Swift：检测精灵套件 SKShapeNode 绘图的相交

I'm drawing with Sprite Kit. I would like to detect when user's drawings are intersecting.

我尝试遵循代码，但它不起作用。看起来精灵套件并没有保存所有的点：

override func touchesMoved(touches: NSSet, withEvent event: UIEvent) {
    /* Called when a touch begins */
    touch = touches.anyObject() as UITouch!        
    for drawingPoint in drawingPoints{
        if(touch.locationInNode(self) == drawingPoint){println(true)}
    }
    drawingPoints.append(touch.locationInNode(self))
}

这是一个快速求线段相交的函数。

func linesIntersect(line1 : CGPointInterval, line2 : CGPointInterval) -> (intersects: Bool, point : CGPoint?)
    {
        //The algorithm is taken from http://www.amazon.com/dp/0672323699/?tag=stackoverfl08-20
        // http://portal.aauj.edu/portal_resources/downloads/programming/windows_game_programming_guru.pdf

        let p0_x = line1.start.x
        let p1_x = line1.end.x
        let p2_x = line2.start.x
        let p3_x = line2.end.x

        let p0_y = line1.start.y
        let p1_y = line1.end.y
        let p2_y = line2.start.y
        let p3_y = line2.end.y

        let s1_x = p1_x - p0_x
        let s1_y = p1_y - p0_y
        let s2_x = p3_x - p2_x
        let s2_y = p3_y - p2_y

        let s = (-s1_y * (p0_x - p2_x) + s1_x * (p0_y - p2_y)) / (-s2_x * s1_y + s1_x * s2_y)
        let t = ( s2_x * (p0_y - p2_y) - s2_y * (p0_x - p2_x)) / (-s2_x * s1_y + s1_x * s2_y)

        if (s >= 0 && s <= 1 && t >= 0 && t <= 1)
        {
            // Collision detected
            let finalX = p0_x + (t * s1_x)
            let finalY = p0_y + (t * s1_y)
            return (true, CGPointMake(finalX, finalY))
        }

        return (false, nil) // No collision
    }