/// 開始動畫
func startRotating() {
 let rotateAnimation = CABasicAnimation(keyPath: "transform.rotation")
 rotateAnimation.isRemovedOnCompletion = false // 避免點擊 Home 鍵返回,動畫停止
 rotateAnimation.fromValue = 0.0
 rotateAnimation.toValue = Double.pi * 2
 rotateAnimation.duration = 20
 rotateAnimation.repeatCount = MAXFLOAT
 self.layer.add(rotateAnimation, forKey: nil)
 isRotating = true
}

/// 停止動畫
func stopRotating() {
 if !isRotating {
 return
 }
 let pausedTime = self.layer.convertTime(CACurrentMediaTime(), from: nil)
 // 讓CALayer的時間停止走動
 self.layer.speed = 0
 // 讓CALayer的時間停留在pausedTime這個時刻
 self.layer.timeOffset = pausedTime
 isRotating = false
}

/// 繼續(xù)動畫
func resumeRotate() {

 if isRotating {
 return
 }

 if self.layer.timeOffset == 0 {
 startRotating()
 return
 }

 let pausedTime = self.layer.timeOffset
 // 1. 讓CALayer的時間繼續(xù)行走
 self.layer.speed = 1.0
 // 2. 取消上次記錄的停留時刻
 self.layer.timeOffset = 0.01
 // 3. 取消上次設置的時間
 self.layer.beginTime = 0.0
 // 4. 計算暫停的時間(這里也可以用CACurrentMediaTime()-pausedTime)
 let timeWhenpause = self.layer.convertTime(CACurrentMediaTime(), from: nil) - pausedTime
 // 5. 設置相對于父坐標系的開始時間(往后退timeSincePause)
 self.layer.beginTime = timeWhenpause
 isRotating = true

}