Here’s an interesting library created in Swift that provides background image downloading with built-in caching called Skeets from Dalton Cherry.

Skeets features a nice simple syntax requiring only one method call to download images with callbacks for progress, completion, and errors.

The feature list according to there readme includes:

• Multi level cache. In-memory and disk caching.
• Nonblocking IO. All HTTP and disk IO happen in the background, thanks to GCD.
• Simple one method to load a remote image.
• Robust, fast, and customizable caching.
• Simple concise codebase at just a few hundred LOC.
• handles redundant image requests, so only one request is sent for multiple queries

You can find Skeets on Github here.