Good time to check out ObjFW [1], it's a cross platform ObjC framework that's just really awesome
How easy is it to port, say, a Leopard-era Objective C app to ObjFW?
- There was a bug or two I had to patch, but the code is readable, so it wasn't a big deal
- OFString, etc aren't intended to be 1:1 replacements for NSString, etc. This wasn't a real problem. They mostly match, and all I needed to do was write a few categories
- The runtime functions are not compatible at all, but most projects wouldn't touch those
- CoreFoundation and the other C APIs are not there at all, so you'll need replacements
- It is a replacement for Foundation framework, not AppKit, so if it is a GUI app you still have a lot of work to do
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.
GNUStep is stuck in 1995 sadly.