CALayerAnimationSync.swift

import UIKit
import PlaygroundSupport

let width: CGFloat = 200
let height: CGFloat = 100
let spacing: CGFloat = 10
let numberOfViews: Int = 5

let canvas = UIView(frame: CGRect(
  x: 0,
  y: 0,
  width: width,
  height: height * CGFloat(numberOfViews) + spacing * CGFloat(numberOfViews - 1)
))
canvas.backgroundColor = .white
PlaygroundPage.current.liveView = canvas

final class SkeletonView: UIView {
  func startAnimation(timeOffset: CFTimeInterval = 0) {
    let animation = CABasicAnimation(keyPath: #keyPath(CALayer.backgroundColor))
    animation.fromValue = UIColor.lightGray.cgColor
    animation.toValue = UIColor.gray.cgColor
    animation.duration = 1
    animation.autoreverses = true
    animation.timingFunction = CAMediaTimingFunction(name: CAMediaTimingFunctionName.easeInEaseOut)
    animation.repeatCount = .infinity
    animation.isRemovedOnCompletion = false
    animation.timeOffset = CFAbsoluteTimeGetCurrent()
    self.layer.add(animation, forKey: nil)
  }
}

for i in 0..<numberOfViews {
  let view = SkeletonView(frame: CGRect(
    x: 0,
    y: CGFloat(i) * (height + spacing),
    width: width,
    height: height
  ))
  canvas.addSubview(view)

  DispatchQueue.main.asyncAfter(deadline: .now() + TimeInterval(i) / 3) {
    view.startAnimation()
  }
}