For a while, iOS developers have complained that the UIKit
framework they use to develop apps isn’t available on the Mac,
making it harder to apply the same tools and techniques and code
they build for iOS to Mac apps.
Today Apple dropped Photos for Mac via a developer release, and
some developers are reporting signs that Apple has built this new
app using something called UXKit, which sits above the Mac’s
familiar AppKit frameworks and strongly resembles UIKit on iOS.