CALayer的动画设置：
extension CALayer {
func pauseAnimate(){
let pausedTime: CFTimeInterval = convertTime(CACurrentMediaTime(), from: nil)
speed = 0.0;
timeOffset = pausedTime;
}
func resumeAnimate(){
let pausedTime: CFTimeInterval = timeOffset
speed = 1.0;
timeOffset = 0.0;
beginTime = 0.0;
let timeSincePause: CFTimeInterval = convertTime(CACurrentMediaTime(), from: nil) - pausedTime
beginTime = timeSincePause;
}
}
