Objective C and magic methods in class

Question

Does objective-c offer a way to intercept calls to class method that does not exist?

Assimilative

Answer 1

The forwardInvocation method is what you are going to want to use. It is called automatically when a non-existent selector is called on an object. The default behavior of this method is to call doesNotRecognizeSelector:(which is what outputs debug information to your console), but you can override it do anything you want. One recommended approach by Apple is to have this method forward the method invocation to another object.

- (void)forwardInvocation:(NSInvocation *)anInvocation

Note that forwardInvocation is a fairly expensive operation. An NSInvocation object needs to be created by the framework and (optionally) used to invoke a selector on another instance. If you are looking for a (relatively) faster method of detecting non-existent selectors then you can choose to implement forwardingTargetForSelector instead.

- (id)forwardingTargetForSelector:(SEL)aSelector

You should Apple's documentation for how to override these methods effectively, there are some gotcha's to watch out for, particularly when overriding the forwardInvocation method on the same object that will have the missing selectors.

Kurzawa

Class method forwarding mechanism requires class methods, not instance methods. +(void)forwardingTargetForSelector:(SEL)sel; +(void)forwardInvocation:(NSInvocation *)anInvocation; The later function uses a supporting function: +(NSMethodSignature *)methodSignatureForSelector:(SEL)sel; – Quintillion

Answer 2

Yes, you can with the resolveClassMethod: class method (which is defined on NSObject):

http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html

Here is also something to watch out for (stumped me the first time): http://iphonedevelopment.blogspot.com/2008/08/dynamically-adding-class-objects.html

Heuristic