Skeleton Loader

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Create the label
        let label = UILabel()
        label.text = "Shimmer"
        label.font = UIFont.systemFont(ofSize: 80)
        label.frame = CGRect(x: 0, y: 80, width: view.frame.width, height: 100)
        label.textAlignment = .center

        // Create the gradient
        let gradient = CAGradientLayer()
        gradient.startPoint = CGPoint(x: 0, y: 0.5)
        gradient.endPoint = CGPoint(x: 1, y: 0.5)

        // Add the gradient to the label. Causes this sublayer to appear on top.
        label.layer.addSublayer(gradient)

        // Create the shimmer animation
        let animationGroup = makeAnimationGroup()
        animationGroup.beginTime = 0.0
        gradient.add(animationGroup, forKey: "backgroundColor")

        // Set the gradients frame to the labels bounds
        gradient.frame = label.bounds
        
        view.addSubview(label)
    }
    
    func makeAnimationGroup(previousGroup: CAAnimationGroup? = nil) -> CAAnimationGroup {
        let animDuration: CFTimeInterval = 1.5
        
        let anim1 = CABasicAnimation(keyPath: #keyPath(CAGradientLayer.backgroundColor))
        anim1.fromValue = UIColor.gradientLightGrey.cgColor
        anim1.toValue = UIColor.gradientDarkGrey.cgColor
        anim1.duration = animDuration
        anim1.beginTime = 0.0

        let anim2 = CABasicAnimation(keyPath: #keyPath(CAGradientLayer.backgroundColor))
        anim2.fromValue = UIColor.gradientDarkGrey.cgColor
        anim2.toValue = UIColor.gradientLightGrey.cgColor
        anim2.duration = animDuration
        anim2.beginTime = anim1.beginTime + anim1.duration

        let group = CAAnimationGroup()
        group.animations = [anim1, anim2]
        group.repeatCount = .greatestFiniteMagnitude
        group.duration = anim2.beginTime + anim2.duration
        group.isRemovedOnCompletion = false

        if let previousGroup = previousGroup {
            // Offset groups by 0.33 seconds for effect
            group.beginTime = previousGroup.beginTime + 0.33
        }

        return group
    }
}

extension UIColor {
    static var gradientDarkGrey: UIColor {
        return UIColor(red: 239 / 255.0, green: 241 / 255.0, blue: 241 / 255.0, alpha: 1)
    }

    static var gradientLightGrey: UIColor {
        return UIColor(red: 201 / 255.0, green: 201 / 255.0, blue: 201 / 255.0, alpha: 1)
    }
}