Can't Cancel Async Task In Android

I need to cancel my asyncthread . In my application I am doing some heavy calculations, and I want to give user ability to cancel calculations(and then retry). I read on forums, th

Solution 1:

Your problem is not that you can't cancel the AsyncTask. You probably get NullPointerException because your call to setContentView() goes through before AsyncTask.cancel() has been successful. A onProgressUpdate() gets called, only to find that the layout is now changed and there is no Viewwith id=R.id.pdf_text1!

From documentation on AsyncTask.

A task can be cancelled at any time by invoking cancel(boolean). Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from doInBackground(Object[]), if possible (inside a loop for instance.)

Since onCancelled() runs on the UI thread, and you are certain that no subsequent calls to onProgressUpdate() will occure, it's is a great place to call setContentView().

Override onCancelled() in you AsyncTask

private class naredi_pdf extends AsyncTask<Void, String, Void> {

    protected Void doInBackground( Void... ignoredParams ) { // YOUR CODE HERE}
    protected void onPostExecute( Void array ) { // YOUR CODE HERE}
    protected void onProgressUpdate(String... values) {// YOUR CODE HERE}

    // ADD THIS
    @Override
    protected void onCancelled() {
        // Do not call super.onCancelled()!

        // Set the new layout
        setContentView(R.id.other_layout);
    }
}

Change close_pdf1()

public void close_pdf1(View view) {
    if(start_pdf!=null) {
        Log.v("not null","not null");

        start_pdf.cancel(true);
    }
}

And you should have an AsyncTask that automatically changes your layout when cancelled. Hopefully you should not encounter any NullPointerException either. Haven't tried the code though :)

Edit

If you feel fancy, follow Rezooms advice on using return.

for(int i = 0; i < 1; i++) {
    if(isCancelled()) {
        return null;
    }
    .
    .
    .
}

Solution 2:

The return statement cancels the execution of the doInBackground method, not break.

Solution 3:

isCancelled is a propietary method of AsyncTask class.

You should define a private boolean property on your extended class, do something like this

private class myAsyncTask extends AsyncTask<Void, String, Void> {
          private boolean isTaskCancelled = false;

          public void cancelTask(){
               isTaskCancelled = true;
          }

          private boolean isTaskCancelled(){
               return isTaskCancelled;
          }

          protected Void doInBackground( Void... ignoredParams ) {
            //Do some stuff
             if (isTaskCancelled()){
                return;
             }
          }


          protected void onPostExecute( Void array )
          {
           //Do something
          }



          protected void onProgressUpdate(String... values)
          {
           //Do something
          }
    }