Informal Protocols
In addition to formal protocols, you can also define an informal protocol by grouping the methods in a category declaration:
@interface NSObject ( MyXMLSupport ) |
- (NSXMLElement *) XMLRepresentation; |
- initFromXMLRepresentation:(NSXMLElement *)XMLElement; |
@end |
Informal protocols are typically declared as categories of the NSObject class, since 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 would also be 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 implementing all the methods is optional, such as for a delegate, but (on Mac OS X v10.5 and later) it is typically better to use a formal protocol with optional methods.
Last updated: 2008-02-05
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