about protocols

Protocols declare methods that can be implemented by any class.

To declare methods that others are expected to implement

To declare the interface to an object while concealing its class

To capture similarities among classes that are not hierarchically related

Formal Protocols

@protocol MyProtocol

- (void)requiredMethod;

@optional
- (void)anOptionalMethod;
- (void)anotherOptionalMethod;

@required
- (void)anotherRequiredMethod;

@end

Informal Protocols

@interface NSObject ( MyXMLSupport )
- initFromXMLRepresentation:(NSXMLElement *)XMLElement;
- (NSXMLElement *)XMLRepresentation;
@end

Adopting a Protocol
@interface ClassName : ItsSuperclass < protocol list >
Categories adopt protocols in much the same way:
@interface ClassName ( CategoryName ) < protocol list >

@interface Formatter : NSObject < Formatting, Prettifying >
@end