Android: visible ListView images flicker when adding data to ArrayAdapter

Question

I have a custom adapter which extends ArrayAdapter, it implements the view holder patterns to show data (text + image) from a web service.

for lazy loading of the images I use the async tasks pattern from the advanced training in the Android's developers site,

I also use disk + ram bitmap cache.

when there is additional data to be retrieved I add a footer view that clicking on it retrieves additional data from the web service and add it to the adapter.

the problem is that when this new data is being added, some of the visible images are changing and immediately changing back, which result in a weird flickering.

other than that everything is working fine and the scrolling is smooth.

as far as I understand those image changes are happening when the visible views are being refreshed when new data is added.

is there a way to bypass this unwanted behavior ?

this is the class doing the downloading and managing the async tasks

public class ImageDownloader {
private ImageCache mCache;
private int reqWidth;
private int reqHeight;

public void download(String url, ImageView imageView, ImageCache imageCache, int reqHeight, int reqWidth) {
    mCache = imageCache;
    this.reqHeight = reqHeight;
    this.reqWidth = reqWidth;

    if (cancelPotentialDownload(url, imageView)) {
        BitmapDownloaderTask task = new BitmapDownloaderTask(imageView);

        DownloadedDrawable downloadedDrawable = new DownloadedDrawable(task);
        imageView.setImageDrawable(downloadedDrawable);
        task.execute(url);
    }
}

private class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {
    private String url;
    private WeakReference<ImageView> imageViewReference;

    public BitmapDownloaderTask(ImageView imageView) {
        imageViewReference = new WeakReference<ImageView>(imageView);
    }

    @Override
    protected Bitmap doInBackground(String... strings) {
        Bitmap bitmap = null;
        try {
            bitmap =  mCache.getBitmapFromURL(strings[0], reqWidth, reqHeight);
        } catch (IOException e) {
        }

        return bitmap;
    }

    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (isCancelled()) bitmap = null;

        if (imageViewReference != null) {
            ImageView imageView = imageViewReference.get();
            BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);

            if (imageView != null) {
                imageView.setImageBitmap(bitmap);
            }
        }
    }
}

private static class DownloadedDrawable extends ColorDrawable {
    private WeakReference<BitmapDownloaderTask> bitmapDownloaderTaskReference;

    public DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask) {
        bitmapDownloaderTaskReference = new WeakReference<BitmapDownloaderTask>(bitmapDownloaderTask);
    }

    public BitmapDownloaderTask getBitmapDownloaderTask() {
        return bitmapDownloaderTaskReference.get();
    }
}

private static BitmapDownloaderTask getBitmapDownloaderTask(ImageView imageView) {
    if (imageView != null) {
        Drawable drawable = imageView.getDrawable();
        if (drawable instanceof DownloadedDrawable) {
            DownloadedDrawable downloadedDrawable = (DownloadedDrawable)drawable;
            return downloadedDrawable.getBitmapDownloaderTask();
        }
    }
    return null;
}

private static boolean cancelPotentialDownload(String url, ImageView imageView) {
    BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);

    if (bitmapDownloaderTask != null) {
        String bitmapUrl = bitmapDownloaderTask.url;
        if ((bitmapUrl == null) || (!bitmapUrl.equals(url))) {
            bitmapDownloaderTask.cancel(true);
        } else {
            return false;
        }
    }
    return true;
}

}

this is the adapter:

private class PlaceAdapter extends ArrayAdapter { final int viewResourceId;

    public PlaceAdapter(Context context, int textViewResourceId, List<PlaceModel> objects) {
        super(context, textViewResourceId, objects);
        viewResourceId = textViewResourceId;
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {
        ViewHolder holder;

        if (convertView == null) {
            LayoutInflater inflater = getLayoutInflater();
            convertView = inflater.inflate(viewResourceId, null);

            holder = new ViewHolder(convertView);
            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        PlaceModel place = getItem(position);

        holder.name.setText(place.getName());
        holder.address.setText(place.getVicinity());
        holder.position = position;

        if (place.getIcon() != null) {
            String url = mImageViewUrls.get(holder.image);
            if (url == null || (url != null && !url.equals(place.getIcon()))) {
                mDownloader.download(place.getIcon(), holder.image, mCache, 100, 100);
                mImageViewUrls.put(holder.image, place.getIcon());
            }
        } else {
            holder.image.setImageBitmap(null);
            mImageViewUrls.remove(holder.image);
        }

        return convertView;
    }
}

private class ViewHolder {
    public final ImageView image;
    public final TextView name;
    public final TextView address;
    public int position;

    public ViewHolder(View row) {
        image = (ImageView) row.findViewById(R.id.placeRow_imageView);
        name = (TextView) row.findViewById(R.id.placeRow_placeName);
        address = (TextView) row.findViewById(R.id.placeRow_placeAddress);
    }
}

mImageViewUrls is a WeakHashMap<ImageView, String> that maps between an ImageView and a url, so redundant async tasks invocations can be reduced by checking if the ImageView is already showing the required image. without this implementation, flickering is happening in all visible images on data change. with this, it happens only with some images.

EDIT: I tried to eliminate possible causes of this issue, first I tried to completely bypass the cache implementation and download each bitmap from the network, then I tried wrapping my adapter with CommonsWare's Endless adapter and with both I got the same result.. so this leaves only the ImageDownloader class and my adapter as possible causes.. I'm completely lost on this.

Answer 1

The reason for this flicker is that, in listview list items are reused. When re-used, the imageviews in the list item retains the old image reference which is displayed first. Later on once new image is downloaded, it starts to show. this causes the flickering behavior. To avoid this flickering issue, always clear the old image reference from the imageview when it is getting reused.

In your case, add holder.image.setImageBitmap(null); after holder = (ViewHolder) convertView.getTag();

So, your getView() method will look like:

@Override
public View getView(final int position, View convertView, ViewGroup parent) {

    ...

    if (convertView == null) {
        LayoutInflater inflater = getLayoutInflater();
        convertView = inflater.inflate(viewResourceId, null);

        holder = new ViewHolder(convertView);
        convertView.setTag(holder);
    } else {
        holder = (ViewHolder) convertView.getTag();
        holder.image.setImageBitmap(null)
    }

    ...

    return convertView;
}