Tells the delegate when a cell is to be the center cell.
On This Page
optional func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, willCenterCellAt indexPath: IndexPath)
func collectionView(UICollectionView, layout: UICollectionViewLayout, didCenterCellAt: IndexPath)
Tells the delegate when a cell has completed the transition and has become the center cell.