pubspec.lock
└── pubspec.yaml 
   

Installing Dependencies

In this article, we use mqtt_client as the MQTT client library and install it by running the following command.

      Shell 
    
    $ dart pub add mqtt_client

This will add a line like this to the project's pubspec.yaml file:

      YAML 
    
    dependencies:
  mqtt_client: ^9.6.2

Using MQTT

We will use the free public MQTT broker that was created based on the MQTT Cloud Service - EMQX Cloud. The server access information is as follows:

Broker: broker.emqx.io
TCP Port: 1883
Websocket Port: 8083

Connecting to the MQTT Server

Edit bin/mqtt_demo.dart file.

      Dart 
    
 
 
    import 'dart:async';
import 'dart:io';
import 'package:mqtt_client/mqtt_client.dart';
import 'package:mqtt_client/mqtt_server_client.dart';

final client = MqttServerClient('broker-cn.emqx.io', '1883');

Future<int> main() async {
  client.logging(on: true);
  client.keepAlivePeriod = 60;
  client.onDisconnected = onDisconnected;
  client.onConnected = onConnected;
  client.pongCallback = pong;

  final connMess = MqttConnectMessage()
      .withClientIdentifier('dart_client')
      .withWillTopic('willtopic') 
      .withWillMessage('My Will message')
      .startClean() 
      .withWillQos(MqttQos.atLeastOnce);
  print('client connecting....');
  client.connectionMessage = connMess;

  try {
    await client.connect();
  } on NoConnectionException catch (e) {
    print('client exception - $e');
    client.disconnect();
  } on SocketException catch (e) {
    print('socket exception - $e');
    client.disconnect();
  }

  if (client.connectionStatus!.state == MqttConnectionState.connected) {
    print('client connected');
  } else {
    print('client connection failed - disconnecting, status is ${client.connectionStatus}');
    client.disconnect();
    exit(-1);
  }
  return 0;
}


/// The unsolicited disconnect callback
void onDisconnected() {
  print('OnDisconnected client callback - Client disconnection');
  if (client.connectionStatus!.disconnectionOrigin ==
      MqttDisconnectionOrigin.solicited) {
    print('OnDisconnected callback is solicited, this is correct');
  }
  exit(-1);
}

/// The successful connect callback
void onConnected() {
  print('OnConnected client callback - Client connection was sucessful');
}

/// Pong callback
void pong() {
  print('Ping response client callback invoked');
} 
   

Then, execute

      Shell 
    
    $ dart run bin/mqtt_demo.dart

We will see that the client has successfully connected to the MQTT broker.

Instructions

MqttConnectMessage: set connection options, including timeout settings, authentication, and last-wish messages.

Example of Certificate Connection:

      Dart 
    
 
 
    /// Security context
SecurityContext context = new SecurityContext()
  ..useCertificateChain('path/to/my_cert.pem')
  ..usePrivateKey('path/to/my_key.pem', password: 'key_password')
  ..setClientAuthorities('path/to/client.crt', password: 'password');
client.secure = true;
client.securityContext = context; 
   

Add the following code.

      Dart 
    
 
 
    client.onSubscribed = onSubscribed;

const topic = 'topic/test';
print('Subscribing to the $topic topic');
client.subscribe(topic, MqttQos.atMostOnce);
client.updates!.listen((List<MqttReceivedMessage<MqttMessage?>>? c) {
  final recMess = c![0].payload as MqttPublishMessage;
  final pt = MqttPublishPayload.bytesToStringAsString(recMess.payload.message);
  print('Received message: topic is ${c[0].topic}, payload is $pt');
});

/// The subscribed callback
void onSubscribed(String topic) {
  print('Subscription confirmed for topic $topic');
} 
   

Then, execute

      Shell 
    
    $ dart run bin/mqtt_demo.dart

We see that we have successfully subscribed to the MQTT topic.

