Ideal way to cancel an executing AsyncTask

Question

I am running remote audio-file-fetching and audio file playback operations in a background thread using AsyncTask. A Cancellable progress bar is sh

Answer 1

If you're doing computations:

• You have to check isCancelled() periodically.

If you're doing a HTTP request:

• Save the instance of your HttpGet or HttpPost somewhere (eg. a public field).
• After calling cancel, call request.abort(). This will cause IOException be thrown inside your doInBackground.

In my case, I had a connector class which I used in various AsyncTasks. To keep it simple, I added a new abortAllRequests method to that class and called this method directly after calling cancel.

Answer 2

Just discovered that AlertDialogs's boolean cancel(...); I've been using everywhere actually does nothing. Great.
So...

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

    private volatile boolean running = true;
    private final ProgressDialog progressDialog;

    public MyTask(Context ctx) {
        progressDialog = gimmeOne(ctx);

        progressDialog.setCancelable(true);
        progressDialog.setOnCancelListener(new OnCancelListener() {
            @Override
            public void onCancel(DialogInterface dialog) {
                // actually could set running = false; right here, but I'll
                // stick to contract.
                cancel(true);
            }
        });

    }

    @Override
    protected void onPreExecute() {
        progressDialog.show();
    }

    @Override
    protected void onCancelled() {
        running = false;
    }

    @Override
    protected Void doInBackground(Void... params) {

        while (running) {
            // does the hard work
        }
        return null;
    }

    // ...

}

Answer 3

With reference to Yanchenko's answer on 29 April '10: Using a 'while(running)' approach is neat when your code under 'doInBackground' has to be executed multiple times during every execution of the AsyncTask. If your code under 'doInBackground' has to be executed only once per execution of the AsyncTask, wrapping all your code under 'doInBackground' in a 'while(running)' loop will not stop the background code (background thread) from running when the AsyncTask itself is cancelled, because the 'while(running)' condition will only be evaluated once all the code inside the while loop has been executed at least once. You should thus either (a.) break up your code under 'doInBackground' into multiple 'while(running)' blocks or (b.) perform numerous 'isCancelled' checks throughout your 'doInBackground' code, as explained under "Cancelling a task" at https://developer.android.com/reference/android/os/AsyncTask.html.

For option (a.) one can thus modify Yanchenko's answer as follows:

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

private volatile boolean running = true;

//...

@Override
protected void onCancelled() {
    running = false;
}

@Override
protected Void doInBackground(Void... params) {

    // does the hard work

    while (running) {
        // part 1 of the hard work
    }

    while (running) {
        // part 2 of the hard work
    }

    // ...

    while (running) {
        // part x of the hard work
    }
    return null;
}

// ...

For option (b.) your code in 'doInBackground' will look something like this:

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

//...

@Override
protected Void doInBackground(Void... params) {

    // part 1 of the hard work
    // ...
    if (isCancelled()) {return null;}

    // part 2 of the hard work
    // ...
    if (isCancelled()) {return null;}

    // ...

    // part x of the hard work
    // ...
    if (isCancelled()) {return null;}
}

// ...

Answer 4

I don't like to force interrupt my async tasks with cancel(true) unnecessarily because they may have resources to be freed, such as closing sockets or file streams, writing data to the local database etc. On the other hand, I have faced situations in which the async task refuses to finish itself part of the time, for example sometimes when the main activity is being closed and I request the async task to finish from inside the activity's onPause() method. So it's not a matter of simply calling running = false. I have to go for a mixed solution: both call running = false, then giving the async task a few milliseconds to finish, and then call either cancel(false) or cancel(true).

if (backgroundTask != null) {
    backgroundTask.requestTermination();
    try {
        Thread.sleep((int)(0.5 * 1000));
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    if (backgroundTask.getStatus() != AsyncTask.Status.FINISHED) {
        backgroundTask.cancel(false);
    }
    backgroundTask = null;
}

As a side result, after doInBackground() finishes, sometimes the onCancelled() method is called, and sometimes onPostExecute(). But at least the async task termination is guaranteed.

Answer 5

Our global AsyncTask class variable

LongOperation LongOperationOdeme = new LongOperation();

And KEYCODE_BACK action which interrupt AsyncTask

   @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            LongOperationOdeme.cancel(true);
        }
        return super.onKeyDown(keyCode, event);
    }

It works for me.

Answer 6

The only way to do it is by checking the value of the isCancelled() method and stopping playback when it returns true.