The most significant change to Apple’s developer ecosystem this decade has been the introduction of the Swift programming language – and we’ll probably see the next big change come during this year’s WWDC with the introduction of third party UIKit apps on the Mac.
As for Swift, the new language was announced at WWDC 2014. With contributions from both Apple engineers and the open-source community, it has seen constant updates and is now in version 4.2.1.
An important aspect of Swift that has been affecting users since its first version is that its application binary interface, or ABI, is not stable. What that means in practice is that Apple can’t include the Swift language support in its operating systems, because an app written with Swift 3 won’t work with the language support binaries for Swift 4. The solution to that is to include the Swift language libraries inside the app bundle that gets downloaded from the App Store, increasing the bandwidth and storage required
