Snap to center collection view cell

CollectionViewController.swift

class CollectionViewController: UIViewController {
  ...
  func snapToCenter() {
      let centerPoint = view.convertPoint(view.center, toView: collectionView)
      guard let centerIndexPath = collectionView.indexPathForItemAtPoint(centerPoint)
      collectionView.scrollToItemAtIndexPath(centerIndexPath, atScrollPosition: .CenteredHorizontally, animated: true)
  }
  ...  
}
    
// MARK: - Scroll view delegate

extension CollectionViewController: UIScrollViewDelegate {
    
    func scrollViewDidEndDecelerating(scrollView: UIScrollView) {
        snapToCenter()
    }
    
    func scrollViewDidEndDragging(scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        if !decelerate {
            snapToCenter()
        }
    }
    
}