How can I make sure my code runs in order in Swift using Firebase?

Question

I am making an app where this block of code runs:

extension LikeOrDislikeViewController: UITableViewDataSource {
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        let uid = Auth.auth().currentUser?.uid
        var numRows = 0

        Database.database().reference().child("Num Liked").child(uid!).observeSingleEvent(of: .value, with: { (snapshot) in
            if let dictionary = snapshot.value as? [String: AnyObject] {
                let numMoviesLiked = ((dictionary["Number of Movies Liked"] as? Int))!

                if numMoviesLiked % 4 == 0 {
                numRows = numMoviesLiked/4
                }
            }
        })
        return numRows
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = UITableViewCell()
        cell.backgroundColor = UIColor.red
        tableView.rowHeight = 113
        cell.textLabel?.text = "\(indexPath.row)"

        return cell
    }
}

I need the "Database.database..." code to run before the return line. How can I do this?

This is a bad setup because `numberOfRowsInSection` will be called many times. Load your data once, not over and over. — rmaddy, Oct 18 '18 at 21:33
The completion handler for `observeSingleEvent` runs **after** your `return numRows`. Place a few logging statements and you can easily see that in the output. You will have to place all the code that needs the data in the completion handler, or pass in your own custom completion handler and call that. See https://stackoverflow.com/questions/42464237/make-code-with-firebase-asynchronous/42464431#42464431, https://stackoverflow.com/questions/46863823/adding-data-and-querying-from-swift-to-firebase/46864260#46864260 — Frank van Puffelen, Oct 19 '18 at 00:46

Mojtaba Hosseini · Answer 1 · 2018-10-18T22:11:25.067

Database operations are heavy and take time a bit to complete. You should update your table view after Database.database... operation completed.

class LikeOrDislikeViewController {
    ...

    lazy var numRows: Int = {
        let uid = Auth.auth().currentUser?.uid
        Database.database().reference().child("Num Liked").child(uid!).observeSingleEvent(of: .value, with: { (snapshot) in
            if let dictionary = snapshot.value as? [String: AnyObject] {
                let numMoviesLiked = ((dictionary["Number of Movies Liked"] as? Int))!

                if numMoviesLiked % 4 == 0 {
                    self.numRows = numMoviesLiked/4
                    self.tableView.reloadData()
                }
            }
        })
        return 0
    }()
}

extension LikeOrDislikeViewController: UITableViewDataSource {

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return numRows
    }

    ...
}

This does not work. I get this error: "Variable used within its own initial value" — Bob Samuels, Oct 18 '18 at 22:01
Code updated. But remember this is just a preview to point out how it's going to work, not the code you can directly use in your production code. ;) — Mojtaba Hosseini, Oct 18 '18 at 22:13

How can I make sure my code runs in order in Swift using Firebase?

1 Answers1