Users accept that not everything can be instantaneous. (Well, most users.) Even so, programmers strive toward perfection.

To approach instantaneity, most real-world apps augment the code you have here in two ways: adding a caching layer and preloading images.

A cache is a place to stash a certain number of Bitmap objects so that they stick around even when you are done using them. A cache can only hold so many items, so you need a strategy to decide what to keep when your cache runs out of room. Many caches use a strategy called LRU, or “least recently used.” When you are out of room, the cache gets rid of the least recently used item.

The Android support library has a class called LruCache that implements an LRU strategy. For the first challenge, use LruCache to add a simple cache to ThumbnailDownloader. Whenever you download the Bitmap for a URL, stick it in the cache. Then, when you are about to download a new image, check the cache first to see whether you already have it around.

Once you have built a cache, you can preload things into it before you actually need them. That way, there is no delay for Bitmaps to download before displaying them.

Preloading is tricky to implement well, but it makes a huge difference for the user. For a second, more difficult challenge, for every GalleryItem you display, preload Bitmaps for the previous 10 and the next 10 GalleryItems.