Drag.swift

func setupCollectionView() {
        self.collectionView.collectionViewLayout = columnLayout
        self.collectionView.dataSource = self
        self.collectionView.dragDelegate = self
        self.collectionView.dragInteractionEnabled = true
}

extension ViewController: UICollectionViewDragDelegate {
    func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
        let attributedString = String(indexPath.item)
        let dragItem = UIDragItem(itemProvider: NSItemProvider(object: attributedString as NSItemProviderWriting))
        dragItem.localObject = attributedString
        return [dragItem]
    }
}