DownloadProgress.swift

class DownloadCircleProgressView: UIView {

    // Interface
    var progress: Float = 0.0 {
        didSet {
            // animate from [oldValue] to [newValue]
            animateProgressLayer(from: CGFloat(oldValue), to: CGFloat(progress), duration: 0.1)
        }
    }

    var color: UIColor = .white

    // start: top center, direction: clockwise
    private let angle = (start: -CGFloat.pi / 2, end: 3 * CGFloat.pi / 2)

    private lazy var outerRingLayer: CAShapeLayer = {
        let outerLayer = CAShapeLayer()
        outerLayer.frame = bounds
        return outerLayer
    }()

    private lazy var progressLayer: CAShapeLayer = {
        let progressLayer = CAShapeLayer()
        progressLayer.frame = bounds
        return progressLayer
    }()

    private let animation = CABasicAnimation()

    override func draw(_ rect: CGRect) {
        let center = CGPoint(x: bounds.width / 2, y: bounds.height / 2)

        outerRingLayer.removeFromSuperlayer()
        outerRingLayer.path = UIBezierPath(arcCenter: center, radius: bounds.height / 2, startAngle: angle.start, endAngle: angle.end, clockwise: true).cgPath
        outerRingLayer.shadowColor = UIColor.black.cgColor
        outerRingLayer.shadowOffset = CGSize(width: 1.0, height: 1.0)
        outerRingLayer.shadowOpacity = 0.5
        outerRingLayer.strokeStart = 0.0
        outerRingLayer.strokeEnd = 1.0
        outerRingLayer.fillColor = UIColor.clear.cgColor
        outerRingLayer.lineWidth = 2.0
        outerRingLayer.strokeColor = color.cgColor
        layer.addSublayer(outerRingLayer)

        progressLayer.removeFromSuperlayer()
        progressLayer.path = UIBezierPath(arcCenter: center, radius: bounds.height / 4, startAngle: angle.start, endAngle: angle.end, clockwise: true).cgPath
        progressLayer.strokeStart = 0.0
        progressLayer.strokeEnd = 0.0
        progressLayer.fillColor = UIColor.clear.cgColor
        progressLayer.strokeColor = color.cgColor
        progressLayer.lineWidth = bounds.height / 2
        layer.addSublayer(progressLayer)
     }

    private func animateProgressLayer(from: CGFloat, to: CGFloat, duration: CFTimeInterval) {
        progressLayer.removeAnimation(forKey: "strokeEnd")
        animation.keyPath = "strokeEnd"
        animation.fromValue = from
        animation.toValue = to
        animation.duration = duration
        animation.isRemovedOnCompletion = false
        animation.isAdditive = true
        animation.fillMode = CAMediaTimingFillMode.forwards
        progressLayer.add(animation, forKey: "strokeEnd")
    }
}