Hi Mike, just listen the podcast and I really enjoyed how clear was your explanation of such a difficult topic. I have one question about the triple callback message dispatching in Objective-C, specifically the forwardInvocation: since the method returns void, is it possible to just do nothing and basically create the equivalent of a "/dev/null" for message dispatching? I'm asking this because I'm trying to understand if there are difference between forwardInvocation and Ruby method_missing. For instance, with method_instance nil is always returned (unless you return something else explicitly). Thank you.

If you do nothing in forwardInvocation: then control will indeed just return to the caller. I don't know about the return value they will see, though. You may need to explicitly zero out the return value in the NSInvocation before handing it back to them if you don't want them to receive garbage.

Note that you need to have an NSMethodSignature to be able to handle these things, which makes it hard to write an arbitrary message handler. If you're doing nothing then you can get away with an NSMethodSignature that doesn't match 100%, but you still need the return type to be correct for the forwarding mechanism to work properly.

I was wondering if you have a copy of this audio? I listened when the podcast first came out and was so amazed how much better I understood what was going on. I've wanted to find it again for years – and just happened upon the description today. I'm sure this is the enlightenment I remember.

But – the MP3 linked at Mobile Orchard is gone. oh no.

http://mobileorchard.com/podcast-mike-ash-on-the-objective-c-runtime-objects-and-the-runtime-message-sending-and-no-such-method/

I'm afraid I don't have a copy. If you manage to dig one up, please let me know and I will post a link to it.