Writing objective-c code for mac os GUI apps was one of those things that finally made "interfaces"/"protocols" really click for me as a young developer. Just implement (some, not even all) method in "FooWidgetDelegate", and wire your delegate implementation into the existing widget. `willFrobulateTheBar` in your delegate is called just before a thing happens in the UI and you can usually interfere or modify with the behavior before the UI does it. Then `didFrobulateTheBar` is called after with the old and new values or whatever other context makes sense and you can hook in here for doing other updates in response to the UI getting an update. If you don't implement a protocol method, the default behavior happens, and preserving the default behavior is baked into the process, so you don't have to re-implement the whole widget's behavior just to modify part of it.

It's probably one of the better UI frameworks I think I've used (though admittedly a lot of that also is in part due to "InterfaceBuilder" magic and auto-wiring. Still I often wish for that sort of elegant "billions of hooks, but you only have to care about the ones you want to touch" experience when I've had to use other UI libraries.