在第二次加载之前，视图不会更新

Question

I have a main view which is a table view with a list of countries. When clicking on any country name (cell), another view is loaded via segue which is passing the name of the country to the next view controller's navigation bar title.

The problem is on the first click the title isn't updated, but when I click back button (dismissing the current view) and click on another country name, the second view loads again and shows the previous title that was suppose to be shown on the first attempt.

The code for the first main view controller:

import UIKit

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {

    var sectionsArray = [String]()
    var sectionsCountries = [Array<AnyObject>]()

    @IBOutlet weak var countries: UITableView!

    internal func numberOfSections(in tableView: UITableView) -> Int {
        // Return the number of sections.
        return self.sectionsArray.count
    }

    internal func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        // Return the number of rows in the section.
            return self.sectionsCountries[section].count
    }

    internal func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
        return self.sectionsArray[section]
    }

    internal func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "CountryCell", for: indexPath)
        cell.textLabel?.text = self.sectionsCountries[indexPath.section][indexPath.row] as? String
        return cell
    }

    var valueToPass:String!

    internal func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        print("You selected cell #\(indexPath.row)!")

        // Get Cell Label
        let indexPath = tableView.indexPathForSelectedRow;
        let currentCell = tableView.cellForRow(at: indexPath!) as UITableViewCell!;

        valueToPass = currentCell?.textLabel?.text
        performSegue(withIdentifier: "cellSegue", sender: self)
        //print(valueToPass)
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "cellSegue" {
            let destination = segue.destination as! CountryViewController
            destination.passedValue = valueToPass
        }
    }

    override func viewDidLoad() {

        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        let url = URL(string: "http://cyber7.co.il/swift/countries/countries-list.json")!
        let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
            if error != nil {
                print(error)
            } else {
                if let urlContent = data {
                    do {
                        let jsonResult = try JSONSerialization.jsonObject(with: urlContent, options: JSONSerialization.ReadingOptions.mutableContainers)
                        for result in jsonResult as! [Dictionary<String, AnyObject>]{
                            self.sectionsArray.append(result["sectionName"] as! String)
                            self.sectionsCountries.append(result["sectionCountries"] as! Array<String> as [AnyObject])
                        }
                    } catch {
                        print("JSON Processing Failed")
                    }
                    DispatchQueue.main.async(execute: { () -> Void in
                        self.countries.reloadData()
                    })
                }
            }
        }
        task.resume()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

the code for the second view controller:

import UIKit

class CountryViewController: UIViewController {

    var passedValue:String!

    @IBOutlet weak var navBar: UINavigationBar!

    @IBAction func backButton(_ sender: AnyObject) {
        self.dismiss(animated: true, completion: nil)
    }

    override func viewWillAppear(_ animated: Bool) {
        self.navBar.topItem?.title = passedValue
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

}

Answer 1

When you have a segue set up from a table view cell to a view controller in the storyboard then it is performed automatically when a cell is selected. Your call to perform a segue in your cell selection method is performing the segue a second time after the first one has already been performed.

Remove your tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) method and do all the data passing logic in prepareForSegue :

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "cellSegue" {
        let destination = segue.destination as! CountryViewController
        let indexPath = countries.indexPathForSelectedRow
        let currentCell = countries.cellForRow(at: indexPath!)
        destination.passedValue = currentCell?.textLabel?.text
    }
}