How do I print preview a page with Gecko2.0 and higher version?

Hello, 

In Gecko1.9, we can print preview a page with nsIWebBrowserPrint::PrintPreview() method only need provide one "printSettings" parameter by: printPreview(printSetting, null, null); 

But in Gecko2.0, above code not work, after reading the source, I know that the second parameter(childWindow) should not be null in Gecko2.0. But I don't know which window should be passed to it? I tried pass current browser window to it, but not work. Anyone can help me out? Any comment is welcome. Thanks a lot.