Flutter is Google’s UI toolkit for building beautiful, natively compiled applications for mobile, web, and desktop from a single codebase. Flutter provides a rich set of components and interfaces, the developer can quickly add native expansion for Flutter. At the same time, Flutter also uses a Native engine to render view. There is no doubt that it can provide a good experience for users.

MQTT is a lightweight IoT communication protocol based on the publish/subscribe model. It can enable stable transmission over severely restricted device hardware and high-latency / low-bandwidth network. Because it is simple and easy to implement, support for QoS, and small size of the packet, it occupies half market of the Internet of Things protocol.

This article mainly introduces how to use MQTT in the Flutter project to implement the connection between client and MQTT broker, subscribe, unsubscribe, send and receive messages and other functions.

Project initialization

Create a project

Create a new project, can refer to the following links:

Install dependencies

Add dependencies into file pubspec.yaml

dependencies: 
  mqtt_client: ^7.2.1

Install dependencies:

flutter pub get

Import

import 'package:mqtt_client/mqtt_client.dart';

Use of MQTT

Connect to MQTT broker

This article will use the MQTT broker which is operated and maintained by EMQX Cloud. EMQX Cloud is the MQTT IoT cloud service platform released by EMQ, it provides the service for accessing MQTT 5.0 with all-in-one operation and maintenance and unique isolation environment.

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

The example of connection

Future<MqttServerClient> connect() async {
  MqttServerClient client =
      MqttServerClient.withPort('broker.emqx.io', 'flutter_client', 1883);
  client.logging(on: true);
  client.onConnected = onConnected;
  client.onDisconnected = onDisconnected;
  client.onUnsubscribed = onUnsubscribed;
  client.onSubscribed = onSubscribed;
  client.onSubscribeFail = onSubscribeFail;
  client.pongCallback = pong;

  final connMessage = MqttConnectMessage()
      .authenticateAs('username', 'password')
      .keepAliveFor(60)
      .withWillTopic('willtopic')
      .withWillMessage('Will message')
      .startClean()
      .withWillQos(MqttQos.atLeastOnce);
  client.connectionMessage = connMessage;
  try {
    await client.connect();
  } catch (e) {
    print('Exception: $e');
    client.disconnect();
  }

  client.updates.listen((List<MqttReceivedMessage<MqttMessage>> c) {
    final MqttPublishMessage message = c[0].payload;
    final payload =
    MqttPublishPayload.bytesToStringAsString(message.payload.message);

    print('Received message:$payload from topic: ${c[0].topic}>');
  });

  return client;
}

The description of callback method

// connection succeeded
void onConnected() {
  print('Connected');
}

// unconnected
void onDisconnected() {
  print('Disconnected');
}

// subscribe to topic succeeded
void onSubscribed(String topic) {
  print('Subscribed topic: $topic');
}

// subscribe to topic failed
void onSubscribeFail(String topic) {
  print('Failed to subscribe $topic');
}

// unsubscribe succeeded
void onUnsubscribed(String topic) {
  print('Unsubscribed topic: $topic');
}

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

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

client.updates.listen: used for monitoring the arrival of messages of the subscribed topics

The example of certificate connection

/// 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;

Other MQTT operations

client.subscribe("topic/test", MqttQos.atLeastOnce)

Publish message

const pubTopic = 'topic/test';
final builder = MqttClientPayloadBuilder();
builder.addString('Hello MQTT');
client.publishMessage(pubTopic, MqttQos.atLeastOnce, builder.payload);

Unsubscribe

client.unsubscribe('topic/test');

Disconnect

client.disconnect();

Test

We write a simple UI interface for this project and use MQTT 5.0 client tool - MQTT X to do the following tests:

connect
subscribe
publish
unsubscribe
disconnect

The interface of the application:

Use MQTT X as another client to send and receive messages:

We can see the log of the whole process

Summary

So far, we have finished that use Flutter to build MQTT applications in the Android platform, implemented the connection between the client and MQTT broker, subscribe, unsubscribe, publish and receive messages, etc.

Flutter makes it easy that develop powerful mobile applications through unified programming language and the feature cross-platform. It may be the most proper solution for developing mobile applications in the future. Using Flutter, MQTT protocol and MQTT cloud service, we can develop more interesting applications.

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