ObjC: is there such a thing as a "class protocol"?

Question

For object instances we can have their class declare some protocol conformance as in:

@protocol P <NSObject>
- (void) someMethod ;
@end

@interface C : NSObject <P>
@end

@implementation C
- (void) someMethod {

}
@end

But what about classes?

I find myself in this situation:

...
Class c = [self modelClass:kind] ;
if (c) {
    model = [c performSelector: @selector(decode:) 
                    withObject: [SExpIO read: [fm contentsAtPath:target]]] ;
}

and I wish there were a way for me to declare that there is such a thing as protocols for class methods.

In the above example, all classes that c can be a class-instance (Hmmm??) of, declare

+ (id) decode: (SExp *) root ;

Is there a way that I could transform the above into:

if (c) {
    model = [c decode: [SExpIO read: [fm contentsAtPath:target]]]
}

by using a suitable "class protocol" declaration?

Answer 1

A protocol is just a list of method declarations. They can be class methods or instance methods. Example:

@protocol MyProtocol

+ (id) aClassMethod;
+ (void) someOtherClassMethod;
- (void) someInstanceMethod;

@end