How to extend protocols / delegates in Objective-C?

Question

The syntax for creating a protocol that implements another protocol is as such:

@protocol NewProtocol <OldProtocol>
- (void)foo;
@end

If you want to call a method in NewProtocol on a pointer typed as OldProtocol you can either call respondsToSelector:

if ([object respondsToSelector:@selector(foo)])
    [(id)object foo];

Or define stub methods as a category on NSObject:

@interface NSObject (NewProtocol)
- (void)foo;
@end
@implementation NSObject (NewProtocol)
- (void)foo
{
}
@end