需求：

need more control over their app’s animations

use layers to improve the drawing performance of their apps

需要：

understand the view architecture of your target platform

be familiar with how to create view-based animations

All you have to do is configure a few animation parameters (such as the start and end points) and tell Core Animation to start

understand the role that Core Animation plays as part of your app’s infrastructure.

一瞥:

manage app content

layer modification trigger animations 层修改触发动画—— 使用core animation的动画都会涉及到layer的属性

layers can be organized into hierarchies

actions let you change a layer’s default behavior——assign you action to layer’s property