private CustomTask mTask;

@Override
public void onActivityCreated(Bundle savedInstanceState)
{
    super.onActivityCreated(savedInstanceState);
    if(this.mTask == null)
    {
        this.mTask = new CustomTask ()
        {
            @Override
            protected void onPostExecute(Response result)
            {
                //Your stuff
                Fragment.this.mTask = null;
            }

            @Override
            protected void onCancelled()
            {
                Fragment.this.mTask = null;
            }
        };
        this.mTask.ExecuteCompat();
    }
}

@Override
public void onDestroyView()
{
    if (this.mTask != null && this.mTask.getStatus() != AsyncTask.Status.FINISHED)
        this.mTask.cancel(true);
    super.onDestroy();
}