The default value is UICollectionViewScrollDirection.Vertical.
By default, cells are laid out with a left-to-right flow direction and wrap top-to-bottom. If the number of cells exceeds screen real estate, the UIKit.UICollectionView will scroll vertically.
If the application developer sets UICollectionViewFlowLayout.ScrollDirection to UICollectionViewScrollDirection.Horizontal, cells will be laid out top-to-bottom and wrap left-to-right.