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

There are such things as Protocols for class methods, and they’re called…. Protocols. For example, it looks like you want a protocol that looks like this:

@protocol MyDecoder
+ (id)decode:(SExp *)root;
@end

You can then use it like this:

Class c = [self modelClass:kind];
if ([c conformsToProtocol:@protocol(MyDecoder)]) {
     model = [c decode: [SExpIO read: [fm contentsAtPath:target]]];
}