cannot understand the following code in objective c?

开发者_开发技巧the following is declared inside the @interface of another class... I don't understand the syntax and what it is trying to say

@interface someclass:NSObject
....
@end

@interface NSObject (HSRequestClassDelegate)

-(void) finishedWithData:(HSRequest *) request data:(NSMutableData *) data;
-(void) failedWithError:(HSRequest *) request error:(NSError *) error;
-(void) noConnection:(HSRequest *) request;

@end

The name of the above file is someclass.h? what does the line @interface NSObject (HSRequestClassDelegate) mean?

thank you in advance.

This is a definition of an informal protocol. Prior to protocols being introduced, protocols were defined as a category on NSObject.

That's called a "category" in cocoa.