What language features does Ruby have that Objective-C lacks? It's a bit more succinct, perhaps, but when you're calling out to Cocoa all the time, that succinctness is going to get thrown out the window.
I did some looking, and I found a few[1] examples[2] of coroutines in Objective-C, though none of them seemed fully fleshed-out. Even if they were, none were, as you say, nearly as nice as "yield".