Subclassing UIView and UIScrollView - iOS Programming: The Big Nerd Ranch Guide - page 186

When the screen is redrawn (which happens after an event is processed), the window's
image is drawn to the screen. Then, all of the subviews of the window draw their images
to the screen, then the subviews of the subviews draw their images, and so on ( Fig-
ure 6.4 ) .
Figure 6.4 Drawing a view hierarchy to the screen
Creating a user interface boils down to this: create each view's image and add each view
to the view hierarchy. Classes like UIButton , MKMapView and UITextField
already know what their images look like. Sometimes, however, you want to create views