broker_setup.dart

Future<MqttServerClient> brokerSetup(Function function) async {
  
    client = MqttServerClient.withPort(broker, clientIdentifier, port);
    client.logging(on: true);
    client.onConnected = onConnected;
    client.onDisconnected = onDisconnected;
    client.onSubscribed = onSubscribed;
    client.onSubscribeFail = onSubscribeFail;
    client.pongCallback = pong;
    client.secure = false;

    final connMessage = MqttConnectMessage()
        .authenticateAs(username, password)
        .keepAliveFor(60)
        .startClean()
        .withWillQos(MqttQos.atLeastOnce)
        .withClientIdentifier(clientIdentifier);

    client.connectionMessage = connMessage;

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

    subscription = client.updates.listen(function); //this will make our widget know when it's time to update!

    return client;
  }
}

//actions
void onConnected() {
  print('connected');
}

void onDisconnected() {
  print('disconnected');
}

void onSubscribed(String topic) {
  print('subscribed to $topic');
}

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

void on() {
  print('disconnected');
}

void pong() {
  print('ping response arrived');
}