Animating split view divider position

[Starfox]

I've been trying to get a working split view with animations based on the code at http://jhaberstro.blogspot.com/2012/...splitview.html - I implemented it with a placeholder that assumed the existence of only one divider (for now), divider zero. Here's my code: http://pastebin.com/FfPLmDS9

It doesn't seem to be working however. setValue:forUndefinedKey: is only called once, and animationForKey: returns nil. What am I doing wrong there?