tablet-app/lib/Helpers/MQTTHelper.dart

import 'package:mqtt_client/mqtt_client.dart';
import 'package:mqtt_client/mqtt_server_client.dart';
import 'package:unique_identifier/unique_identifier.dart';

class MQTTHelper {
  MQTTHelper._privateConstructor();
  static final MQTTHelper instance = MQTTHelper._privateConstructor();

  void onConnected(dynamic tabletAppContext) {
    print('Connected !!!!!!!!!!!! ----------------------------------');

    tabletAppContext.clientMQTT.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}');
    });
  }

  // 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');
  }

  Future<MqttServerClient> connect(dynamic tabletAppContext) async {
    var identifier = await UniqueIdentifier.serial;
    tabletAppContext.clientMQTT = MqttServerClient.withPort(tabletAppContext.host.replaceAll('http://', ''), 'tablet_app_'+identifier, 1883);

    tabletAppContext.clientMQTT.logging(on: false);
    tabletAppContext.clientMQTT.keepAlivePeriod = 20;
    tabletAppContext.clientMQTT.onDisconnected = onDisconnected;
    tabletAppContext.clientMQTT.onConnected = () => onConnected(tabletAppContext);
    tabletAppContext.clientMQTT.onSubscribed = onSubscribed;

    final connMessage = MqttConnectMessage().authenticateAs('admin', 'mdlf2021!') // TODO ONLINE
        .keepAliveFor(60)
        .withWillTopic('willtopic')
        .withWillMessage('Will message')
        .withClientIdentifier('tablet_app_'+identifier)
        .startClean()
        .withWillQos(MqttQos.atLeastOnce);

    tabletAppContext.clientMQTT.connectionMessage = connMessage;
    tabletAppContext.clientMQTT.autoReconnect = true;
    try {
      await tabletAppContext.clientMQTT.connect();
      tabletAppContext.clientMQTT.subscribe("#", MqttQos.atLeastOnce);

      const pubTopic = 'player/status';
      final builder = MqttClientPayloadBuilder();
      builder.addString('tablet_app_'+identifier);
      tabletAppContext.clientMQTT.publishMessage(pubTopic, MqttQos.atLeastOnce, builder.payload);
    } catch (e) {
      print('Exception: $e');
      tabletAppContext.clientMQTT.disconnect();
    }

    return tabletAppContext.clientMQTT;
  }
}