Protocols in Objective C

Protocols are like interface in Java. It is having all methods. The implementation of the methods of this protocols are implemented in other class in which they are derived/subclassed.

Syntax / Protocols are declared as follows:-

@protocol NSObserver

- (void) response: (NSString *) str;

@end

When class is derived from protocol then that class must be implement all the methods of that protocol.

Deriving / adopting protocol 

@interface NSHttpRequest : NSObject < NSObserver , NSRequestSend >

{

// instance variables

}

// methods

@end 

You can specify the protocol name in the datatype like declaration or return type.
Example:-
id< NSObserver > obj;

It is telling that obj will hold anytime of object which has adopted NSObserver protocol.

There are two different types of methods in protocol.
i.Optional
ii. Required

When class derive/ adopted protocol need not compulsory to implements optional methods.
Required methods are compulsory to implement.
Method without any directive are compulsory to implement.

Ex:.

@protocol NSHttpObserver

- (void) response: (NSString *) str;// compulsory to implement

@optional 

-(NSString*) getHeader; // optional to implement

@required

- (void)setUri(NSString*) uri;// compulsory to implement

- (void)setProxy(NSString*) proxy;// compulsory to implement

@end // BaseballPlayer