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How to do the documentation in objective C?
Is it possible to write class/method documentation in Objective-C in a way that Xcode 4 displays them in the quick help, and/or generates a doc document from it in the style of Apple's own documentation?
Displaying it live - not that I know of.
But as for generating Apple like documentation, the best I have found is appledoc which I've been using for a year now. If accepts a wide range of commenting styles including Javadoc styles and can generate very Apple like documentation which it can also install directly into your Xcode help system.
I don't know if it still works for Xcode 4 but for Xcode 3 you could generate API doc sets using doxygen.
Apple has a set by step guide: Using doxygen to Create Xcode Documentation Sets on how to do it.
There is also the appledoc tool available from GitHub. Makes nice docsets.