How to hide the pointer(mouse) when it's idle ?

[Wang Lei <wanglei 198112 gmail com>]

I'm going to write a function to hide the pointer when it's idle.

FUNCTIONALITY:
When the mouse has been idle for N secs, save the current position,
then move it to the corner of the focused window.When the mouse moves
again, I want it start from saved position.

But there is a problem. I wanted to find something like
pointer-motion-hook, but it doesn't exist. How can I know when the
mouse starts moving?

 Thanks!

-- 
Regards
Lei