ViewController+UICollectionViewDragDelegate.swift

import UIKit

extension ViewController: UICollectionViewDragDelegate {
    
    func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
        let n = "\(numbers[indexPath.item])"
        let itemProvider = NSItemProvider(object: n as NSString)
        let dragItem = UIDragItem(itemProvider: itemProvider)
        
        return [dragItem]
    }
}