我怎样才能使球落下只与圆圈发生碰撞，如果它与部分的颜色不同？

Question

我正在创建一个应用程序，其中不同颜色的球落下，你必须通过旋转圆圈来抓住它们，以便圆圈那部分的颜色与球匹配。我遇到的一个问题是，我不知道如何仅为圆的一部分制作物理体，而不是整圆。什么是实现这一目标的最佳方式。谢谢，任何帮助表示赞赏。

func dropBall() {

    let ball = SKShapeNode(circleOfRadius: 40)
    ball.fillColor = colors[Int(arc4random_uniform(UInt32(level)))]
    ball.strokeColor = ball.fillColor
    ball.position = CGPoint(x: 0, y: self.size.height / 2.1)
    addChild(ball)

    let ballBody = SKPhysicsBody(circleOfRadius: 30)
    ballBody.mass = 1.5
    ballBody.categoryBitMask = PhysicsCategory.ball
    ballBody.collisionBitMask = 4
    ball.physicsBody = ballBody

    mostRecentBallColor = ball.fillColor

}




func addCircleObstacle() {


    var degreesSectionAngle = 360 / (level)
    var radiansSectionAngle = Double(degreesSectionAngle) * (M_PI / 180.0)

    var secondXCoordinate = cos(radiansSectionAngle) * 40

    var secondYCoordinate = sin(radiansSectionAngle) * 40

    let path = UIBezierPath()

    path.addArc(withCenter: CGPoint.zero,
                radius: 160,
                startAngle: CGFloat(radiansSectionAngle),
                endAngle: CGFloat(0),
                clockwise: false)

    path.addArc(withCenter: CGPoint.zero,
                radius: 200,
                startAngle: CGFloat(0.0),
                endAngle: CGFloat(radiansSectionAngle),
                clockwise: true)

    circleball = obstacleByDuplicatingPath(path, clockwise: true)
    obstacles.append(circleball)
    circleball.position = CGPoint(x: 0, y: self.size.height / -6)
    addChild(circleball)

    print(level)
    print(degreesSectionAngle)
    print(radiansSectionAngle)

    circleBody.isDynamic = false

    circleBody.categoryBitMask = PhysicsCategory.Edge

    circleball.physicsBody = circleBody

}





func obstacleByDuplicatingPath(_ path: UIBezierPath, clockwise: Bool) -> SKNode {

    let container = SKNode()

    var degreesSectionAngle = 360 / level
    var radiansSectionAngle = Double(degreesSectionAngle) * (M_PI / 180.0)


    var rotationFactor = CGFloat(radiansSectionAngle)
    if !clockwise {
        rotationFactor *= -1
    }

    for i in 0...level - 1 {

        if i == mostRecentBallColor {

        }

        section = SKShapeNode(path: path.cgPath)
        section.fillColor = colors[i]
        section.strokeColor = colors[i]
        section.zRotation = rotationFactor * CGFloat(i);

        let sectionBody = SKPhysicsBody(polygonFrom: path.cgPath)
        sectionBody.categoryBitMask = PhysicsCategory.Obstacle
        sectionBody.collisionBitMask = 0
        sectionBody.contactTestBitMask = PhysicsCategory.ball
        sectionBody.affectedByGravity = false
        section.physicsBody = sectionBody

        container.addChild(section)   

    }
    return container
}