How do I do mask preloader?

Hi guys,

I’m just wondering how to do mask preloader? I’ve created a 100 frame preloader but i do not know how to write the mask preloader…

anyone might to help out?

I’ve successfully created a width preloader which is start from left to the end of right. but is not masking.

here is my 100 frame preloader…

http://www.c-cltd.com.tw/preloader.rar

Thank you very much. Appreciated a lot.