Displaying Images Efficiently on Android

In this blog post, we demonstrate some simple ways to efficiently display images.

The Straightforward Approach

The simplest way to load an image (e.g. from resource), is to load it in an AsyncTask, and updates the ImageView in onPostExecute():

class ImageLoadingTask extends AsyncTask {
    ImageLoadingTask(Resources resources, ImageView imageView) {
        mResources = resources;
        mImageView = new WeakReference(imageView);
    }
 
    @Override
    protected Bitmap doInBackground(Integer[] params) {
        return BitmapFactory.decodeResource(mResources, params[0]);
    }
 
    @Override
    protected void onPostExecute(Bitmap result) {
        final ImageView imageView = mImageView.get();
        if (imageView != null)
            imageView.setImageBitmap(result);
    }
 
    private final Resources mResources;
    private final WeakReference mImageView;
}

This works fine until you try to load some large image, when the image refuses to be loaded with this error from logcat: W/OpenGLRenderer﹕ Bitmap too large to be uploaded into a texture (4000×3726, max=2048×2048)

And of course, it might even throw an OutOfMemory error on low-end devices.

Load a Scaled Image

To solve the problem, we should request the decoder to subsample the original image, as shown below with the updated doInBackground():

@Override
protected Bitmap doInBackground(Integer... params) {
    // extracts the size of the original image first
    final int resourceId = params[0];
    final BitmapFactory.Options options = new BitmapFactory.Options();
    options.inJustDecodeBounds = true;
    BitmapFactory.decodeResource(mResources, resourceId, options);
 
    // calculates the inSampleSize
    if (options.outWidth > mTargetWidth || options.outHeight > mTargetHeight) {
        options.inSampleSize
            = Math.min(Math.round((float) options.outWidth / (float) mTargetWidth),
                Math.round((float) options.outHeight / (float) mTargetHeight));
    }
 
    // decodes the image
    options.inJustDecodeBounds = false;
    return BitmapFactory.decodeResource(mResources, resourceId, options);
}

This works pretty fine in most cases, until the ImageView is used e.g. inside a ListView, which recycles child views for performance reasons. In this scenario, each time the ImageView is displayed, it will trigger an image load task. However, the sequence when the tasks are finished is undefined. So there is a chance that the image displayed actually comes from the previous item, which for some reason takes longer to load.

ListView

To solve this issue, the ImageView should remember the last image it’s supposed to load, so we extends the class and introduces a simple loadImageResource() method as below:

public void loadImageResource(int resId) {
    // if loading or already loaded the same resource, do nothing
    // otherwise cancel the current loading task
    if (mResId == resId)
        return;
    if (mImageLoadingTask != null) {
        final ImageLoadingTask loadingTask = mImageLoadingTask.get();
        if (loadingTask != null)
            loadingTask.cancel(true);
    }
 
    mResId = resId;
    setImageBitmap(null); // or some placeholder image
 
    final ImageLoadingTask loadingTask = new ImageLoadingTask(getResources(), this);
    mImageLoadingTask = new WeakReference(loadingTask);
    loadingTask.execute(resId);
}

Cache Images

Now everything works fine, except that whenever our ImageView get recycled, it needs to load the image again, so we introduce a simple image cache:

public class ImageCache extends LruCache<Integer, Bitmap> {
    public ImageCache(int maxSize) {
        super(maxSize);
    }
 
    @Override
    protected int sizeOf(Integer key, Bitmap bitmap) {
        // uses byte sizes of the bitmaps
        return bitmap.getRowBytes() * bitmap.getHeight();
    }
}
 
// sample code to create an ImageCache using 1 / 8th of the memory
final int cacheSize = (int) (Runtime.getRuntime().maxMemory() / 8L);
ImageCache imageCache = new ImageCache(cacheSize);

With the above simple ImageCache, one can easily caches images in memory, and improves the performance significantly.