Animate a bezier path drawn in drawRect() Swift

UIBezierPath to draw border on UIButton with animation like loader (Swift 5)

func animation(){
    CATransaction.begin()

    let layer : CAShapeLayer = CAShapeLayer()
    layer.strokeColor = UIColor.purple.cgColor
    layer.lineWidth = 3.0
    layer.fillColor = UIColor.clear.cgColor

    let path : UIBezierPath = UIBezierPath(roundedRect: CGRect(x: 0, y: 0, width: self.btn.frame.size.width + 2, height: self.btn.frame.size.height + 2), byRoundingCorners: .allCorners, cornerRadii: CGSize(width: 5.0, height: 0.0))
    layer.path = path.cgPath

    let animation : CABasicAnimation = CABasicAnimation(keyPath: "strokeEnd")
    animation.fromValue = 0.0
    animation.toValue = 1.0

    animation.duration = 7.0

    CATransaction.setCompletionBlock{ [weak self] in
        print("Animation completed")
    }

    layer.add(animation, forKey: "myStroke")
    CATransaction.commit()
    self.btn.layer.addSublayer(layer)
}

Note:Animation Output

To animate CGPath you cannot use UIView.animation methods. I created custom UIView subclass to show you how can you animate CGPaths shape, please refer to the comments and amend it for your requirements:

class MyView: UIView {

let shapeLayer = CAShapeLayer()
let maskLayer = CAShapeLayer()
var rectanglePath = UIBezierPath()

override func didMoveToSuperview() {
    super.didMoveToSuperview()

    backgroundColor = UIColor.clear

    // initial shape of the view
    rectanglePath = UIBezierPath(rect: bounds)

    // Create initial shape of the view
    shapeLayer.path = rectanglePath.cgPath
    shapeLayer.strokeColor = UIColor.black.cgColor
    shapeLayer.fillColor = UIColor.clear.cgColor
    layer.addSublayer(shapeLayer)

    //mask layer
    maskLayer.path = shapeLayer.path
    maskLayer.position =  shapeLayer.position
    layer.mask = maskLayer
}

func prepareForEditing(editing:Bool){

    let animation = CABasicAnimation(keyPath: "path")
    animation.duration = 2

    // Your new shape here
    animation.toValue = UIBezierPath(ovalIn: bounds).cgPath
    animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseOut)

    // The next two line preserves the final shape of animation,
    // if you remove it the shape will return to the original shape after the animation finished
    animation.fillMode = kCAFillModeForwards
    animation.isRemovedOnCompletion = false

    shapeLayer.add(animation, forKey: nil)
    maskLayer.add(animation, forKey: nil)
  }
}