Share Sheets in iOS have another name behind the scenes — activity view controllers. That's fitting because, in addition to providing sharing options, they also provide for other "activities", like action options. Traditionally, that's been system options such as copy, save, print, etc. However, just like custom sharing options have now been made available in iOS 8, so too have custom action options. They're part of the new Extensibility feature set, one of the most important additions to iOS since the advent of the App Store. Extensibility enables Widgets, custom keyboards, and so much more, including custom action extensions. So, how do action extensions work?