how to get what tells fileURL has been changed by others

FROM : norio
DATE : Fri Apr 18 12:53:34 2008

Hi,

I'd like to know who tells NSDocument that its location has been 
changed by other apps and how to get the notification.

I'd like to hack before you notice something like "the document's 
location was changed, so do you want to save any other location?" when 
you open the file by an app and move the file to the other directory 
in Finder while editing the file.

Thank you,
Norio