Publish Message

      Dart 
    
 
 
    client.published!.listen((MqttPublishMessage message) {
  print('Published topic: topic is ${message.variableHeader!.topicName}, with Qos ${message.header!.qos}');
});

const pubTopic = 'test/topic';
final builder = MqttClientPayloadBuilder();
builder.addString('Hello from mqtt_client');

print('Subscribing to the $pubTopic topic');
client.subscribe(pubTopic, MqttQos.exactlyOnce);

print('Publishing our topic');
client.publishMessage(pubTopic, MqttQos.exactlyOnce, builder.payload!); 
   

We see that the message has been published successfully and we receive it.

Complete Test

We use the following code for the complete test.

      Dart 
    
 
 
    import 'dart:async';
import 'dart:io';
import 'package:mqtt_client/mqtt_client.dart';
import 'package:mqtt_client/mqtt_server_client.dart';

final client = MqttServerClient('broker-cn.emqx.io', '1883');

Future<int> main() async {
  client.logging(on: false);
  client.keepAlivePeriod = 60;
  client.onDisconnected = onDisconnected;
  client.onConnected = onConnected;
  client.onSubscribed = onSubscribed;
  client.pongCallback = pong;

  final connMess = MqttConnectMessage()
      .withClientIdentifier('dart_client')
      .withWillTopic('willtopic') 
      .withWillMessage('My Will message')
      .startClean() 
      .withWillQos(MqttQos.atLeastOnce);
  print('Client connecting....');
  client.connectionMessage = connMess;

  try {
    await client.connect();
  } on NoConnectionException catch (e) {
    print('Client exception: $e');
    client.disconnect();
  } on SocketException catch (e) {
    print('Socket exception: $e');
    client.disconnect();
  }

  if (client.connectionStatus!.state == MqttConnectionState.connected) {
    print('Client connected');
  } else {
    print('Client connection failed - disconnecting, status is ${client.connectionStatus}');
    client.disconnect();
    exit(-1);
  }

  const subTopic = 'topic/sub_test';
  print('Subscribing to the $subTopic topic');
  client.subscribe(subTopic, MqttQos.atMostOnce);
  client.updates!.listen((List<MqttReceivedMessage<MqttMessage?>>? c) {
    final recMess = c![0].payload as MqttPublishMessage;
    final pt = MqttPublishPayload.bytesToStringAsString(recMess.payload.message);
    print('Received message: topic is ${c[0].topic}, payload is $pt');
  });

  client.published!.listen((MqttPublishMessage message) {
    print('Published topic: topic is ${message.variableHeader!.topicName}, with Qos ${message.header!.qos}');
  });

  const pubTopic = 'topic/pub_test';
  final builder = MqttClientPayloadBuilder();
  builder.addString('Hello from mqtt_client');

  print('Subscribing to the $pubTopic topic');
  client.subscribe(pubTopic, MqttQos.exactlyOnce);

  print('Publishing our topic');
  client.publishMessage(pubTopic, MqttQos.exactlyOnce, builder.payload!);

  print('Sleeping....');
  await MqttUtilities.asyncSleep(80);

  print('Unsubscribing');
  client.unsubscribe(subTopic);
  client.unsubscribe(pubTopic);

  await MqttUtilities.asyncSleep(2);
  print('Disconnecting');
  client.disconnect();

  return 0;
}

/// The subscribed callback
void onSubscribed(String topic) {
  print('Subscription confirmed for topic $topic');
}

/// The unsolicited disconnect callback
void onDisconnected() {
  print('OnDisconnected client callback - Client disconnection');
  if (client.connectionStatus!.disconnectionOrigin ==
      MqttDisconnectionOrigin.solicited) {
    print('OnDisconnected callback is solicited, this is correct');
  }
  exit(-1);
}

/// The successful connect callback
void onConnected() {
  print('OnConnected client callback - Client connection was sucessful');
}

/// Pong callback
void pong() {
  print('Ping response client callback invoked');
} 
   

Summary

Now we've finished connecting to the public MQTT server using the mqtt_client library in Dart and implemented the connection, message publishing, subscription, and test between the client and the MQTT server.

MQTT Dart (programming language)

Published at DZone with permission of Zhiwei Yu. See the original article here.

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