[favorited]

You rarely have to add @objc annotations or use NSObject directly. You only need it for target/action, KVO, etc.

I rarely see that annotation or class name in my codebases too.

[dep_b]

UIButton only? Maybe binding UITextField events?

[favorited]

Buttons, selector-based Notifications (though the block version is finally good, so we mostly just use that now), & gesture recognizers look like our biggest users of @objc. Basically all to support target/action.

A quick search through one of my apps shows that about 4% of functions are marked as @objc (and we're not using the old compatibility mode where more methods were implicitly @objc either).

[regera]

@IBAction can replace @ObjC for all selectors. We put it a mandate in our code reviews. There is also NSObjectProtocol which forces (in most cases) devs to subclass NSObject.