删除Quartz2D Swift

Question

我正在创建一个绘图应用程序，并且能够绘制线条并更改画笔宽度和不透明度。我现在不能删除（撤消）在UIImageView上绘制的行。下面是我的代码，如果你知道任何帮助，那将是伟大的或知道如何让我朝着正确的方向:)

func drawLineFrom(fromPoint: CGPoint, toPoint: CGPoint) {

    // 1
    UIGraphicsBeginImageContext(view.frame.size)

    tempImageView.image?.draw(in: CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height))
    let context = UIGraphicsGetCurrentContext()
    // 2

    context?.move(to: CGPoint(x: fromPoint.x, y: fromPoint.y))
    context?.addLine(to: CGPoint(x: toPoint.x, y: toPoint.y))

    // 3

    context?.setBlendMode(CGBlendMode.normal)
    context?.setLineCap(CGLineCap.round)
    context?.setLineWidth(brushWidth)
    context?.setStrokeColor(UIColor(red:red, green:green, blue:blue, alpha: 1.0).cgColor)

    // 4
    context?.strokePath()

    // 5
    tempImageView.image = UIGraphicsGetImageFromCurrentImageContext()
    tempImageView.alpha = opacity
    UIGraphicsEndImageContext()

}

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    swiped = true
    if let touch = touches.first {
        let currentPoint = touch.location(in: view)
        drawLineFrom(fromPoint: lastPoint, toPoint: currentPoint)


        lastPoint = currentPoint
    }
}

override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
    if !swiped {
        // draw a single point
        drawLineFrom(fromPoint: lastPoint, toPoint: lastPoint)
    }

    // Merge tempImageView into mainImageView
    UIGraphicsBeginImageContext(mainImageView.frame.size)
    mainImageView.image?.draw(in: CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height),blendMode: CGBlendMode.normal, alpha: 1.0)

    tempImageView.image?.draw(in: CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: CGBlendMode.normal, alpha: opacity)

    mainImageView.image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    tempImageView.image = nil
}