1. Install Homebrew if you haven't.
2. Run brew install ctags. The version of ctags that ship of OS X is an antique.
3. Run echo "--langdef=objc" >> ~/.ctags && echo "--langmap=objc:.m" >> ~/.ctags && echo "--regex-objc=/^[\t ]*(\-|\+)[\t ]*\([_a-zA-Z0-9][_a-zA-Z0-9]*\)([_a-zA-Z0-9][_a-zA-Z0-9]*)/\2/f/" >> ~/.ctags
4. cd to the directory to write the tags file in
5. Run ctags -R mytargetdirectory
6. Press ^] in MacVim over a function name to go to declaration/definitions

PS: Creating the .ctags file is so convoluted because I can't figure out how to put a multiliner without ruining the formatting in the instructions.

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