问如何在没有SKShapeNode的情况下实现循环加载器

EN

import UIKit

class CircularLoadingView: UIView {

    @IBOutlet weak var rotatingImage: UIImageView!
    let duration: Double = 1

    func startAnimation() {
        let animation = CABasicAnimation(keyPath: "transform.rotation.z")
        animation.toValue = NSNumber(floatLiteral:  M_PI * 2.0 * duration)
        animation.duration = duration
        animation.isCumulative = true
        animation.repeatCount = Float(CGFloat.greatestFiniteMagnitude)
        rotatingImage.layer.add(animation, forKey: "animationKey")

        UIView.animate(withDuration: duration, animations: {
            self.alpha = 1
        })
    }
}