start AsyncTask from one activity, get result in another

Question

I'm new to Android programming, and I'd like to create a central database service class which will take care of user data exchange with an external database. For this, I created a service which is started after successful login. I created another class that extends AsyncTask to do the data retrieval. Now, I wanted the methods for the data retrieval to be stored in the service. I would fire intents to the service from different activities, and with .setAction() I would determine which method to call, or which data to retrieve.

I also created an interface class for handling the AsyncTask results. Now, from this question I thought that it would be possible to have multiple listeners to one and the same AsyncTask result. But now this seems impossible to achieve: I'd like to retrieve the AsyncTask results in the MainMenuActivity, but I can't create an instance of AsyncUserData there as a delegate for the UserData class. In my example below, the missing piece is a valid instance of AsyncUserData for the UserData class to work with. How could I do it?

Here's the example: MainMenuActivity

public class MainMenuActivity extends ActionBarActivity implements AsyncUserData {
  TextView tvUsername;

  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main_menu);
    tvUsername =
      (TextView) findViewById(R.id.tvUsername);
    TelephonyManager tManager = (TelephonyManager) this.getSystemService(Context.TELEPHONY_SERVICE);
    String uid = tManager.getDeviceId();
    getDataFromUserSessionService(this, uid);
  }

  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main_menu, menu);
    return true;
  }

  @Override
  public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();
    if (id == R.id.action_settings) {
      return true;
    }
    return super.onOptionsItemSelected(item);
  }

  @Override
  public void retrieveResult(String result) throws JSONException {
    JSONObject jsonObject = new JSONObject(result);
    String joName;
    joName = jsonObject.getJSONObject("name").toString();
    user.setName(joName);
    tvUsername.setText(joName);
  }

  public void getDataFromUserSessionService(Context context, String uid) {
    Intent intent = new Intent(context, UserSession.class);
    intent.setAction(UserSession.ACTION_FETCH_USER_DATA);
    intent.putExtra(UserSession.UID, uid);
    context.startService(intent);
  }

UserSession Service

public class UserSession extends IntentService {
  public static final String ACTION_FETCH_USER_DATA = "com.example.blahblah.services.action.read_user_data";

  @Override
  protected void onHandleIntent(Intent intent) {
    if (intent != null) {
      utils = new Utils(this);
      final String action = intent.getAction();
      uid = intent.getStringExtra(UID);
      if (ACTION_FETCH_USER_DATA.equals(action)) {
        handleUserDataFetch(uid);
      }
    }
  }

  private void handleUserDataFetch(String uid) {
    String[] parameters = new String[2];
    parameters[0] = uid;
    parameters[1] = Constants.USER_DATA_FETCH;
    UserData userData = new UserData(this);
    userData.execute(parameters);
  }

UserData AsyncTask Class (the Utils class just has another post method):

public class UserData extends AsyncTask < String, Void, String > {
  public AsyncUserData delegate = null;
  private Context myContext;

  public UserData(Context context) {
    myContext = context;
  }

  @Override
  protected String doInBackground(String...params) {
    String serverResponse = "";
    String uid = params[0];
    Utils utils = new Utils(myContext);
    String phpName = params[1];
    List < NameValuePair > nameValuePairs = new ArrayList < NameValuePair > ();
    nameValuePairs.add(new BasicNameValuePair("uid", uid));

    try {

      serverResponse = utils.passDataToServer(phpName, nameValuePairs);
    } catch (IOException e) {
      e.printStackTrace();
    }
    return serverResponse;
  }

  protected void onPostExecute(String result) {
    try {
      delegate.retrieveResult(result);
    } catch (JSONException e) {
      e.printStackTrace();
    }
  }

};

And the AsyncUserData interface:

public interface AsyncUserData {
  void retrieveResult(String result) throws JSONException;
}

Answer 1

You can use a Singleton that stores a reference to the activity

    public class ServiceToActivity
    {

                public ActionBarActivity mainactivity = null;
                private static ServiceToActivity singleton = null;
                public Class<?> cl = null;

                private ServiceToActivity()
                {

                }

                public static ActionBarActivity getSingleton()
                {
                    if(singleton==null)
                           return null;
                    return singleton.mainactivity;
                }

               public static Class<?> getSingletonClass()
                {
                    if(singleton==null)
                           return null;
                    return singleton.cl;
                }

                public static void setSingleton(ActionBarActivity mainactivity, Class<?> cl)
                {
                      if(singleton==null)
                             singleton = new ServiceToActivity();
                      singleton.mainactivity = mainactivity;
                      singleton.cl = cl;
                }
    }

Then create the singleton before the service is started

  public void getDataFromUserSessionService(Context context, String uid) {
Intent intent = new Intent(context, UserSession.class);
intent.setAction(UserSession.ACTION_FETCH_USER_DATA);
intent.putExtra(UserSession.UID, uid);
ServiceToActivity.setSingleton(this,this.getClass());   //create Singleton to store a reference to the activity
context.startService(intent);
}

In UserData retrieve data to the main activity by:

 protected void onPostExecute(String result) {
try {
  Class<?> cl = ServiceToActivity.getSingletonClass();
  Method met = cl.getMethod("retrieveResult", String); //String because result is of type String: you can use result.getClass() instead
  met.invoke(cl.cast(ServiceToActivity.getSingleton()), result); // compare it to this ServiceToActivity.getSingleton().retrieveResult(result);
} catch (JSONException e) {
  e.printStackTrace();
}
}

Answer 2

It sounds like you might want to use an event bus such as otto