The Core iOS Developer’s Cookbook, Fifth Edition

View Hierarchies

A tree-based hierarchy orders what you see on your iOS screen. Starting with the main window, views are laid out in a specifically hierarchical way. All views may have children, called subviews. Each view, including the window, owns an ordered list of these subviews. Views might own many subviews; they might own none. Your application determines how views are laid out and who owns whom.

Subviews display in order, always from back to front. This works something like a stack of animation cels—the transparent sheets used to create cartoons. Only the parts of the sheets that have been painted show through. The clear parts allow any visual elements behind the sheet to be seen. Views, too, can have clear and painted parts and can ...