class SomeViewController: UIViewController {
  let collectionView: UICollectionView = UICollectionView(frame: CGRect.zero, collectionViewLayout: SomeLayout())

  override func viewDidLoad() {
    super.viewDidLoad()

    let toLayout: UICollectionViewLayout = AnotherLayout()

    collectionView.startInteractiveTransition(to: toLayout, completion: { [weak self] (_, _)  in
      guard let slf = self else { return }
      let isInTransition = (slf.collectionView.collectionViewLayout as? UICollectionViewTransitionLayout) != nil // false
    })
    let isInTransition = (slf.collectionView.collectionViewLayout as? UICollectionViewTransitionLayout) != nil // true
  }
}