Martin Pilkington: Appreciating AppKit, Part 1:

“AppKit is Apple's UI framework for building apps for the Mac. It has existed in one form or another for around 30 years and is the basis for many of the concepts and features of UIKit on iOS. Understandably, given its age, it has quite a few quirks and dated features. Some can simply be ignored, such as drawers. Others are still core to how parts of AppKit function, such as NSCell. These features can make AppKit seem daunting and difficult to work with, especially for those who have only known UIKit.

“However, the upside of AppKit's age is that it has an incredibly rich feature set. If you get over the initial hurdle you find a framework far more powerful than UIKit has ever been. Indeed it is this power that has played a significant role in allowing small teams to build the sorts of apps the Mac has been celebrated for.”