method_missing-like functionality in objective-c (i.e. dynamic delegation at run time)

Question

I'm trying to transform one method call into another dynamically (at runtime).

For instance, I'd like the following:

[obj foo]

to delegate to:

[obj getAttribute: @"foo"]

(I'd like to do this dynamically as I don't know ahead of time what those method names or attributes are going to be).

I see that there's a hook into:

 - (id) forwardingTargetForSelector: (SEL) aSelector

That only seems to work for delegation, though, I want to keep the object as "self" and transform the method arguments.

Where should I look for this sort of behavior? Is it even possible in obj-c?

Answer 1

You can use the method -forwardInvocation: for that. It takes a full NSInvocation object which represents the method call, and you can handle it however you wish. If you do this, you should also override -methodSignatureForSelector: to return the correct NSMethodSignature (required for -forwardInvocation: to work on unknown selectors). It's also recommended that you override -respondsToSelector: to declare that you can handle the selector in question.