Dequeue reusable cell with delay

Question

I have a tableView where I insert 20 rows with delay using DispatchQueue method. First 10 rows appear fine. Problem starts with 11th one when Xcode begins to dequeue reusable rows. In simulator it looks like it starts to insert by 2 rows nearly at the same time (11th+12th, then 13th+14th).

I wonder why is that. Do DispatchQueue and tableView.dequeueReusableCell methods conflict? And if yes how to organize things properly?

var numberOfCells = 0

//My TableView
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "TextCell")! as UITableViewCell
    return cell
}

//Function that inserts rows
func updateTableView(nextPassageID: Int) {
    for i in 0...numberOfCells - 1 {
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Double(i)) {
            self.numberOfCells += 1
            let indexPath = IndexPath(row: i, section: 0)
            self.tableView.insertRows(at: [indexPath], with: .fade)
            }
    }
}

Answer 1

I think using a Timer is the better solution in your use case:

private var cellCount = 0

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

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath)
    cell.textLabel?.text = "Cell \(indexPath.row)"
    return cell
}

func addCells(count: Int) {
    guard count > 0 else { return }

    var alreadyAdded = 0
    Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] t in
        guard let self = self else {
            t.invalidate()
            return
        }

        self.cellCount += 1

        let indexPath = IndexPath(row: self.cellCount - 1, section: 0)
        self.tableView.insertRows(at: [indexPath], with: .fade)

        alreadyAdded += 1
        if alreadyAdded == count {
            t.invalidate()
        }
    }
}

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    addCells(count: 20)
}