The NSProtocolChecker and NSProxy classes provide message filtering and forwarding capabilities. If you wish to ensure at runtime that a given object will only be sent messages in a certain protocol, you create an NSProtocolChecker instance with the protocol and the object as arguments-

    id versatileObject = [[ClassWithManyMethods alloc] init];
    id narrowObject = [NSProtocolChecker protocolCheckerWithTarget: versatileObject
                                         protocol: @protocol(SomeSpecificProtocol)];
    return narrowObject;

This is often used in conjunction with distributed objects to expose only a subset of an objects methods to remote processes