如何在启动Cocoa应用程序时确定按下的修饰键？

In an app I'm developing, a database is used to store all user data, usually located somewhere in Library/Application Support. In order to enable the user to switch the database, I want to implement a functionality similar to iTunes or iPhoto, where the app asks for the library's or database's location if the option key is pressed upon starting the app.In an app I'm developing, a database is used to