Return value from AsyncTask class onPostExecute method

Question

Ok so now I have Class A that contains some spinners that values will be populated by Class B that extends AsnycTask which grabs the spinner values from a web service. In class B i manage to retrieve the values, showing in a Toast. The problem now is how do I pass those spinner values back to Class A?

I've tried

Can OnPostExcecute method in AsyncTask RETURN values?

by passing Class A to Class B and store the value in a public variable of Class A like below

@Override
protected void onPostExecute(String result)
{
  classA.classAvariable = result;
}

However whenever I try to read the classAvariable i always get a NullPointer Exception. Seems like the variable was never assigned with the result. For readability purpose I needed to seperate Class B instead of using as an inline class.

Any ideas my fellow Java programmers?

Answer 1

I guess you are trying to read class A variable before it is being set.. Try to do it using callbacks..in the callback function pass the values and refresh your spinners..

You can create interface, pass it to AsyncTask (in constructor), and then call method in onPostExecute

For example:

Your interface:

public interface OnTaskCompleted{
    void onTaskCompleted(values);
}

Your Activity:

public YourActivity implements OnTaskCompleted{
    //your Activity
    YourTask  task = new YourTask(this); // here is the initalization code for your asyncTask
}

And your AsyncTask:

public YourTask extends AsyncTask<Object,Object,Object>{ //change Object to required type
    private OnTaskCompleted listener;

    public YourTask(OnTaskCompleted listener){
        this.listener=listener;
    }

    //required methods

    protected void onPostExecute(Object o){
        //your stuff
        listener.onTaskCompleted(values);
    }
}

Answer 2

Problem here is that when you execute your AsynchTask, its doInBackground() methode run in separate thread and the thread that have started this AsynchTask move forward, Thereby changes occur on your variable by AsynchTask does not reflect on parent thread (who stated this AsynchTask) immediately.

Example --

class MyAsynchTask
{

doInbackground()
{
a = 2;
}

}


int a = 5;

new MyAsynchTask().execute();

// here a still be 5