/**
當(dāng)前時(shí)間2秒以后開始動畫
*/
keyFrameAnim.beginTime = CACurrentMediaTime() + 2;
/**
截止到當(dāng)前時(shí)間，動畫已經(jīng)執(zhí)行了2秒，
注意，如果執(zhí)行的時(shí)間大于動畫時(shí)長，則表示動畫已經(jīng)執(zhí)行過。
*/
keyFrameAnim.beginTime = CACurrentMediaTime() - 2;

 /**
 The receiver does not appear until it begins and is removed from the presentation when it is completed.
 */
 kCAFillModeRemoved; // （默認(rèn)）動畫模型的呈現(xiàn)效果直至開始時(shí)才顯示，并在動畫結(jié)束后移除。
 /**
 The receiver does not appear until it begins but remains visible in its final state when it is completed.
 */
 kCAFillModeForwards; // 動畫模型的呈現(xiàn)效果直至開始時(shí)才顯示，但在動畫結(jié)束后仍然顯示最后的狀態(tài)。
 /**
 The receiver appears in its initial state before it begins but is removed from the presentation when it is completed.
 */
 kCAFillModeBackwards; // 動畫開始之前，動畫模型顯示其初始呈現(xiàn)效果，但在動畫結(jié)束后移除。
 /**
 The receiver appears in its initial state before it begins and remains visible in its final state when it is completed.
 */
 kCAFillModeBoth; // 動畫開始之前，動畫模型顯示其初始呈現(xiàn)效果，并且在動畫結(jié)束后仍然顯示最后的狀態(tài)。

- (IBAction)pauseBtnClicked:(id)sender {
  /**
   判斷當(dāng)前圖層對象是否有針對postion屬性的動畫效果
   */
  if ([self.layer.presentationLayer animationForKey:@"position"]) {
    // 通過絕對時(shí)間獲取圖層的本地時(shí)間
    CFTimeInterval localTime = [self.layer convertTime:CACurrentMediaTime() fromLayer:nil];
    /**
     將圖層的時(shí)間流逝速度設(shè)置為0，以暫停動畫
     */
    self.layer.speed = 0;
    // 設(shè)置圖層的時(shí)間軸偏移量，為繼續(xù)動畫做準(zhǔn)備
    self.layer.timeOffset = localTime;

  }
}

- (IBAction)continueBtnClicked:(id)sender {
  /**
   判斷當(dāng)前圖層對象是否有針對postion屬性的動畫效果
   */
  if ([self.layer.presentationLayer animationForKey:@"position"]) {
    // 獲取上次暫停時(shí)的時(shí)間軸偏移量
    CFTimeInterval timeOffset = self.layer.timeOffset;

    // 重置時(shí)間軸偏移量
    self.layer.timeOffset = 0;
    // 速度還原為1
    self.layer.speed = 1;

    // 重置開始時(shí)間
#warning 此處嚴(yán)重不理解。
    self.layer.beginTime = 0;

    // 計(jì)算暫停時(shí)間和當(dāng)前時(shí)間的差值
    CFTimeInterval localTime = [self.layer convertTime:CACurrentMediaTime() fromLayer:nil];
    CFTimeInterval timeSincePause = localTime - timeOffset;

    // 從上一次暫停處開始
    self.layer.beginTime = timeSincePause;
  }
}