Animating change to mask layer for iOS UIView

CircleFillView.swift

import UIKit

class CircleFillView: UIView {

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    
    let circleLayer = CAShapeLayer()
    let circleMaskLayer = CAShapeLayer()
    
    private func commonInit() {
        self.backgroundColor = .clear
        circleLayer.fillColor = UIColor.black.cgColor
        self.layer.addSublayer(circleLayer)
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        circleLayer.path = UIBezierPath(ovalIn: self.bounds).cgPath
    }
    
    private func fillPath(for percent: Double) -> CGPath {
        let height = self.bounds.height * CGFloat(percent)
        let rect = CGRect(x: 0, y: height, width: self.bounds.width, height: self.bounds.height - height)
        return UIBezierPath(rect: rect).cgPath
    }
        
    /// Change fill color using implicit animation.    
    func implicitChange() {
        circleLayer.fillColor = UIColor.blue.cgColor
    }
    
    /// Alter a mask to hide more of the circle. Animate the change with a custom duration.
    func animatedChange() {
        circleMaskLayer.path = fillPath(for: 0.0)
        circleLayer.mask = circleMaskLayer
        let animation = CABasicAnimation(keyPath: "path")
        animation.duration = 0.5
        animation.fromValue = fillPath(for: 0.0)
        animation.toValue = fillPath(for: 0.75)
        animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
        circleLayer.mask?.add(animation, forKey: "path")
        (circleLayer.mask as? CAShapeLayer)?.path = fillPath(for: 0.75)
    }
}