How to create login window before main window appears?

[tiplip]

Hi,

State my case first, I want to create one login window, which will be closed if user account info is verified then new main window shows, the application exits if user choose to close the login window directly, of course, main window won't appear this moment.

I have no idea what to start, for instance, how to be notified to show or hide(close) another main window after login window closed?

any idea will be appreciated, thanks.