The View Hierarchy
Every application has a single instance of UIWindow that serves as the container for all the views in the application. UIWindow is a subclass of UIView, so the window is itself a view. The window is created when the application launches. Once the window is created, other views can be added to it.
When a view is added to the window, it is said to be a subview of the window. Views that are subviews of the window can also have subviews, and the result is a hierarchy of view objects with the window at its root (Figure 3.2).
Figure 3.2 An example view hierarchy and the interface that it creates
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