Sure there are (for some definition of "most important parts). Take a look at GNUstep for a start. Some MacOS frameworks like WebKit are also open sourced from Apple itself.
As an example of how this works in practice, the disassembler/decompiler Hopper has both MacOS and Linux versions that are compiled from the same code.
Now, how successful they'll be extending the open source frameworks to made this useful in compiling non-trivial software that wasn't written with "dual compatibility" is a fair question, but the starting pieces are there, and patching the compilers and creating a copy of MacOS's filesystem layout would go a long way in making it easier to compile MacOS software on FreeBSD/GNUstep.
And in any event, so far as doing it legally, I don't see why this would be any less legal than Wine's recreation of the Win32 API/libraries. Only difference is Apple might be more litigious than Microsoft, but Microsoft doesn't exactly lie down to software theft...
> Sure there are (for some definition of "most important parts). Take a look at GNUstep for a start.
I was following and trying to use GNUstep 15 years ago, and they were constantly chasing a moving target and were not managing to catch up. (Plus naturally the reimplementation bugs, plus the differences in interpretation of the "spec" which forced the programmer to be very careful and not take shortcuts.)
That's the huge issue when you are trying to reproduce an API which is not under your control by any mean, and is still alive and changing.
GNUStep is not current with the frameworks that it corresponds to on macOS (and was never complete).
Those frameworks are also a tiny part of the full set that you need to build non-trivial software on macOS.
Audio? Video? so much more stuff that nobody has ever tried to reimplement the APIs for.
Yes, such a reimplementation would be legal,but Wine has been a project for 20+ years and has a huge headstart compared to any idea of doing this for macOS.
As an example of how this works in practice, the disassembler/decompiler Hopper has both MacOS and Linux versions that are compiled from the same code.
Now, how successful they'll be extending the open source frameworks to made this useful in compiling non-trivial software that wasn't written with "dual compatibility" is a fair question, but the starting pieces are there, and patching the compilers and creating a copy of MacOS's filesystem layout would go a long way in making it easier to compile MacOS software on FreeBSD/GNUstep.
And in any event, so far as doing it legally, I don't see why this would be any less legal than Wine's recreation of the Win32 API/libraries. Only difference is Apple might be more litigious than Microsoft, but Microsoft doesn't exactly lie down to software theft...