1）iOS动画的模型：三层树模型；

https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CoreAnimation_guide/CoreAnimationBasics/CoreAnimationBasics.html#//apple_ref/doc/uid/TP40004514-CH2-SW12

Layer Trees Reflect Different Aspects of the Animation State

An app using Core Animation has three sets of layer objects. Each set of layer objects has a different role in making the content of your app appear onscreen:

• Objects in the model layer tree (or simply “layer tree”) are the ones your app interacts with the most. The objects in this tree are the model objects that store the target values for any animations. Whenever you change the property of a layer, you use one of these objects. 

• Objects in the presentation tree contain the in-flight values for any running animations. Whereas the layer tree objects contain the target values for an animation, the objects in the presentation tree reflect the current values as they appear onscreen. You should never modify the objects in this tree. Instead, you use these objects to read current animation values, perhaps to create a new animation starting at those values. 

• Objects in the render tree perform the actual animations and are private to Core Animation.

2）动画帧图片组；

3）呈现动画帧的时间控制；

4）动画的组合；

caanimationgroup：同一个layer；

CATransaction：不同layer；

5）自定义动画与帧自绘；

iOS动画的要素