SKShapeNode跟随触摸

Question

我试图了解如何在触摸时创建SKShapeNode线并使终点跟随触摸。我已经弄清楚如何反复创建线，但不是根据当前触摸位置实际动态地改变其终点。有谁知道如何做到这一点？谢谢。 这是我目前的代码：

func touchDown(atPoint pos : CGPoint) {
    if captain.contains(pos){
        selectedUnit = "p1"
    }
}

func touchMoved(toPoint pos : CGPoint) {

    if selectedUnit == "p1" {
        let line = SKShapeNode()

        startPoint = CGPoint(x:captain.position.x,y:captain.position.y)
        endPoint = pos

        pathToDraw.move(to: CGPoint(x: startPoint.x, y: startPoint.y))
        //            pathToDraw.addLine(to: CGPoint(x: endPoint.x, y: endPoint.y))
        pathToDraw.addLine(to: CGPoint(x: pos.x, y: pos.y))

        line.zPosition = 100
        line.path = pathToDraw
        line.strokeColor = UIColor.red
        line.lineWidth = 5
        self.addChild(line)
    }


}