Question

I’ve been trying to implement a simple component-based game object architecture using Objective-C, much along the lines of the article ‘Evolve Your Hierarchy’ by Mick West. To this end, I’ve successfully used a some ideas as outlined in the article ‘Objective-C Message Forwarding’ by Mike Ash, that is to say using the -(id)forwardingTargetForSelector: method.

The basic setup is I have a container GameObject class, that contains three instances of component classes as instance variables: GCPositioning, GCRigidBody, and GCRendering. The -(id)forwardingTargetForSelector: method returns whichever component will respond to the relevant selector, determined using the -(BOOL)respondsToSelector: method.

All this, in a way, works like a charm: I can call a method on the GameObject instance of which the implementation is found in one of the components, and it works. Of course, the problem is that the compiler gives ‘may not respond to …’ warnings for each call. Now, my question is, how do I avoid this? And specifically regarding the fact that the point is that each instance of GameObject will have a different set of components? Maybe a way to register methods with the container objects, on a object per object basis? Such as, can I create some kind of -(void)registerMethodWithGameObject: method, and how would I do that?

Now, it may or may not be obvious that I’m fairly new to Cocoa and Objective-C, and just horsing around, basically, and this whole thing may be very alien here. Of course, though I would very much like to know of a solution to my specific issue, anyone who would care to explain a more elegant way of doing this would additionally be very welcome.

Much appreciated, -Bastiaan

Answer 1

I don’t think that sending the container object all of its components’ messages is what Mick West was suggesting–that doesn’t help to remove the idea of a “monolithic game entity object”.

The eventual goal is to have the components communicate directly with one another, with no container object at all. Until then, the container object acts as glue between old code that expects a single object for each game entity and the new component-to-component system.

That is, you shouldn’t need to use message forwarding at all in the final product, so ignoring the warnings, or declaring variables as id for now to quiet them, isn’t all that ugly. (The plan as laid out by the article is to eventually remove the very code that is causing your warnings!)