Most applications display images in some way, shape, or form. Those images are often fetched from a remote server, introducing a number of interesting challenges. Performing a request to a remote server takes time and it requires resources. It is therefore important to consider solutions to minimize the number of requests an application makes.

We added the ability to cancel image requests in the previous episode. This and the next episode focus on caching images. We start simple by caching images in memory.

In this episode, we continue to improve the solution we implemented in this series by caching images on disk. Caching images on disk has a number of benefits. It reduces the number of requests the application makes and it improves the performance of the application. The user experiences the application as fast and snappy.

The previous episodes have illustrated that caching images can result in significant performance improvements. In the previous episode, I stressed the importance of writing cached images to disk on a background thread to prevent the image service from blocking the main thread.

Earlier in this series, you learned that a cache on disk has a number of benefits. It persists the cache across launches and it can be used to seed a cache in memory. Even though modern devices have plenty of disk space, we need to be mindful of the space the cache on disk takes up on the user's device. Applications like Twitter and Instagram fetch hundreds if not thousands of images. Even if those images are optimized and small in size, the cache on disk can grow quickly, taking up a non-trivial amount of space on the user's device. In this episode, we add the ability to limit the size of the cache on disk, similar to how the image service limits the size of the cache in memory.

In the previous episodes, we implemented a service to fetch and cache remote images. Even though the service we built is pretty flexible, some applications require a more powerful solution and more options to fit their needs. This episode focuses on Kingfisher, a popular, open source library to fetch and cache remote images. You learn how to integrate Kingfisher in a project using CocoaPods. I show you how Kingfisher differs from the service we created earlier in this series and we take a peek under the hood to learn how Kingfisher does its magic.