A Protocol in Objective-C is identical in functionality to an interface in Java, or a purely virtual class in C++.
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.
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.
In addition to formal protocols, you can also define an informal protocol by grouping the methods in a category declaration:
@interface NSObject (MyProtocol)
//someMethod();
@end
Informal protocols are typically declared as categories of the NSObject class, because that broadly associates the method names with any class that inherits from NSObject. Because all classes inherit from the root class, the methods aren’t restricted to any part of the inheritance hierarchy. (It is also possible to declare an informal protocol as a category of another class to limit it to a certain branch of the inheritance hierarchy, but there is little reason to do so.)
When used to declare a protocol, a category interface doesn’t have a corresponding implementation. Instead, classes that implement the protocol declare the methods again in their own interface files and define them along with other methods in their implementation files.
An informal protocol bends the rules of category declarations to list a group of methods but not associate them with any particular class or implementation.
Being informal, protocols declared in categories don’t receive much language support. There’s no type checking at compile time nor a check at runtime to see whether an object conforms to the protocol. To get these benefits, you must use a formal protocol. An informal protocol may be useful when all the methods are optional, such as for a delegate, but (in Mac OS X v10.5 and later) it is typically better to use a formal protocol with optional methods.
- A protocol is means to define a list of required and/or optional methods that a class implements. If a class adopts a protocol, it must implement all required methods in the protocols it adopts.
- Cocoa uses protocols to support interprocess communication through Objective-C messages. In addition, since Objective-C does not support multiple inheritance, you can achieve similar functionality with protocols, as a class can adopt more than one protocol.
- A good example of a protocol is NSCoding, which has two required methods that a class must implement. This protocol is used to enable classes to be encoded and decoded, that is, archiving of objects by writing to permanent storage.
- 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.
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.
In addition to formal protocols, you can also define an informal protocol by grouping the methods in a category declaration:
@interface NSObject (MyProtocol)
//someMethod();
@end
Informal protocols are typically declared as categories of the NSObject class, because that broadly associates the method names with any class that inherits from NSObject. Because all classes inherit from the root class, the methods aren’t restricted to any part of the inheritance hierarchy. (It is also possible to declare an informal protocol as a category of another class to limit it to a certain branch of the inheritance hierarchy, but there is little reason to do so.)
When used to declare a protocol, a category interface doesn’t have a corresponding implementation. Instead, classes that implement the protocol declare the methods again in their own interface files and define them along with other methods in their implementation files.
An informal protocol bends the rules of category declarations to list a group of methods but not associate them with any particular class or implementation.
Being informal, protocols declared in categories don’t receive much language support. There’s no type checking at compile time nor a check at runtime to see whether an object conforms to the protocol. To get these benefits, you must use a formal protocol. An informal protocol may be useful when all the methods are optional, such as for a delegate, but (in Mac OS X v10.5 and later) it is typically better to use a formal protocol with optional methods.