Question

I use AsyncTask to change text of TextView like this:

    private class LongOperation extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... urls) {
        String response = "";
        for (String url : urls) {
            response += url;
        }
        return response;
    }

    @Override
    protected void onPostExecute(String result) {
        textView.setText(result);
    }
}

Everything will fine if I call it in OnClick event:

 public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    textView = (TextView) findViewById(R.id.txt);
    Button button = (Button)this.findViewById(R.id.button);
    button.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
          new LongOperation().execute(new String[]{"Hello"});
        }
      });
   }

But the problem when I called it in my thread, the program forced close

this.closeButton.setOnClickListener(new OnClickListener() {
         @Override
        public void onClick(View v) {
          Thread t= new Thread(){               
           @Override
             public void run(){                                     
              try{
                    //Do something
                    //Then call AsyncTask
                new LongOperation().execute(new String[]{"Hello"});

               }catch(Exception e){}
         }
        };      
         t.start();
        }
      });

Where am I wrong? I dont’ understand how difference call AsyncTask in thread or not.

Answer 1

I recommend you consult the AsyncTask documentation and Processes and Threads for a better understanding of how it works. Essentially, you should create your AsyncTask subclass on the main thread.

When you call AsyncTask.execute(), your provided, AsyncTask.onPreExecute is called on the main thread, so you can do UI setup.

Next AsyncTask.doInBackground method is called, and runs in its own thread.

Finally, when your AsyncTask.doInBackground method completes, a call is made to AsyncTask.onPostExecute on the main thread, and you can do any UI cleanup.

If you need to update the UI from within your AsyncTask.doInBackground method, call AsyncTask.publishProgress, which will invoke onProgressUpdate in the main thread.