Unexpected behavior in scaling CAShapeLayer

I am using the following code to scale a CAShapelayer instance (i.e., self.view.layer.sublayers![0]):

class ViewController: UIViewController {   
override func viewDidLoad()
{
    super.viewDidLoad()
    self.view = Map(frame: CGRect(x: 0, y: 0, width: self.view.bounds.width, height: self.view.bounds.height))
    let pinchGR = UIPinchGestureRecognizer(target: self, action: #selector(self.didPinch(_:)))
    self.view.addGestureRecognizer(pinchGR)
}

@objc func didPinch(_ pinchGR: UIPinchGestureRecognizer)
{
    let transformation = CGAffineTransform(a: pinchGR.scale, b: 0, c: 0, d: pinchGR.scale, tx: 0, ty: 0)
    if pinchGR.state == .began || pinchGR.state == .changed
    {
        self.view.layer.sublayers![0].setAffineTransform(transformation)
    }
}}

class Map: UIView{
override init(frame: CGRect)
{
    super.init(frame: frame)
    self.backgroundColor = UIColor.blue
    drawTestShape()
}
required init?(coder aDecoder: NSCoder)
{
    super.init(coder: aDecoder)
}
func drawTestShape()
{
    let shapeLayer = CAShapeLayer()
    shapeLayer.frame = self.frame
    let path = UIBezierPath()
    shapeLayer.lineWidth = 5.0
    shapeLayer.fillColor = nil
    shapeLayer.strokeColor = UIColor.white.cgColor
    shapeLayer.backgroundColor = UIColor.red.cgColor
    path.move(to: CGPoint(x:200,y:200))
    path.addLine(to: CGPoint(x:300,y:300))
    path.addLine(to: CGPoint(x:100,y:300))
    path.close()
    shapeLayer.path = path.cgPath
    self.layer.addSublayer(shapeLayer)
}}

The scaling is performed correctly in the first pinch gesture. However, when a second pinch gesture is applied, the CAShapelayer instance starts again from its initial size, and not from the size it had after the first gesture.

I can't really get behind the idea of scaling a layer, but if you must do it, try something more like this; this is the standard pattern for implementing a pinch gesture recognizer, so it's really best to stick with it:

@IBAction func didPinch(_ pinchGR: UIPinchGestureRecognizer) {
    self.view.layer.sublayers![0].setAffineTransform(
        self.view.layer.sublayers![0].affineTransform().scaledBy(
            x:pinchGR.scale, y:pinchGR.scale))
    pinchGR.scale = 1.0;
}