Discover subclasses of a given class in Obj-C

Question

Is there any way to discover at runtime which subclasses exist of a given class?

Edit: From the answers so far I think I need to clarify a bit more

Answer 1

Rather than try to automatically register all the subclasses of MYCommand, why not split the problem in two?

First, provide API for registering a class, something like +[MYCommand registerClass:].

Then, create code in MYCommand that means any subclasses will automatically register themselves. Something like:

@implementation MYCommand
+ (void)load
{
    [MYCommand registerClass:self];
}
@end