setValueについて教えてください

関数circeAnimationの中のsetValueはanimationLayerとしてlayerに何の値をセットしているんですか？

swift
1
2  func circleAnimation(layer:CAShapeLayer){
3    //アニメーションを行うプロパティ＝今回はstrokeend(起点と終点を設定)
4    var drawAnimation = CABasicAnimation(keyPath: "strokeEnd")
5    
6    drawAnimation.setValue(layer, forKey: "animationLayer")
7    drawAnimation.delegate = self
8    
9    drawAnimation.duration = 1.0
10    drawAnimation.repeatCount = 1.0
11    drawAnimation.fromValue = 0.0
12    drawAnimation.toValue = 1.0
13    
14    drawAnimation.timingFunction = CAMediaTimingFunction(name:kCAMediaTimingFunctionEaseOut)
15    
16    layer.add(drawAnimation, forKey: "circleAnimation")
17    
18    
19    
20  }
21  override func animationDidStop(anim:CAAnimation!, finished flag:Bool ){
22    let  layer:CAShapeLayer = anim.valueForKey("animationLayer") as CAShapeLayer
23    countDownNum -= 1
24    countNumberLabel.text = String(countDownNum)
25    
26    if countDownNum <= 0 {
27      
28    }else{
29      circleAnimation(layer: layer)
30    }
31  }
32  
33
34    override func didReceiveMemoryWarning() {
35        super.didReceiveMemoryWarning()
36        // Dispose of any resources that can be recreated.
37    }
38    
39
40    /*
41    // MARK: - Navigation
42
43    // In a storyboard-based application, you will often want to do a little preparation before navigation
44    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
45        // Get the new view controller using segue.destinationViewController.
46        // Pass the selected object to the new view controller.
47    }
48    */
49
50}
51