private class TransitionAnimator {
  let groupName = "transition.group"
  weak var layer: CALayer?
  
  init(_ view: UIView) {
    self.layer = view.layer
  }
  
  private var group: CAAnimationGroup? {
    return layer?.animation(forKey: groupName) as? CAAnimationGroup
  }
  
  private func addGroup(_ animations: [CAAnimation], duration: TimeInterval) {
    guard let layer = layer else {
      return
    }
    layer.speed = 1
    layer.timeOffset = 0
    let t = CACurrentMediaTime()
    layer.beginTime = t
    layer.setValue(t, forKey: "lastStartedAt")
    layer.setValue(0, forKey: "currentDuration")
    layer.removeAllAnimations()
    
    let g = CAAnimationGroup()
    g.animations = animations
    g.duration = duration
    layer.add(g, forKey: groupName)
  }
  
  func start(_ animations: [CAAnimation], duration: TimeInterval) {
    addGroup(animations, duration: duration)
    pause()
  }
  
  func seek(_ progress: CGFloat) {
    guard let layer = layer, let duration = group?.duration else {
      return
    }
    
    pause()
    
    let s = CACurrentMediaTime()
    layer.setValue(s, forKey: "lastStartedAt")
    layer.beginTime = s
    let currentDuration = duration * 2 * CFTimeInterval(progress)
    layer.setValue(currentDuration, forKey: "currentDuration")
    layer.timeOffset = currentDuration
  }
  
  func pause() {
    guard let layer = layer else {
      return
    }
    
    let t = CACurrentMediaTime()
    layer.beginTime = t
    let currentDuration = (layer.value(forKey: "currentDuration") as! TimeInterval) + Double(layer.speed) * (t - (layer.value(forKey: "lastStartedAt") as! TimeInterval))
    layer.setValue(currentDuration, forKey: "currentDuration")
    layer.speed = 0
    layer.timeOffset = currentDuration
  }
  
  func resume() {
    guard let layer = layer else {
      return
    }
    
    layer.speed = 1
    let s = CACurrentMediaTime()
    layer.setValue(s, forKey: "lastStartedAt")
    layer.beginTime = s
  }
  
  func cancel(isAnimated: Bool = true) {
    guard let layer = layer, let group = self.group else {
      return
    }
    
    guard isAnimated else {
      layer.removeAnimation(forKey: groupName)
      return
    }
    
    let newDuration = layer.timeOffset
    let newAnimations: [CABasicAnimation] = group.animations!.compactMap { $0 as? CABasicAnimation }.map {
      let a = CABasicAnimation()
      a.keyPath = $0.keyPath
      a.toValue = layer.value(forKeyPath: $0.keyPath!)
      a.fromValue = layer.presentation()!.value(forKeyPath: $0.keyPath!)
      return a
    }
    
    layer.removeAnimation(forKey: groupName)
    addGroup(newAnimations, duration: newDuration)
  }
  
  func finish() {
    guard let layer = layer, let group = self.group else {
      return
    }
    
    group.animations!.compactMap { $0 as? CABasicAnimation }.forEach {
      layer.setValue($0.toValue, forKeyPath: $0.keyPath!)
    }
    
    resume()
  }
}