如何防止关闭贝塞尔路径

我想创建一条贝塞尔曲线，并为我的对象创建碰撞边界。

let firstSurfacePoint = CGPoint(x: 30, y: 120)
let secondSurfacePoint = CGPoint(x: 20, y: 200)
let thirdSurfacePoint = CGPoint(x: 200, y: 300)

let center = CGPoint(x: 150, y: 120)
let radius: CGFloat = 120.00
let arcWidth: CGFloat = 20.00    
let fourthSurfacePoint = CGPoint(x: 240, y: 300)

func createCollisionPath(collision: UICollisionBehavior) {

    let line =  UIBezierPath()

    line.moveToPoint(firstSurfacePoint)
    line.addCurveToPoint(thirdSurfacePoint, controlPoint1: secondSurfacePoint, controlPoint2: secondSurfacePoint)
    line.addLineToPoint(fourthSurfacePoint)

    let currentPath = CAShapeLayer()
    currentPath.path = line.CGPath
    currentPath.strokeColor = UIColor.blackColor().CGColor
    currentPath.fillColor = UIColor.clearColor().CGColor
    currentPath.lineWidth = 1
    view.layer.addSublayer(currentPath)

    collision.addBoundaryWithIdentifier("curve", forPath: line)

}

如果我选择 addArcWithCente，我会得到同样糟糕的结果

line.moveToPoint(firstSurfacePoint)
line.addArcWithCenter(center, radius: radius, startAngle: CGFloat(M_PI), endAngle: CGFloat(M_PI/2), clockwise: false)
line.addLineToPoint(fourthSurfacePoint)

在这两次尝试中，收到了奇怪的结果，这是至关重要的，因为我无法创建适当的碰撞。在第一次尝试中，我的对象穿过路径，就好像它是从firstPoint到thirdPoint的直线一样，在第二次尝试中也是如此

我究竟做错了什么？

我的图（无弧）

我的身材（带弧线）

currentPath.fillColor = nil

or

currentPath.fillColor = UIColor.clearColor().CGColor