问题:

I'm writing a Cocoa API for a project and the API takes a delegate. The protocol that I came up with declares all the methods as optional, but why would I do that instead of just documenting the delegate methods in a header file and taking a plain id as a parameter?

回答1:

For the benefit of your users. If the object takes delegates conforming to some protocol and they pass something else in, the compiler can tell them. That isn't possible if you take an id and use a category as the delegate method interface.

回答2:

Because having "all of these methods" optional isn't quite the same as permitting "anything you care to send".

回答3:

It also produces code that is more usable in the IDE. For example if I'm looking at

@interface MyController : NSObject <FooBarDelegate> {
}
@end

I can command+double click in Xcode to jump to the definition of FooBarDelegate. With a category there's no formal declaration of intent to be a delegate.

Also, @required can be a problem for future plans with regard to backward binary compatibility and a new preferred method signature.