How can I get the checkbox' value in the header?

I am using the jQuery plugin Flexigrid which has the checkbox attached with every row, and the checkbox in the header can make the rows all checked.

Now I开发者_开发问答 am making a dialog which has a Flexigrid in it, when I press a button. I want the dialog popup and make the checkbox in the header to be "checked" and when I leave the dialog, make it unchecked. But how can I do it?

All right, I've got it. Just use:

$('th input[type="checkbox"]',$( the dialog)).attr("checked",false);