Custom property animation with actionForKey: how do I get the new value for the property?

In a CALayer subclass I'm working on I have a custom property that I want to animate automatically, that is, assuming the property is called "myProperty", I want the following code: [myLayer setMyProperty:newValue]; To cause a smooth animation from the current value to "newValue". Using the approach of overriding actionForKey: and needsDisplayForKey: (see following code) I was able to get it to ru