I am writing a text editor for Mac OS X. I need to display hidden characters in an NSTextView (such as spaces, tabs, and special characters). I have spent a lot of time searching for how to do this but so far I have not found an answer. If anyone could point me in the right direction I would be grateful.
Have a look at the NSLayoutManager class. Your NSTextView will have a layout manager associated with it, and the layout manager is responsible for associating a character (space, tab, etc.) with a glyph (the image of that character drawn on the screen).
In your case, you would probably be most interested in the replaceGlyphAtIndex:withGlyph: method, which would allow you to replace individual glyphs.