Apple’s beta of their upcoming Photos for OS X app, which was included in yesterday’s initial beta release of OS X Yosemite 10.10.3, reportedly uses a private, unreleased UXKit framework, based on the iOS UIKit framework.
While Apple often uses private frameworks that never become accessible to developers, the UXKit framework is notable as it appears to be a version of UIKit based on AppKit.
Mac developers have longed for an OS X version of UIKit, which is the framework upon which apps are constructed and managed on iOS devices. Such a framework would offer developers a powerful tool for creating software for porting software over from iOS devices to the OS X desktop.
Currently, Apple has no known plans to release UXKit to third-party developers, although the framework could be a handy incentive to iOS developers as Apple attempts to encourage them to port their apps for the OS X desktop.