TableViewController.swift

import UIKit

class TableViewController: UITableViewController {

    var elements : [String] = ["A", "B", "C"]

    override func viewDidLoad() {
        super.viewDidLoad()
        NotificationCenter.default.addObserver(self, selector: #selector(TableViewController.showAlert(_:)), name:NSNotification.Name(rawValue: "showAlert"), object: nil)
    }
    
    // MARK: - Table view data source
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return elements.count
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "reuseIdentifier", for: indexPath)

        cell.textLabel?.text = elements[indexPath.row]

        return cell
    }

    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        NotificationCenter.default.post(Notification(name: Notification.Name(rawValue: "showAlert")))
    }

    //MARK: - Notifications

    func showAlert(_ notification: Notification) {
        let alertController = UIAlertController(title: "Reset Seen Questions", message: "Are you sure you want to reset all questions to unseen?", preferredStyle: .alert)
        let cancelAction = UIAlertAction(title: "Cancel", style: .cancel) { action in
            // ...
        }
        alertController.addAction(cancelAction)

        let OKAction = UIAlertAction(title: "Reset", style: .default, handler:{(action:UIAlertAction) -> Void in
            // fake synchronous database-fetch
            self.elements = ["1", "2", "3", "4"]
            self.reloadData()
        })

        alertController.addAction(OKAction)

        self.present(alertController, animated: true, completion: nil)
    }

    func reloadData() {
        DispatchQueue.main.async(execute: {
            self.tableView.reloadData()
        })
    }

}