That is more or less how UIs using Cocoa/Cocoa Touch are working from a developer standpoint of view. Most standard views/controls have a delegate property which is informed about certain things happening or asked when an important decision has to be made. This works extremely well and by implementing a delegate you get around the problems related to subclassing UI controls/views. Now that Objective-C/C has blocks Apple began to replace (or add) block based "callbacks" which makes life easier in most cases.
