Cocoa Tutorial: Get The Most Out of Key Value Coding and Observing

by Marcus Zarra

Key Value Observing/Key Value Coding (KVO/KVC) is probably one of the most powerful and most under-utilized features of Objective-C. Here are a couple of examples of how to get the most out of it

When a call is made on an object through Key Value Coding such as [self valueForKey:@”someKey”], numerous attempts are made to resolve the call. First, the method someKey is looked for. If that is not found then the iVar someKey is looked for. If neither of those are found, then one last attempt is made before presenting an error. That last attempt is a call to the method -(id)valueForUndefinedKey:. If that method is not implemented then an NSUndefinedKeyException is raised.

valueForUndefinedKey: is designed so that when you request a value from an object using -(id)valueForKey: the object has a last chance to respond to that request before an error occurs. This has many benefits and I have included two examples of those benefits in this post–Core Data Parameters and Data Formatting.