Question

For various Android applications, I need large ListViews, i.e. such views with 100-300 entries.

All entries must be loaded in bulk when the application is started, as some sorting and processing is necessary and the application cannot know which items to display first, otherwise.

So far, I’ve been loading the images for all items in bulk as well, which are then saved in an ArrayList<CustomType> together with the rest of the data for each entry.

But of course, this is not a good practice, as you’re very likely to have an OutOfMemoryException then: The references to all images in the ArrayList prevent the garbage collector from working.

So the best solution is, obviously, to load only the text data in bulk whereas the images are then loaded as needed, right? The Google Play application does this, for example: You can see that images are loaded as you scroll to them, i.e. they are probably loaded in the adapter’s getView() method. But with Google Play, this is a different problem, anyway, as the images must be loaded from the Internet, which is not the case for me. My problem is not that loading the images takes too long, but storing them requires too much memory.

So what should I do with the images? Load in getView(), when they are really needed? Would make scrolling sluggish. So calling an AsyncTask then? Or just a normal Thread? Parametrize it?

I could save the images that are already loaded into a HashMap<String,Bitmap>, so that they don’t need to be loaded again in getView(). But if this is done, you have the memory problem again: The HashMap stores references to all images, so in the end, you could have the OutOfMemoryException again.

I know that there are already lots of questions here that discuss “Lazy loading” of images. But they mainly cover the problem of slow loading, not too much memory consumption.

Edit: I’ve now decided to start AsyncTasks in getView() which load the image into the ListView in the background. But this causes my application to run into an RejectedExecutionException. What should I do now?

Answer 1

I took the approach of loading the images with an AsyncTask and attaching the task to the view in the adapter’s getView function to keep track of which task is loading in which view. I use this in an app of mine and there’s no scroll lag and all images are loaded in the proper position with no exceptions being thrown. Also, because the task does no work if it’s canceled, you can perform a fling on your list and it should lag up at all.

The task:

public class DecodeTask extends AsyncTask<String, Void, Bitmap> {

private static int MaxTextureSize = 2048; /* True for most devices. */

public ImageView v;

public DecodeTask(ImageView iv) {
    v = iv;
}

protected Bitmap doInBackground(String... params) {
    BitmapFactory.Options opt = new BitmapFactory.Options();
    opt.inPurgeable = true;
    opt.inPreferQualityOverSpeed = false;
    opt.inSampleSize = 0;

    Bitmap bitmap = null;
    if(isCancelled()) {
        return bitmap;
    }

    opt.inJustDecodeBounds = true;
    do {
        opt.inSampleSize++;
        BitmapFactory.decodeFile(params[0], opt);
    } while(opt.outHeight > MaxTextureSize || opt.outWidth > MaxTextureSize)
    opt.inJustDecodeBounds = false;

    bitmap = BitmapFactory.decodeFile(params[0], opt);
    return bitmap;
}

@Override
protected void onPostExecute(Bitmap result) {
    if(v != null) {
        v.setImageBitmap(result);
    }
}

}

The adapter stores an ArrayList that contains the file paths of all the images that need loaded. The getView function looks like this:

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ImageView iv = null;
    if(convertView == null) {
        convertView = getLayoutInflater().inflate(R.id.your_view, null); /* Inflate your view here */
        iv = convertView.findViewById(R.id.your_image_view);
    } else {
        iv = convertView.findViewById(R.id.your_image_view);
        DecodeTask task = (DecodeTask)iv.getTag(R.id.your_image_view);
        if(task != null) {
            task.cancel(true);
        }
    }
    iv.setImageBitmap(null);
    DecodeTask task = new DecodeTask(iv);
    task.execute(getItem(position) /* File path to image */);
    iv.setTag(R.id.your_image_view, task);

    return convertView;
}

NOTE: Just a caveat here, this might still give you memory problems on versions 1.5 – 2.3 since they use a thread pool for AsyncTask. 3.0+ go back to the serial model by default for executing AsyncTasks which keeps it to one task running at a time, thus using less memory at any given time. So long as your images aren’t too big though, you should be fine.

UPDATE: While this solution will still work, there have been great additions to the open source community for solving this problem in a cleaner way. Libraries like Glide or Picasso both handle loading items in a list quite well and I’d recommend you look into one of those solutions if possible.