适用于Android的MQTT Java应用程序中的setCallback抛出错误

Question

I am trying to build an MQTT "listener" app for Android and have used this sample application as a model. However the original sample throws a "Network on Main Activity" runtime error and so I am forced to an AsyncTask class.

The main App runs as a service : LWMQService.java [ relevant part of code here ]

package org.iothub.lightwatchmq;

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.MqttPersistenceException;
import org.eclipse.paho.client.mqttv3.MqttTopic;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

import org.iothub.lightwatchmq.PushCallback;

import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.os.AsyncTask;
import android.os.IBinder;
import android.util.Log;
import android.widget.Toast;
import android.provider.Settings.Secure;

public class LWMQService extends Service implements SensorEventListener {

    -----

    String BROKER_URL = "tcp://test.mosquitto.org:1883";
    String TOPIC = "iothub";
    String TOPIC2 = "iothub2";
    String ClientID ;

    private MqttClient client;

   @Override
   public IBinder onBind(Intent arg0) {
      return null;
   }

   @Override
   public int onStartCommand(Intent intent, int flags, int startId) {

      ------

       ClientID = Secure.getString(getContentResolver(), Secure.ANDROID_ID); 

       try {

           client = new MqttClient(BROKER_URL, ClientID, new MemoryPersistence());
       } catch (MqttException e) {
           e.printStackTrace();
           System.exit(1);
       }
       new mConnect ().execute();
     //  new mTransmit ().execute();
       new mListen().execute();;

      Toast.makeText(this, "Service started", Toast.LENGTH_LONG).show();
      Log.d(LogMsgID, "LightWatch Service started");
      return START_STICKY;
   }

   @Override
   public void onDestroy() {
      -----
      return;
   }

   private class mConnect extends AsyncTask<Void, Void, Void> {
         protected Void doInBackground(Void...dummy) {
             try {

                 client.connect();

             } catch (MqttException e) {
                 e.printStackTrace();
                 Log.d(LogMsgID, "Connect Failed");
                 System.exit(1);
             }
            return null;
         }
    }

   public class mListen extends AsyncTask<Void, Void, Void> {
         protected Void doInBackground(Void...dummy) {
             try {
                 client.setCallback(new PushCallback(this));
                 client.subscribe(TOPIC2);

             } catch (MqttException e) {
                 e.printStackTrace();
                 Log.d(LogMsgID, "Listen Failed");
                 System.exit(1);
             }
            return null;
         }
    }


   private class mTransmit extends AsyncTask<Void, Void, Void> {
         protected Void doInBackground(Void...dummy) {

            --------
         }

     }

   @Override
      public final void onAccuracyChanged(Sensor sensor, int accuracy) {
        // Do something here if sensor accuracy changes.
      }

    @Override
      public final void onSensorChanged(SensorEvent event) {
        // The light sensor returns a single value.
        ---
      }
}

the mConnect and the mTransmit( publish) classes compile correctly and work fine BUT the mListen ( subscribe) class i get a compile error that states "The constructor PushCallback(LWMQService.mListen) is undefined" showing on the following line in the class

client.setCallback(new PushCallback(this));

the PushCallback class is available in a different file : PushCallback.java

package org.iothub.lightwatchmq;

import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.ContextWrapper;
import android.content.Intent;
import android.util.Log;

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.MqttTopic;
import org.iothub.lightwatchmq.LWMQService.mListen;

public class PushCallback implements MqttCallback {

    String LogMsgID = "iothub: ";
    private ContextWrapper context;

    public PushCallback(ContextWrapper context) {

        this.context = context;
        Log.d(LogMsgID, "PCB-1");
    }

    @Override
    public void connectionLost(Throwable cause) {
        //We should reconnect here
    }

    @Override
    public void deliveryComplete(IMqttDeliveryToken arg0) {
        // TODO Auto-generated method stub
    }

    @Override
    public void messageArrived(String Topic, MqttMessage message) throws Exception {
        // TODO Auto-generated method stub

        final Intent intent = new Intent(context, MainActivity.class);
        final PendingIntent activity = PendingIntent.getActivity(context, 0, intent, 0);
        String RecdMessage = new String(message.getPayload());
        Log.d(LogMsgID, RecdMessage);

    }
}

I would admit that I am not very good at Java and maybe making some obvious error but would be grateful if someone can please help me out with this error.

Answer 1

In the scope where you call client.setCallback(new PushCallback(this)); this is the AsyncTask , as AsyncTask class does not extend ContextWrapper it will not match the constructor.

Change the line to be

client.setCallback(new PushCallback(LWMQService.this));

to grab the right context object