Unexpected behavior in scaling CAShapeLayer

Question

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.

Matt you are right this is not necessary, I removed it, but the problem I have posted is still there. — ckc, Feb 18 '18 at 16:46
Okay, but I can only go on the information you show. You need to provide enough relevant code to allow the problem to be reproduced, and explain how to reproduce it. Scaling a bare layer like this is a very odd thing to do. Why don't you scale a view? Why don't you use a scroll view which already knows how to respond to a pinch by zooming? — matt, Feb 18 '18 at 16:51
I have added the complete code. My target is the creation of a map. — ckc, Feb 18 '18 at 17:05
I have reached to the decision to scale the layer from "Core Animation Programming Guide". I also used this way since: - I will add a sublayer that will show the user's position using a circle, and this sublayer must not be scaled, - I have the impression that the layer scaling is faster, and - I want all the screen to have the pinch gesture capability, even if the layer has been zoomed to a smaller size. (I am new in IOS programming, so the aforementioned may be not correct) — ckc, Feb 18 '18 at 17:19
Well, I'm not persuaded, but I've provided code that translates the standard tried-and-true pattern for a pinch gesture on a view, adapted to a layer instead. — matt, Feb 18 '18 at 17:58

matt · Accepted Answer · 2018-02-18T17:55:57.247

1

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;
}

edited Feb 18 '18 at 17:55

answered Feb 18 '18 at 17:48

matt

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It works perfectly. Matt thank you very much for your help! I will also try the case of scaling a view, as you advised me (probably by adding a subview, adding the shapelayer on it and performing the animation on this subview instead of the layer). I appreciate your interest, have a nice day :) – ckc Feb 18 '18 at 18:07
You do not add the shape layer as a sublayer of the view; you make the view _host_ the shape layer _as its primary layer_. – matt Feb 18 '18 at 18:15
Thank you for the advice. I will try it and probably come back with new questions! – ckc Feb 18 '18 at 18:18

Unexpected behavior in scaling CAShapeLayer

1 Answers1