This is why I went with GNUstep. All of the APIs are 1:1 with Apple's, including the runtime (which is very important imo). It has AppKit support as well; its AppKit implementation is kind of half-baked, but at least it exists. Also, a lot of the APIs are kind of old. But looking at how Swift evolved over the years, maybe that's not a bad thing.

My main complaint with GNUstep is the licensing. The runtime itself is MIT which is great, but its implementation of Foundation/AppKit is LGPL. ObjFW, including its runtime, is LGPL. At least with GNUstep one day I can create my own version of Foundation based on Cocotron or swift-corelibs-foundation or something, and not need to muck with rpaths + ship a bunch of .dll/.dylib/.so files with my app in order to comply with the license.