通过以固定间隔更改的path属性SKShapeNode，可以创建逐帧动画序列。要创建动画，设置该path属性为形状的顺序，与一个圆，两端开始什么也没有。您可以使用UIBezierPath（用于的包装器）CGPath通过以下步骤为动画创建形状：

1. 将路径的“笔”移动到圆心
2. 弧添加到与路径addArcWithCenter从startAngle到endAngle
3. 在圆弧上与终点角相对应的点到中心的路径上添加一条线
4. 更改endAngle固定金额

5. 重复步骤1-4
   这是上述步骤的实现：

   override func didMove(to:SKView) {

   let circle = SKShapeNode(circleOfRadius: 50)
   circle.fillColor = SKColor.blue
   circle.strokeColor = SKColor.clear
   circle.zRotation = CGFloat.pi / 2
   addChild(circle)
   
   countdown(circle: circle, steps: 20, duration: 5) {
       print("done")
   }
   

   }

   // Creates an animated countdown timer
   func countdown(circle:SKShapeNode, steps:Int, duration:TimeInterval, completion:@escaping ()->Void) {
   guard let path = circle.path else {
   return
   }
   let radius = path.boundingBox.width/2
   let timeInterval = duration/TimeInterval(steps)
   let incr = 1 / CGFloat(steps)
   var percent = CGFloat(1.0)

   let animate = SKAction.run {
       percent -= incr
       circle.path = self.circle(radius: radius, percent:percent)
   }
   let wait = SKAction.wait(forDuration:timeInterval)
   let action = SKAction.sequence([wait, animate])
   
   run(SKAction.repeat(action,count:steps-1)) {
       self.run(SKAction.wait(forDuration:timeInterval)) {
           circle.path = nil
           completion()
       }
   }
   

   }

   // Creates a CGPath in the shape of a pie with slices missing
   func circle(radius:CGFloat, percent:CGFloat) -> CGPath {
   let start:CGFloat = 0
   let end = CGFloat.pi * 2 * percent
   let center = CGPoint.zero
   let bezierPath = UIBezierPath()
   bezierPath.move(to:center)
   bezierPath.addArc(withCenter:center, radius: radius, startAngle: start, endAngle: end, clockwise: true)
   bezierPath.addLine(to:center)
   return bezierPath.cgPath
   }