How to use AsyncTask inside instantiateItem method of PagerAdapter

Question

I wanted to use ViewPager to display some encrypted images. For that I created an extension of PagerAdapter and put my decryption code inside instantiateItem method. But the problem is that the process of decryption takes too long. So, I used AsyncTask inside instantiateItem method to show a progress dialog.

Now, ViewPager displays a blank screen at first and I should swipe to view the first image. Moreover setCurrentItem() does not work.

My Code:

public class FullScreenImageAdapter extends PagerAdapter {

    private Activity _activity;
    private ArrayList<String> _imagePaths;
    private LayoutInflater inflater;
    ImageView imgDisplay;

    // constructor
    public FullScreenImageAdapter(Activity activity,
            ArrayList<String> imagePaths) {
        this._activity = activity;
        this._imagePaths = imagePaths;
    }

    @Override
    public int getCount() {
        return this._imagePaths.size();
    }

    @Override
    public boolean isViewFromObject(View view, Object object) {
        return view == ((RelativeLayout) object);
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        inflater = (LayoutInflater) _activity
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View viewLayout = inflater.inflate(R.layout.layout_fullscreen_image, container,
                false);

        imgDisplay = (ImageView) viewLayout.findViewById(R.id.img_axew);

        new DisplayFile().execute(_imagePaths.get(position));

        ((ViewPager) container).addView(viewLayout);

        return viewLayout;
    }

    private class DisplayFile extends AsyncTask<String, Integer, Integer> {

        ProgressDialog progress;
        String path;
        Bitmap bitmap;

        @Override
        protected Integer doInBackground(String... arg0) {
            path = arg0[0];

            File file = new File(path);
            try {
                bitmap = AxerProcessor.getBitmapFromByte(AxerProcessor.decryptBytes(AxewActivity.password, AxerProcessor.decompressBytes(AxerProcessor.getFileBytes(file))));
            } catch (Exception e) {
                Toast.makeText(_activity, _activity.getString(R.string.error_wrong_password).replace("%s", file.getName()), Toast.LENGTH_LONG).show();
            }

            return null;
        }

        @Override
        protected void onPreExecute() {
            progress = ProgressDialog.show(_activity, _activity.getString(R.string.alert_decrypt_progress), 
                    _activity.getString(R.string.alert_decrypt_progress_msg), false);
        }

        @Override
        protected void onPostExecute(Integer result) {
            progress.dismiss();
            imgDisplay.setImageBitmap(bitmap);
        }

    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        ((ViewPager) container).removeView((RelativeLayout) object);

    }
}

Answer 1

I think you are missing a call to PagerAdapter.notifyDataSetChanged() - http://developer.android.com/reference/android/support/v4/view/PagerAdapter.html#notifyDataSetChanged()

You can call notifyDataSetChanged():

• Right after your call to .addView() [in which case you have a blank screen at first], or
• You can move more code (inflate view, addView) into onPostExecute so that nothing happens until the image is fully ready to be rendered.

Answer 2

Thanks to @rtsai2000, I finally used get() method of AsyncTask to return the true result. The solution was to wait for the result (viewLayout). So my codes changed to:

public class FullScreenImageAdapter extends PagerAdapter {

    private Activity _activity;
    private ArrayList<String> _imagePaths;
    private LayoutInflater inflater;
    ImageView imgDisplay;
    View viewLayout;
    private ViewGroup container;

    // constructor
    public FullScreenImageAdapter(Activity activity,
            ArrayList<String> imagePaths) {
        this._activity = activity;
        this._imagePaths = imagePaths;
    }

    @Override
    public int getCount() {
        return this._imagePaths.size();
    }

    @Override
    public boolean isViewFromObject(View view, Object object) {
        return view == ((RelativeLayout) object);
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        this.container = container;

        DisplayFile df = new DisplayFile();
        df.execute(_imagePaths.get(position));

        View result = null;

        try {
            result = df.get();
        } catch (InterruptedException e) {
        } catch (ExecutionException e) {
        }

        return result;
    }

    private class DisplayFile extends AsyncTask<String, View, View> {

        ProgressDialog progress;
        String path;
        Bitmap bitmap;

        @Override
        protected View doInBackground(String... arg0) {
            path = arg0[0];

            File file = new File(path);
            try {
                bitmap = AxerProcessor.getBitmapFromByte(AxerProcessor.decryptBytes(AxewActivity.password, AxerProcessor.decompressBytes(AxerProcessor.getFileBytes(file))));
            } catch (Exception e) {
                Toast.makeText(_activity, _activity.getString(R.string.error_wrong_password).replace("%s", file.getName()), Toast.LENGTH_LONG).show();
            }

            return viewLayout;
        }

        @Override
        protected void onPreExecute() {
            progress = ProgressDialog.show(_activity, _activity.getString(R.string.alert_decrypt_progress), 
                    _activity.getString(R.string.alert_decrypt_progress_msg), false);

            inflater = (LayoutInflater) _activity
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            viewLayout = inflater.inflate(R.layout.layout_fullscreen_image, container,
                    false);
        }

        @Override
        protected void onPostExecute(View result) {
            progress.dismiss();

            imgDisplay = (ImageView) result.findViewById(R.id.img_axew);

            imgDisplay.setImageBitmap(bitmap);
            ((ViewPager) container).addView(result);
        }

    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        ((ViewPager) container).removeView((RelativeLayout) object);

    }
}