Creating anonymous delegate objects in Objective-C

Question

Cocoa uses delegates extensively to provide (among other things) callback methods for asynchronous operations. However, I personally hate the delegate model and how it pollutes the current class with handlers for very specific child operations. UIAlertView is a perfect example.

Therefore, I'm wondering if it's possible to simply create an anonymous delegate object that meets the requirements of a delegate protocol (such as UIAlertViewDelegate) via blocks and pass this anonymous object wherever a delegate reference is expected.

I imagine something like so:

id myDelegate = @{
    alertView:alertView didDismissWithButtonIndex:index = ^ {
        ...
    }
};

UIAlertView* alertView = [[UIAlertView alloc] initWithTitle:... message:... delegate:myDelegate cancelButtonTitle:... otherButtonTitles:...;
[alertView show];

I've heard Objective-C has some good dynamic language features but I don't know how to simply add a method/selector to an object. Can this be done in a relatively straightforward manner?

Answer 1

Yes, the language features you mention are exposed via the objective-c runtime, although there's no built-in facility to dynamically create delegate classes and the runtime api is not the friendliest of things.

There is a library called A2DynamicDelegate, which does exactly what you're talking about. I haven't used it, but it may be worth investigating.

EDIT: A problem with this approach is that delegates are not retained, so you'll need to either keep a strong reference somewhere else, or an add an associative reference to the UIAlertView. You may find that all this fuss isn't worth it and just adding an extra method to your view controller works better (you can conform to the delegate protocol in a class extension to avoid polluting your interface).