His initial goal was to not have to port it -- it was to get it compiling from the same source on both, since they're both Objective-C applications written atop

His initial goal was to not have to port it -- it was to get it compiling from the same source on both, since they're both Objective-C applications written atop OS X. What he found was that similar functionality was implemented with an arbitrarily different interface. UIKit is not exactly the same as AppKit (I think it's a little bit cleaner) and breaks AppKit code even for features that are supported on both sides. If you're used to stable APIs and expect strong justification for changing an interface, especially when there is no apparent change for features supported or efficiency, going from AppKit to UIKit is like having someone flick vinegar off of their finger tips into your eyes. And if you're used to being able to develop software (using other open source software) on a computer you paid for and run it on a device you paid for without having to pay again and sign a contract limiting your freedom, then it's like getting a right hook to the abdomen.