DragDrop2.swift

func collectionView(_ collectionView: UICollectionView, dropSessionDidUpdate session: UIDropSession, withDestinationIndexPath destinationIndexPath: IndexPath?) -> UICollectionViewDropProposal
{
    if session.localDragSession != nil
    {
        if collectionView.hasActiveDrag
        {
            return UICollectionViewDropProposal(operation: .move, intent: .insertAtDestinationIndexPath)
        }
        else
        {
            return UICollectionViewDropProposal(operation: .copy, intent: .insertAtDestinationIndexPath)
        }
    }
    else
    {
        return UICollectionViewDropProposal(operation: .forbidden)
    }
}