animation ios stroke swift uibezierpath

Swift: UIBezierPath Stroke Animation from Center

I’ve been making a simple UIBezierPath animation with Swift. This path consists on creating a rounded rectangle with a colored border. The animation must be the drawing of the colored border. To do so, I’ve created a CAShapeLayer with a UIBezierPath(roundedRect:, cornerRadius: )

let layer = CAShapeLayer()
var viewPrueba = UIView()
override func viewDidLoad() {
// Do any additional setup after loading the view, typically from a nib.
viewPrueba = UIView(frame: CGRectMake(self.view.frame.width/2-100, self.view.frame.height/2 - 100, 200, 200))
let path = UIBezierPath(roundedRect: CGRectMake(0, 0, 200, 200), cornerRadius: 40.0)
layer.path = path.CGPath
layer.fillColor = UIColor.clearColor().CGColor
layer.strokeColor = UIColor.blueColor().CGColor
layer.strokeStart = 0.0
layer.strokeEnd = 0.0
layer.lineWidth = 4.0
layer.lineJoin = kCALineJoinRound
let tapGR = UITapGestureRecognizer(target: self, action: #selector(ViewController.anim))
func anim() {
let anim1 = CABasicAnimation(keyPath: "strokeEnd")
anim1.fromValue = 0.0
anim1.toValue = 1.0
anim1.duration = 4.0
anim1.repeatCount = 0
anim1.autoreverses = false
anim1.removedOnCompletion = false
anim1.additive = true
anim1.fillMode = kCAFillModeForwards
self.layer.addAnimation(anim1, forKey: "strokeEnd")

It works well. The only problem is that the animation starts from the top-left part of the square and not from the top-center. How can I do that?

The only thing I’ve found in order to achieve this is by doing it with a circle and not a rectangle, which is not what we want.

This is where the animation starts