问题
Say, if we already have aScreen that points to the main screen, by
UIScreen *aScreen = [UIScreen mainScreen];
can we get to the main UIWindow object from this UIScreen object? (do they have any connection at all? That is, instead of using [UIApplication sharedApplication].keyWindow)
回答1:
No.
The application has a list of windows, usually just one. Each window has a screen property that indicates where it's currently located. The screen object holds information about the physical device.
It seems inspired on OSX NSScreen, there is no a list of windows per screen. Each app has windows, each window knows in which screen is located.
回答2:
A UIWindow knows which screen it's assigned to, but a UIScreen doesn't know what UIWindows are using it. That's why the standard pattern is to save the main window as a property of the ApplicationDelegate, so you'll have a reference to it.
来源：https://stackoverflow.com/questions/10743740/on-ios-can-we-get-to-the-main-uiwindow-object-from-the-uiscreen-object