Xcode is written in Objective-C and takes advantage of a number of OS X frameworks, so porting it to Windows would require porting all the frameworks on which Xcode relies. Furthermore, Xcode also uses a number of programming tools that would have to be ported to Windows as well (some of them already are, of course).
There are several reasons that Objective-C isn't readily available on Windows:
- Most development of Objective-C frameworks takes place on OS X, and a lot of the frameworks aren't open-source and thus can't be ported to Windows (they'd have to be rewritten).
- There are some open-source frameworks that could be used on Windows -- for example, OS X's AppKit and Foundation frameworks are (mostly) available as part of the GNUstep project -- but these frameworks aren't widely used or supported on Windows, and sometimes lack capabilities found in their OS X counterparts.
That said, GCC is available on Windows, and since GCC is an Objective-C compiler, you could compile Objective-C code on Windows if you had the right libraries available (or didn't use any third-party libraries). But Objective-C isn't terribly useful without supporting frameworks, and those are rare or nonexistent on Windows.
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