Formal and Informal Protocols

There are two varieties of protocol, formal and informal:

• 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.

• 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.

Adopting and Conforming to a Formal Protocol

A class can either declare adoption of a formal protocol or inherit adoption from a superclass. The adoption syntax uses angle brackets in the @interface declaration of the class. In the following example, the CAAnimation class declares its superclass to be NSObject and then formally adopts three protocols.

@interface CAAnimation : NSObject <NSCopying, CAMediaTiming, CAAction>

A class—and any instance of that class—are said to conform to a formal protocol if the class adopts the protocol or inherits from another class that adopts it. Conformance to a protocol also means that a class implements all the required methods of the protocol. You can determine at runtime whether an object conforms to a protocol by sending it a conformsToProtocol: message.

Creating Your Own Protocol

You may also declare your own protocol and implement the code that communicates with adopters of that protocol. For a description of how to do this, see the document that definitively describes protocols.