Working with Windows and Views
iPad apps have a single window, so you won’t find additional document windows for displaying content. When your application is running — even though other apps may be hibernating or running in the background — your app’s interface takes over the entire screen.
Looking out the window
The single window that you see displayed on the iPad is an instance of the
UIWindow class. This window is created at launch time, either programmatically by you or automatically by
UIKit when you use a storyboard. In general, after you create the
Window object (that is, if you create it instead of having it done for you), you never really have to think about it again.
Although your application never creates more than one window at a time, iOS can support additional windows on top of your window. The system status bar is one example. You can also display alerts on top of your window by using the supplied Alert views.
Admiring the view
In an iPad app world, view objects are responsible for the view functionality in the Model-View-Controller architecture. A view is a rectangular area on the screen (on top of a window).