Paho-mqtt callbacks in different classes android java

Question

I've just started Java Android programming, or even Java programming in general and I wanted to implement the Paho MQTT Android Service using a certain MqttHandler class and I want to set the callback as a parameter for the MqttHandler class. Other answers regarding callbacks in general suggested using an interface class but I don't know how that works. This is what I have tried:

public interface InterfaceMqttCallback extends MqttCallbackExtended{
    @Override
    public void connectComplete(boolean b, String s);

    @Override
    public void connectionLost(Throwable throwable);

    @Override
    public void messageArrived(String topic, MqttMessage mqttMessage) throws Exception ;

    @Override
    public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken);

}

the MqttHandler class constructor:

public MqttHandler(Context context, InterfaceMqttCallback mqttCallbackExtended){
    mqttAndroidClient = new MqttAndroidClient(context, serverUri, clientId);
    mqttAndroidClient.setCallback(mqttCallbackExtended);
    connect();
}

a function in the Activity that initializes the MqttHandler:

private void startMqtt(){

    mqttHandler = new MqttHandler(getApplicationContext(), new InterfaceMqttCallback() {
        @Override
        public void connectComplete(boolean b, String s) {
            Log.w("Anjing", s);
        }

        @Override
        public void connectionLost(Throwable throwable) {

        }

        @Override
        public void messageArrived(String topic, MqttMessage mqttMessage) throws Exception {
            Log.w("Anjing", mqttMessage.toString());
        }

        @Override
        public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken) {

        }
    });

}

But as I've tested it, I think the callbacks was not set because none of the messages have been received. So then I tried setting the callbacks directly in the MqttHandler and it works, the messages are received.

public MqttHandler(Context context, InterfaceMqttCallback mqttCallbackExtended){
    mqttAndroidClient = new MqttAndroidClient(context, serverUri, clientId);
    mqttAndroidClient.setCallback(new InterfaceMqttCallback() {
        @Override
        public void connectComplete(boolean b, String s) {
            Log.w("mqtt", s);
        }

        @Override
        public void connectionLost(Throwable throwable) {

        }

        @Override
        public void messageArrived(String topic, MqttMessage mqttMessage) throws Exception {
            Log.w("Anjing", mqttMessage.toString());
        }

        @Override
        public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken) {

        }
    });
    connect();
}

So what's the problem? Thanks in advance..

EDIT: Of course I can set the mqttAndroidClient variable in the MqttHandler class as public, but is it safe?

Answer 1

The proper way to set callback handler is setting it in MQTT client, like following:

        public class MTTConnector { 
            public void connect {
              MqttAndroidClient mqttClient = new MqttAndroidClient(BaseApplication.getAppContext(), broker, MQTT_CLIENT_ID); 
              mqttClient.setCallback(new MqttCallbackHandler(BaseApplication.getAppContext()));
          }
        }

    public class MqttCallbackHandler implements MqttCallbackExtended {
    @Override
        public void connectComplete(boolean b, String s) {
            Log.w("mqtt", s);
        }

        @Override
        public void connectionLost(Throwable throwable) {

        }

        @Override
        public void messageArrived(String topic, MqttMessage mqttMessage) throws Exception {
            Log.w("Anjing", mqttMessage.toString());
        }

        @Override
        public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken) {

        }
  }

If you are using latest version your call back need to implement MqttCallbackExtended class, see following link for more details on online GitHub project.

MqttCallbackExtended.java

Inside your activity initialize MTTConnector and call connect.

Hoping this help.

Cheers !