AsyncTask block UI thread and show progressbar with delay

Question

My AsyncTask is blocking block button element while downloading image and progress dialog is shown with delay - its shows for a while before image is shown, but downloading takes long time and button is blocked (orange) and dialog is not shown.

 public  Bitmap download(String url, ProgressBar progressbar) throws InterruptedException, ExecutionException {
     BitmapDownloaderTask task = new BitmapDownloaderTask(progressbar);
     task.execute(url);
     return task.get();
}

class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {



    public BitmapDownloaderTask(ProgressBar progressbar) {

    }
    @Override
    protected void onPreExecute() {
        dialog = new ProgressDialog(ShowActivity.this);
        dialog.setMessage("Loading");
        dialog.setIndeterminate(true);
        dialog.setCancelable(false);
        dialog.show();
    }

    @Override
    protected Bitmap doInBackground(String... Params) {
        return imageLoader.getBitmap(params[0]);

    }
    @Override
    protected void onPostExecute(Bitmap bitmap) {
         dialog.dismiss();


    }
}    

In button listener, simply call download function, the progress parameter is because I have progress bar circle in imageview - the dialog is for testing only, to found why is there the delay and block. In another app I use runable and thread and element is not blocked, but in tutorials is AsyncTask mentioned as better solution for this.

Answer 1

The image download is indeed executed in the background thread, but with return task.get(); you're just waiting for it to finish, and that's what's blocking your main thread.

You should use onPostExecute() as a callback for when the task has finished, so not just to dismiss the dialog but also to do what you need with the bitmap returned by doInBackground().

Answer 2

It is because you are calling AsyncTask#get:

Waits if necessary for the computation to complete, and then retrieves its result.

You should perform whatever operation you need to use the image for within onPostExecute()