I was wondering whether to use HeaderDoc or Doxygen for Objective-C project, and after a bit of googling I get the impression that HeaderDoc is on its way out.
I found a nice article on the Apple website here which describes how to get XCode to run Doxygen for you as part of your build, and turn the results into a DocSet which XCode knows about.
It works rather well, and the documentation for my test iPhone project now shows up in XCode along with the standard Apple sets.
One slight issue - I had to edit the script a bit to cope with having a space in the path to your project. It also has rather verbose output - adding –silent to the make command helps a bit, as does altering the doxygen.config to run quietly. Unfortunately the commands in the make file that Doxygen generates still seem to spit out a fair amount of crap even if everything is working.
One other problem - XCode can’t seem to find the documentation for a type when I option-click on it, in the way that it would do for an Apple type. This may just be a question of needing to rebuild my indexes though…