How to get results of Async task in Android

Question

I am trying AsyncTask. I cannot understand how to return the result.I am also confused on doInBackground() has return type Void . Why does it need return null; if I return null how will I get value from this method.

package com.example.shikkok_services;

import android.app.Dialog;
import android.content.Context;
import android.os.AsyncTask;
import android.os.Handler;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.Window;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;

public class MyTask extends AsyncTask<Void, Integer, Void> {

    //progressBar holo UI te kaj kore tai Context dorkar tai MyTask e construtor decler korlam.Inner class hole eita dorkar silo na

    Context context;
    Handler handler;
    Dialog dialog;
    TextView txtprogrss;
    ProgressBar progress;
    Button btnCancel;

    MyTask(Context context, Handler handler){
        this.context=context;
        this.handler=handler;

    }

    MyTask(Context context){
      this.context=context;
      this.handler=handler;
    }

    //--------------------------------onPreExecute()............................................
    @Override
    protected void onPreExecute() {

        super.onPreExecute();
        // create dialog

        dialog=new Dialog(context);
        dialog.setCancelable(true);
        dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
        dialog.setContentView(R.layout.pogressdialog);
        dialog.show();

        txtprogrss=(TextView) dialog.findViewById(R.id.txtProgress);
        progress=(ProgressBar)dialog.findViewById(R.id.progressBar2);
        btnCancel=(Button)dialog.findViewById(R.id.btnProgress);
        //progress=new ProgressBar(context);

        btnCancel.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub

                MyTask.this.cancel(true);
                dialog.dismiss();
            }
        });
        dialog.show();

    }

    //--------------------------------doInBackground()...........................

    @Override
    protected Void doInBackground(Void... arg0) {
     //Kaj hobe backgournd e not UI e
        // do tasks and update msg
        for (int i = 0; i < 10; i++) {
            if(isCancelled()){
            break;
            }else{
            Log.e("In Background","current value;"+ i);
            publishProgress(i);
            //joto bar publishProgress() call hobe toto bar OnProgressUpdate hobe..
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            }

        }

        return null;    
    }

    //------------------------------onProgressUpdate().....................................

    @Override
    protected void onProgressUpdate(Integer... values) {
        //dowonload hole koto percent hosse seita dhore dhore UI te dekhalte parbo

        super.onProgressUpdate(values);
        // update dialog progress

        progress.setProgress(values[0]);
        txtprogrss.setText("progress update"+ values[0]+"%");

    }

    //-----------------------------OonPostExecute()...........................................
    @Override
    protected void onPostExecute(Void result) {
        // jokhon kaj ses hoe jabe tokhon ei method e asben then UI te chole asben
        super.onPostExecute(result);

        // handler.sent message

        dialog.dismiss();
        Toast.makeText(context, "Finished", Toast.LENGTH_LONG).show();

    }





}

Answer 1

I am also confuse doInBackground is return type Void .Why it is need return null; if I return null how get value from this method.please tell me.

This is because that is what you have made it. You can return a type you just have to change the class definition ex.

public class MyTask extends AsyncTask<Void, Integer, String> {  // changed last param

With this, doInBackground() will be expected to return a String type to onPostExecute(). Which means you would also have to change that method to

 @Override
protected void onPostExecute(String result) {

You would then obviously change your return statement in doInBackground() to return someString.

I used String as an example but you could change that.

As far as returning value after AsyncTask finishes

See this answer about using an interface. It is really easy to implement and will update your Activity when the task finishes.

Also please read (or re-read) the AsyncTask docs

Answer 2

AsyncTask must return null because you said so declaring this: AsyncTask<Void, Integer, Void>. The third parameter determines what doInBackground() returns and therefore what onPostExecute() gets.

Change this datatype to make it return whetever you want, this way you'll receive it as a parameter in the onPostExecute() method and you can even update Views in the main UI Thread from within this method (you can even update them from anywhere besides doInBackground()).

Answer 3

For getting value from asynchronous task call it like this:

String result= new MyTask ().execute().get();


class MyTask extends AsyncTask<String, String, String>
        {
        protected String doInBackground(String... params)


                  return somevalue; 
                      }

                }