Круглый индикатор выполнения в iOS с длительным альфа-прогрессом

Один из подходов — замаскировать CAGradientLayerCAShapeLayer. Слой фигуры должен иметь дугу в качестве path и lineCap из .round.

class FadingProgressView: UIView {
    let shapeLayer = CAShapeLayer()
    let gradientLayer = CAGradientLayer()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = .clear
        layer.addSublayer(gradientLayer)
        gradientLayer.mask = shapeLayer
        gradientLayer.colors = [UIColor.blue.cgColor, UIColor.white.cgColor, UIColor.white.cgColor]
        gradientLayer.type = .conic
        gradientLayer.startPoint = CGPoint(x: 0.5, y: 0.5)
        gradientLayer.endPoint = CGPoint(x: 0.4, y: 0)
        gradientLayer.locations = [0, 0.75, 1]
        gradientLayer.backgroundColor = UIColor.clear.cgColor
        
        shapeLayer.fillColor = nil
        shapeLayer.strokeColor = CGColor(gray: 0, alpha: 1)
        shapeLayer.lineWidth = 5
        shapeLayer.lineCap = .round
        
        let animation = CABasicAnimation(keyPath: "transform.rotation")
        animation.fromValue = CGFloat.pi * 2
        animation.toValue = 0
        animation.duration = 1
        animation.repeatCount = .greatestFiniteMagnitude
        layer.add(animation, forKey: "transform.rotation")
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        gradientLayer.frame = bounds
        shapeLayer.frame = bounds
        let path = UIBezierPath(
            arcCenter: .init(x: bounds.midX, y: bounds.midY),
            radius: bounds.width / 2 - 20,
            startAngle: 1.55 * .pi, endAngle: .pi, clockwise: true)
        shapeLayer.path = path.cgPath
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

Использование:

class MyViewController: UIViewController {
    override func viewDidLoad() {
        let progress = FadingProgressView(frame: CGRect(x: 200, y: 200, width: 100, height: 100))
        view.addSubview(progress)
    }
}