Knowing when a dragging session starts in Finder

I'm trying to fade a window in when the user initiates a drag in Finder. I'm guessing I'll have to use the Accessibility API, but don't know how to get started.

There seem to be lots of notifications in the AX* and NSAccessibility* AP开发者_如何学运维Is, but I can't find one for dragging items.

By the way, this is a sandboxed Lion app. This might make things harder or impossible, but you probably know more about that than me.