1. The Layer Tree
Core Animation's original name is Layer Kit
Core Animation is a compositing engine; its job is to compose different pieces of visual content on the screen, and to do so as fast as possible. The content in question is divided into individual layers stored in a hierarchy known as the
layer tree. This tree forms the underpinning for all of UIKit, and for everything that you see on the screen in an iOS application.
In UIView, tasks such as rendering, layout and animation are all managed by a Core Animation class called
CALayer. The only major feature of UIView that isn’t handled by CALayer is user interaction.
There are four hierarchies, each performing a different role: