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;