Question

is it possible to do something like this in objective-c (using categories or something similar):
(pseudo code)

   abstract class Element extends Node {}

   class Object extends Element (and uses Sprite instead of Node) {}

   class Sprite extends Node {}

One solution would be to put Node as a member variable to Element, but then i would have to write all the wrapper functions for Node class. Is it possible to extend it somehow?

thanks!

Answer 1

This used to be possible, but the necessary function (class_setSuperclass) was deprecated a long time ago and cannot be used anymore (which is probably a good thing).

There are a couple of different ways to go about this, some better than others.

The best, I think, would be to make a new Element subclass, called SpriteElement, which has a Sprite member variable, and then forwards on all the relevant sprite-ly methods to the internal Sprite instance.

You could also define a new protocol that captures all of the things you want about a Sprite, like so:

@protocol Sprite <NSObject>

- (void)doSpritelyThing;

@end

(I’m assuming that the Sprite class declares a -doSpritelyThing method)

Then, with a category, you could declare that the Element class conforms to the <Sprite> protocol:

@interface Element (Sprite) <Sprite>

@end

And them implement the method like so:

@implement Element (Sprite)

- (void)doSpritelyThing {
  ...
}

@end

You would probably also want to declare that the Sprite class conforms to the Sprite protocol:

@interface Sprite () <Sprite>

@end

Then in your code, instead of declaring your variables as type Element *, you’d declare them as id<Sprite> (any object that understands messages from the <Sprite> protocol).

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The first approach is fine if you can be flexible with an Element being a different instance from a Sprite. If you can’t, then the second approach works great, too.