ios animation

CAAnimation
↓
CAPropertyAnimation
↓ ↓
CABasicAnimation CAKeyframeAnimation

CABasicAnimation
Simple animations involve changing the properties(from a start value to an end value) of a layer
and letting Core Animation animate those changes over time. Simple animation 也被称为property-based animation.
implicit simple animation: all you have to do is update the properties of your layer object.
explicit simple animation: create a CABasicAnimation object and use that object to configure the animation parameters