Category or Protocol? (objc newbie)

FROM : Rick Kitts
DATE : Tue Apr 19 04:36:33 2005

Hello. I'm working on a class that uses a delegate. I was looking at
how Apple does this sort of thing and ran into NSXMLParser. It takes a
delegate, of course, and the delegate interface is declared as follows:

// The parser's delegate is informed of events through the methods in
the NSXMLParserDelegateEventAdditions category.
@interface NSObject (NSXMLParserDelegateEventAdditions)

To be honest, I'm not sure that this means. Does
NSXMLParserDelegateEventAdditions
define a type that I can subclass?

Coming from Java world my inclination is to use a Protocol and do:

-(void)setDelegate:(id<MyProtocol>)delegate;

This never seems to be done in any of the Apple stuff so I'm guessing
my inclination is at least idiomatically wrong but, again, I'm not sure
why.

So, I guess what I'm asking is can anyone either a) explain what this
stuff is trying to do or b) point me to some docs or a book that can
help me understand this sort of thing? I have read the objc manual
(PDF) from Apples site and I didn't see this sort of thing covered.

Many thanks in advance for any assistance,
---Rick