private func reorderItems(coordinator: UICollectionViewDropCoordinator, destinationIndexPath: IndexPath, collectionView: UICollectionView)
    {
        let items = coordinator.items
        if items.count == 1, let item = items.first, let sourceIndexPath = item.sourceIndexPath
        {
            var dIndexPath = destinationIndexPath
            if dIndexPath.row >= collectionView.numberOfItems(inSection: 0)
            {
                dIndexPath.row = collectionView.numberOfItems(inSection: 0) - 1
            }
            collectionView.performBatchUpdates({
                if collectionView === self.collectionView2
                {
                    self.items2.remove(at: sourceIndexPath.row)
                    self.items2.insert(item.dragItem.localObject as! String, at: dIndexPath.row)
                }
                else
                {
                    self.items1.remove(at: sourceIndexPath.row)
                    self.items1.insert(item.dragItem.localObject as! String, at: dIndexPath.row)
                }
                collectionView.deleteItems(at: [sourceIndexPath])
                collectionView.insertItems(at: [dIndexPath])
            })
            coordinator.drop(items.first!.dragItem, toItemAt: dIndexPath)
        }
    }

func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator)
    {
        let destinationIndexPath: IndexPath
        if let indexPath = coordinator.destinationIndexPath
        {
            destinationIndexPath = indexPath
        }
        else
        {
            // Get last index path of table view.
            let section = collectionView.numberOfSections - 1
            let row = collectionView.numberOfItems(inSection: section)
            destinationIndexPath = IndexPath(row: row, section: section)
        }

        switch coordinator.proposal.operation
        {
        case .move:
            self.reorderItems(coordinator: coordinator, destinationIndexPath:destinationIndexPath, collectionView: collectionView)
            break

        case .copy:
            self.copyItems(coordinator: coordinator, destinationIndexPath: destinationIndexPath, collectionView: collectionView)

        default:
            return
        }
    }