Model支撑：(依附对象)
从presentLayer获取数据；
渲染树为私有；
-(void)addAnimation:(CAAnimation *)anim forKey:(NSString *)key
key：动画帧的关联要素；
timingFunction：控制动画运行的节奏；
duration：动画的持续时间；
delegate：动画代理，用来监听动画的执行过程；
主要用于自定义动画；
CAMediaTiming：动画的暂停、继续控制
@interface CALayer : NSObject <NSCoding, CAMediaTiming>
@protocol CAMediaTiming
/* The begin time of the object, in relation to its parent object, if
* applicable. Defaults to 0. */
@property CFTimeInterval beginTime;
/* The basic duration of the object. Defaults to 0. */
@property CFTimeInterval duration;
/* The rate of the layer. Used to scale parent time to local time, e.g.
* if rate is 2, local time progresses twice as fast as parent time.
* Defaults to 1. */
@property float speed;
/* Additional offset in active local time. i.e. to convert from parent
* time tp to active local time t: t = (tp - begin) * speed + offset.
* One use of this is to "pause" a layer by setting `speed' to zero and
* `offset' to a suitable value. Defaults to 0. */
@property CFTimeInterval timeOffset;