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android – how do i send data back from onPostExecute in an AsyncTask?

Posted by: admin June 15, 2020 Leave a comment

Questions:

my issue is the same as this
Instance variable of Activity not being set in onPostExecute of AsyncTask or how to return data from AsyncTask to main UI thread
but i want to send the data back to the same calling activity. Doesnt startActivity for intents always restart the activity

How to&Answers:

On option is to use listeners, where you create an interface that your activity implents, something like:

public interface AsyncListener {
    public void doStuff( MyObject obj );
}

That way, if you’re subclassing AsyncTask, it is easy to add this listener, then in onPostExecute(), you could do something like:

protected void onPostExecute( MyObject obj ) {
   asyncListener.doStuff(obj);
}

Answer:

This depends on your class structure, but if your AsyncTask is a class within your Activity then you can reference methods of that activity.
What you would do is in your onPostExecute method call a function of your Activity that passes some data that was retrieved in the AsyncTask to the activity where you can then use it..

The code would look like this

class YourActivity extends Activity {
   private static final int DIALOG_LOADING = 1;

   public void onCreate(Bundle savedState) {
     setContentView(R.layout.yourlayout);
     showDialog(DIALOG_LOADING);     
     new LongRunningTask1().execute(1,2,3);

   } 

   protected Dialog onCreateDialog(int dialogId) {
     switch(dialogId) {
       case DIALOG_LOADING:
           ProgressDialog pDialog = new ProgressDialog(this);
           pDialog.setTitle("Loading Data");
           pDialog.setMessage("Loading Data, please wait...");
           return pDialog;
        default:
            return super.onCreateDialog(dialogId);
      }      
   }

   private void onBackgroundTaskDataObtained(List<String> results) {
      dismissDialog(DIALOG_LOADING);
     //do stuff with the results here..
   }

   private class LongRunningTask extends AsyncTask<Long, Integer, List<String>> {
        @Override
        protected void onPreExecute() {
          //do pre execute stuff
        }

        @Override
        protected List<String> doInBackground(Long... params) {
            List<String> myData = new ArrayList<String>(); 
            for (int i = 0; i < params.length; i++) {
                try {
                    Thread.sleep(params[i] * 1000);
                    myData.add("Some Data" + i);
                } catch(InterruptedException ex) {
                }                
            }
            return myData;
        }

        @Override
        protected void onPostExecute(List<String> result) {
            YourActivity.this.onBackgroundTaskDataObtained(result);
        }    
    }

}

So the typical flow is like this, set the view of the current page, and then show a progress dialog. Right after that start the async task (or whenever, it doesn’t matter really).

After your async task is complete, call a function of the activity and pass it the data.
Don’t use shared data within the async task or you risk issues with threading.. Instead once you are done with it pass it to the activity. If you want to update the view progressively while doing work you can use on onProgressUpdate