How to make an update loop?

I have read through the cocoa programming guide (the first half, atleast), and the "Your first iPhone App" tutorial from Apple. From what I understand so far iPhone Apps are event driven, like Flash programs (when a button is pushed, a function is called to handle it). But I want to make games, and that requires updating every frame, regardless of whether any events occurred or not. Is there an event that occurs every frame that I can use to update game sprites and run AI, etc? In Flash I would do it with a object.onEnterFrame() function, if that helps anyone understand what I'm talking about.