How do you tell if a key exists for an object using Key-Value Coding?

Question

I\'d like to test whether an object has a writeable @property in the iPhone SDK.

One possible way of doing this is to check the -valueForKey: method, but that seems rat

Answer 1

There is no way, in general, to determine if an object has a value for a given key. An instance may decide to return a value for an otherwise undefined key from its -valueForUndefinedKey: method. Or it may let the default implementation throw an exception. Modern Objective-C (2.0) objects often declare relevant properties in their public API. For these objects, you can use the Objective-C runtime's class_copyPropertyList or protocol_copyPropertyList to get a list of the available properties. You can also emulate the KVC search list, testing via repondsToSelector: if the instance responds to an appropriate getter method. Finally you could use the runtime's class_copyIvarList to check the ivars for an appropriate ivar. This is a lot of work that you shouldn't be doing. It won't guarantee that you know whether an object instance will raise an exception when sent a valueForKey: message and it indicates a bigger issue...

If you have a collection of objects that must all provide a value for a given key, but some don't, you have a design problem. You should define an appropriate interface (a @protocol in Objective-C) that declares the needed properties. You can then test for conformance to this protocol, or make use of the compiler to do some compile-time type checking for you to make sure instance conform to the protocol (if you're passing around single instances).