Protocol versus Category

Question

Can anyone explain the differences between Protocols and Categories in Objective-C? When do you use one over the other?

Answer 1

Categories:

A category is a way of adding new methods to all instances of an existing class without modifying the class itself.

You use a category when you want to add functionality to an existing class without deriving from that class or re-writing the original class.

Let's say you are using NSView objects in cocoa, and you find yourself wishing that all instances of NSView were able to perform some action. Obviously, you can't rewrite the NSView class, and even if you derive from it, not all of the NSView objects in your program will be of your derived type. The solution is to create a category on NSView, which you then use in your program. As long as you #import the header file containing your category declaration, it will appear as though every NSView object responds to the methods you defined in the catagory source file.

Protocols:

A protocol is a collection of methods that any class can choose to implement.

You use a protocol when you want to provide a guarantee that a certain class will respond to a specific set of methods. When a class adopts a protocol, it promises to implement all of the methods declared in the protocol header. This means that any other classes which use that class can be certain that those methods will be implemented, without needing to know anyting else about the class.

This can be useful when creating a family of similar classes that all need to communicate with a common "controller" class. The communication between the controller class and the controlled classes can all be packaged into a single protocol.

Side note: the objective-c language does not support multiple inheritance (a class can only derive from one superclass), but much of the same functionality can be provided by protocols because a class can conform to several different protocols.