我可以在没有动画的情况下更改strokeEnd属性吗？

我似乎无法从strokeEndCAShapeLayer的属性中删除动画。

文档说该属性是animatable默认设置的，但不是默认设置的，我无法查明问题所在。有什么建议在哪里看？

这是我的代码：

class ViewController: UIViewController {

    let circle = CAShapeLayer()

    override func viewDidLoad() {
        super.viewDidLoad()

        // Circle
        circle.fillColor = UIColor.clearColor().CGColor
        circle.strokeColor = UIColor.blackColor().CGColor
        circle.lineWidth = 10
        circle.strokeEnd = 0
        circle.lineJoin = kCALineJoinRound
        circle.path = UIBezierPath(ovalInRect: CGRectMake(60, 140, 200, 200)).CGPath
        circle.actions = ["strokeEnd" : NSNull()]

        // Show Button
        let showButton = UIButton(frame: CGRectMake(40, 40, 240, 40))
        showButton.addTarget(self, action: "showButton", forControlEvents: UIControlEvents.TouchUpInside)
        showButton.setTitle("Show circle", forState: UIControlState.Normal)
        showButton.backgroundColor = UIColor.greenColor()

        // Add to view
        self.view.layer.insertSublayer(circle, atIndex: 1)
        self.view.addSubview(showButton)
    }

    func showButton() {
        circle.strokeEnd = 1
    }
}