如何在Objective-C中扩展协议/委托

If i want to extend a class like AVAudioPlayer, whats the best way to also add another method to AVAudioPlayerDelegate ?. Do I make a category for it, do I extend it? If I extend it do I then also have to make sure to overwrite the actual delegate getter/setter ? How would I extend the protocol ? The following gives me errorsIf i want to extend a class like AVAudioPlayer,