What's the difference between formal and informal protocols in Objective-C?

Question

What's the difference between formal and informal protocols in Objective-C?

Answer 1

From http://developer.apple.com/iphone/library/documentation/General/Conceptual/DevPedia-CocoaCore/Protocol.html

Formal and Informal Protocols

There are two varieties of protocol, formal and informal:

• An informal protocol is a category on NSObject, which implicitly makes almost all objects adopters of the protocol. (A category is a language feature that enables you to add methods to a class without subclassing it.) Implementation of the methods in an informal protocol is optional. Before invoking a method, the calling object checks to see whether the target object implements it. Until optional protocol methods were introduced in Objective-C 2.0, informal protocols were essential to the way Foundation and AppKit classes implemented delegation.

• A formal protocol declares a list of methods that client classes are expected to implement. Formal protocols have their own declaration, adoption, and type-checking syntax. You can designate methods whose implementation is required or optional with the @required and @optional keywords. Subclasses inherit formal protocols adopted by their ancestors. A formal protocol can also adopt other protocols.

Formal protocols are an extension to the Objective-C language.