Question

Is it possible to implement protocol chaining?

For example, I’m defining a new protocol (ProtocolA), and I’d like to make sure that it can only be applied to objects that conform to ProtocolB. So something like…

@protocol ProtocolA <NSObject<ProtocolB>>
@end

Question

Does objective C allow this type of behaviour? If not is there a design pattern I should be aware of to achieve a similar effect?

Answer 1

Instead of using inscribed protocols (which is improper format), simply separate each individual protocol with a comma like so:

@protocol ProtocolA <NSObject, ProtocolB>
@end

It’s perfectly legal. In fact, several hundred UIKit protocols are declared along these lines.