The Animation Proxy Object
So what is the animator proxy object? The animator proxy object is available in both NSView and NSWindow. It implements the protocol NSAnimatablePropertyContainer. This container uses Key-Value Coding to set the actual value of whatever parameter was specified while doing the value interpolation and animation behind the scenes.
As the name implies, the animator proxy acts as an agent that takes the value you give it and handles animating the property from the starting or current value to the value specified. It then sets the property as if you had called set on the property explicitly.