CABasicAnimation on transform.scale translates CAShapeLayer on the X & Y axis - swift

Question

I am trying to add a pulsating effect around a button, however, the code I am using translates the CAShapeLayer as well as increasing its size.
How do I only increase the scale of a CAShapeLayer during this animation whilst keeping its position in the view static?
I have isolated the code out into a simple project which performs this animation and it is still occurring.
See effect in a video here: https://imgur.com/a/AbTtLKe
To test this:
Create a new project
Add a button into the centre of the view
Link it to the viewControllers code file as an IBOutlet with the name beginButton
Here is my code:
let pulsatingLayer = CAShapeLayer()
override func viewDidLoad() {
super.viewDidLoad()
let circularPath = UIBezierPath(arcCenter: beginButton.center, radius: beginButton.bounds.midX, startAngle: -CGFloat.pi / 2, endAngle: 2 * .pi, clockwise: true)
pulsatingLayer.path = circularPath.cgPath
pulsatingLayer.strokeColor = UIColor.clear.cgColor
pulsatingLayer.lineWidth = 10
pulsatingLayer.fillColor = UIColor.red.cgColor
pulsatingLayer.lineCap = kCALineCapRound
view.layer.addSublayer(pulsatingLayer)
animatePulsatingLayer()
}
private func animatePulsatingLayer() {
let pulseAnimation = CABasicAnimation(keyPath: "transform.scale")
pulseAnimation.toValue = 1.5
pulseAnimation.duration = 1
pulseAnimation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseOut)
pulseAnimation.autoreverses = true
pulseAnimation.repeatCount = Float.infinity
pulsatingLayer.add(pulseAnimation, forKey: "pulsing")
}
Thanks!

Answer 1

Your animation is relative to the origin of the frame of the view.
By changing the center of the circular path to be CGPoint.zero, you get an animation that pulses centered on the origin of the layer. Then by adding that to a new pulsatingView whose origin is centered on the button, the pulsing layer is centered on the button.
override func viewDidLoad() {
super.viewDidLoad()
let circularPath = UIBezierPath(arcCenter: CGPoint.zero, radius: beginButton.bounds.midX, startAngle: -CGFloat.pi / 2, endAngle: 2 * .pi, clockwise: true)
pulsatingLayer.path = circularPath.cgPath
pulsatingLayer.strokeColor = UIColor.clear.cgColor
pulsatingLayer.lineWidth = 10
pulsatingLayer.fillColor = UIColor.red.cgColor
pulsatingLayer.lineCap = kCALineCapRound
let pulsatingView = UIView(frame: .zero)
view.addSubview(pulsatingView)
view.bringSubview(toFront: beginButton)
// use Auto Layout to place the new pulsatingView relative to the button
pulsatingView.translatesAutoresizingMaskIntoConstraints = false
pulsatingView.leadingAnchor.constraint(equalTo: beginButton.centerXAnchor).isActive = true
pulsatingView.topAnchor.constraint(equalTo: beginButton.centerYAnchor).isActive = true
pulsatingView.layer.addSublayer(pulsatingLayer)
animatePulsatingLayer()
}

Answer 2

You need to read up on transformation matrixes. (Any book on 3D computer graphics should have a section that covers it.)
A scale transformation is centered on the current origin. So if your shape is not centered on the origin, it will be drawn in towards the origin. To scale a shape in toward it's own center you need to do something like this:
Create an identity transform
Translate it by (-x, -y) of your shape's center
Add the desired scale to the transform
Add a Translate by (x, y) to shift the shape back
Concat your transform to the layer's transform.
(I always have to think really hard to get this stuff right. I wrote the above off the top of my head, before 7:00 AM local time on a weekend, without sufficient caffeine. It probably isn't exactly right, but should give you the idea.)