Async Worker Crashes App

Question

I have written the following code within the main class for an async worker

private class Renderer extends AsyncTask<Void, Void, Void>{

        @Override
        protected Void doInBackground(Void... params) {
            // TODO Auto-generated method stub

              Toast.makeText(getApplicationContext(), "works... ", Toast.LENGTH_LONG).show();
            return null;
        }





         protected void onProgressUpdate(Integer... progress) {

         }

         protected void onPostExecute(Long result) {

         }
     }

But calling this crashes the app

public void render(View v)
    {
        new Renderer().execute();
    }

Can some one tell me what's going wrong?

score 1 · Accepted Answer · edited May 23 '17 at 11:56

1

You can't do a Toast in doInBackground, you will get Can't create handler inside thread that has not called Looper.prepare().

If it's just for testing at this stage (which your code seems to imply), consider instead just writing some log output for now.

Alternatively put your Toast either onPreExecute or onPostExecute

More info: How to raise a toast in AsyncTask, I am prompted to used the Looper

edited May 23 '17 at 11:56

Community

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answered Jul 26 '13 at 11:20

Ken Wolf

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does not work even after putting the toast in Onpostexecute.App does not crash but nothing happens – techno Jul 26 '13 at 15:00
Your `AsyncTask` is wrong. Put `@Override` before all the methods and follow what eclipse tells you to do. `onPostExecute()` is not being called at all as the method signature should be `protected void onPostExecute(Void result) {` according to your `AsyncTask` declaration. **SHORT ANSWER**: Change onPostExecute to `(Void result)` NOT `(Long result)` and it will work – Ken Wolf Jul 26 '13 at 15:04

score 0 · Answer 2 · answered Jul 26 '13 at 11:20

You are showing toast in doInbackground which is not possible. Do it in onPostExecute

  protected void onPostExecute(Long result) {
            Toast.makeText(getApplicationContext(), "works... ", Toast.LENGTH_LONG).show();
     }

doInbackground is invoked on the background thread. You should update ui on the ui thread. You can alos display toast in onProgressUpdate.

More info at

http://developer.android.com/reference/android/os/AsyncTask.html

Check the topic under the section The 4 steps.

okay,but does not work even after putting the toast in Onpostexecute.App does not crash but nothing happens — techno, Jul 26 '13 at 14:54

score 0 · Answer 3 · answered Jul 26 '13 at 11:21

0

You can't access UI thread from background thread.

answered Jul 26 '13 at 11:21

Geralt_Encore

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score 0 · Answer 4 · answered Jul 26 '13 at 11:25

An AsyncTask is a helper around Threads and Handlers. It helps you to perform a background task and update the UI regularly.

Thus, the methods of AsyncTask run on different Threads: doInBackground runs in a background Thread onProgressUpdate and onPostExecute run on the UI Thread.

Now, you can only modify the UI in the UI Thread (=main Thread). This is why your code leads to a crash. Simply moving your Toast into onPostExecute will solve your problem.

score 0 · Answer 5 · edited Mar 03 '15 at 07:57

0

 runOnUiThread(new Runnable() {
                   @Override
                   public void run() { 
                    Toast.makeText(getApplicationContext(), "works... ", Toast.LENGTH_LONG).show();
                   }
                });`

Try to do all ui work on main thread.

edited Mar 03 '15 at 07:57

Arun A S

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answered Mar 03 '15 at 07:11

Meenaxi

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Async Worker Crashes App

5 Answers5