Android：AsyncTask - 无法在doInBackground中设置适配器？

Question

I am a bit confused about this so I would really appreciate somebody's help.

I have an activity - MainActivity, inside of the activity I have two classes - RetrieveThumbnailsTask (AsyncTask) and an adapter ImageAdapter (extends BaseAdapter). I am basically just retrieving the thumbnails of the images from the external storage and showing them in a gridview. Now this proved to be quite slow (for hundreds of images it takes quite a time) so I decided to create the thumbnails in an AsyncTask.

The code is /irrelevant parts left out/:

public class MainActivity extends Activity {

    private Cursor cursor;
    private Bitmap[] thumbnails;
    private ProgressDialog pd;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        init();
    }

    private void init() {
        createProgressDialog();
        new RetrieveThumbnailsTask(this).execute();
    }

    private void createProgressDialog() {
        //...
    }

    private void createThumbnails() {
        //...
    }

    private class ImageAdapter extends BaseAdapter {

        private Context mContext;

        public ImageAdapter(Context c) {
            mContext = c;
        }

        @Override
        public int getCount() {
            // TODO Auto-generated method stub
            return thumbnails.length;
        }

        @Override
        public Object getItem(int position) {
            // TODO Auto-generated method stub
            return null;
        }

        @Override
        public long getItemId(int position) {
            // TODO Auto-generated method stub
            return 0;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ImageView imageView;
            if (convertView == null) {  // if it's not recycled, initialize some attributes
                imageView = new ImageView(mContext);
                imageView.setLayoutParams(new GridView.LayoutParams(120, 120));
                imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
                imageView.setPadding(8, 8, 8, 8);
            }
            else {
                imageView = (ImageView) convertView;
            }

            imageView.setImageBitmap(thumbnails[position]);
            return imageView;   
        }
    }

    private class RetrieveThumbnailsTask extends AsyncTask<Void, Void, Boolean> {

        private MainActivity activity;

        public RetrieveThumbnailsTask(MainActivity a) {
            activity = a;
        }

        protected void onPreExecute() {
            pd.show();
        }

        protected Boolean doInBackground(Void... params) {
            activity.createThumbnails(); //This is the slow process that creates the thumbnails
            return true;
        }

        protected void onPostExecute(Boolean b) {
            pd.dismiss();
            GridView gridview = (GridView) findViewById(R.id.gridview);
            gridview.setAdapter(new ImageAdapter(activity));

            gridview.setOnItemClickListener(new OnItemClickListener() {
                public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
                    Toast.makeText(MainActivity.this, "" + position, Toast.LENGTH_SHORT).show();
                }
            });
        }
    }
}

The part I don't understand is this:

GridView gridview = (GridView) findViewById(R.id.gridview);
gridview.setAdapter(new ImageAdapter(activity));

gridview.setOnItemClickListener(new OnItemClickListener() {
public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
                Toast.makeText(MainActivity.this, "" + position, Toast.LENGTH_SHORT).show();
            }
});

Now when this code is inside of onPostExecute, everything works alright. But when it is inside doInBackground (right above the return true statement), the code hangs on gridview.setAdapter(new ImageAdapter(activity)); and the app force closes. So my question is - why does this happen? What is the explanation? I guess I must be missing something big time. Thank you!

Answer 1

gridview is a UI element and you can't update the UI on a background Thread which is what you are doing with setAdapter() and is why it works in onPostExecute() .

All methods of AsyncTask run on the UI Thread except for doInBackground() so that is the only one which you can't update UI elements. So if you have a problem with setting it in onPostExecute() then please explain what it is, although you said it works. If there is no problem then just set it there.

AsyncTask Docs