Xcode (11.2) iOS Simulator shows a black screen upon Launch

Question

Everything was working fine, but when I was trying to figure out how to implement a segue programmatically without the use of a storyboard, I started running into this issue. When I try to run the app, all I can see is a black screen and no errors in the log.

I tried commenting out the segue code,I tried deleting all content from the simulator and rebooting the simulator, Xcode, and my mac, but to no avail.

Some help would be much appreciated.

Here is my code...

import UIKit
import RealmSwift

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.

        do { _ = try Realm()
        } catch { print("Error initializing new realm, \(error)") }

        if let window = window {
            let navController = UINavigationController(rootViewController: FirstViewController())
            window.backgroundColor = UIColor.white
            window.rootViewController = navController
            window.makeKeyAndVisible()
        }
        return true
    }
}

import UIKit
import RealmSwift

class FirstViewController: UITableViewController {

    let realm = try! Realm()

    var days : Results<Days>?
    var workouts : Results<Workouts>?

    var daysOfWeek : [String] = ["Monday", "Tuesday", "Wednsday", "Thursday", "Friday", "Saturday", "Sunday"]

    var indexCheck : Int = 0

    let picker = UIPickerView()
    var textField1 = UITextField()
    var textField2 = UITextField()



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

        picker.delegate = self
        picker.dataSource = self


        navigationItem.title = "Workouts"
        navigationController?.navigationBar.prefersLargeTitles = true

        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Cell")

        let addBarButton = UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(addWorkout))
        self.navigationItem.rightBarButtonItem = addBarButton


        loadDays()
    }

    //MARK: - Add a New Workout
    @objc func addWorkout() {

        var containsDay = false
        var counter = 0

        let alert = UIAlertController(title: "New Workout", message: "Please name your workout...", preferredStyle: .alert)

        let addAction = UIAlertAction(title: "Add Workout", style: .default) { (UIAlertAction) in
            //Add day and workout to database
            //if day exists, append workout to the existing day.
            //if day doesn't exist, create a day and append workout to newly created day object.

            //First, we have to create an initial days object...
            //Need to check if ANY of the weekdays == picked day, only execute then.
            if self.days?.isEmpty == false {
                for i in 0...(self.days!.count - 1) {

                    if self.days?[i].weekday == self.daysOfWeek[self.picker.selectedRow(inComponent: 0)] {
                        containsDay = true
                        counter = i
                        break
                    }
                }

                if containsDay == true {
                    let newWorkout = Workouts()
                    newWorkout.title = self.textField2.text!

                    try! self.realm.write {
                        self.days?[counter].workout.append(newWorkout)
                        self.loadDays()
                    }
                } else {
                    let dow = Days()
                    let newWorkout = Workouts()
                    dow.weekday = self.daysOfWeek[self.picker.selectedRow(inComponent: 0)]
                    newWorkout.title = self.textField2.text!
                    dow.workout.append(newWorkout)
                    self.save(newDay: dow)
                }
            } else {
                let dow = Days()
                let newWorkout = Workouts()
                dow.weekday = self.daysOfWeek[self.picker.selectedRow(inComponent: 0)]
                newWorkout.title = self.textField2.text!
                dow.workout.append(newWorkout)

                self.save(newDay: dow)
            }
        }

        alert.addTextField { (alertTextField1) in
            alertTextField1.placeholder = "Day of Week"
            alertTextField1.text = self.textField1.text
            self.textField1 = alertTextField1
            alertTextField1.inputView = self.picker
        }

        alert.addTextField { (alertTextField2) in
            alertTextField2.placeholder = "Muscle Group"
            self.textField2 = alertTextField2
            alertTextField2.inputView = nil
        }


        alert.addAction(addAction)
        present(alert, animated: true, completion: nil)
    }


    //MARK: - TableView DataSource and Delegate Methods
    override func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
        let label = UILabel()
        label.text = days?[section].weekday ?? "Section Header"
        label.backgroundColor = UIColor.lightGray
        return label
    }

    override func numberOfSections(in tableView: UITableView) -> Int {
        return days?.count ?? 0
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return days?[section].workout.count ?? 0
    }


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

        let workout = days?[indexPath.section].workout[indexPath.row].title ?? "Workout"

        cell.textLabel?.text = "\(workout)  Section:\(indexPath.section) Row:\(indexPath.row)"

        return cell
    }


    //MARK: - Swipe To Delete
    override func tableView(_ tableView: UITableView, canEditRowAt indexPath: IndexPath) -> Bool {
        return true
    }

    override func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCell.EditingStyle, forRowAt indexPath: IndexPath) {

        if editingStyle == .delete {
            try! realm.write {
                realm.delete((days?[indexPath.section].workout[indexPath.row])!)

                tableView.beginUpdates()

                tableView.deleteRows(at: [indexPath], with: .automatic)

                if days?[indexPath.section].workout.isEmpty == true {
                    realm.delete((days?[indexPath.section])!)
                    let indexSet = IndexSet(arrayLiteral: indexPath.section)
                    tableView.deleteSections(indexSet, with: .automatic)
                }

                tableView.endUpdates()
            }
        }
    }


    //MARK: - Load Data
    func loadDays() {
        days = realm.objects(Days.self)
        tableView.reloadData()
    }

    //MARK: - Save Data
    func save(newDay : Days) {
        do {
            try realm.write {
                realm.add(newDay)
            }
        } catch {
            print("Error saving day \(error)")
        }
        self.loadDays()
    }


    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {

        let vc = SecondViewController()
        self.navigationController?.pushViewController(vc, animated: true)
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

        let destinationVC = segue.destination as! SecondViewController

        if let indexPath = tableView.indexPathForSelectedRow {
            destinationVC.selectedWorkout = days?[indexPath.section].workout[indexPath.row]
        }
    }
}


//MARK: - PickerView Delegate Methods
extension FirstViewController : UIPickerViewDelegate, UIPickerViewDataSource {

    func numberOfComponents(in pickerView: UIPickerView) -> Int {
        return 1
    }

    func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
        return 7
    }

    func pickerView(_ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? {
        return daysOfWeek[row]
    }

    func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
        textField1.text = daysOfWeek[row]
    }

}

Answer 1

I suppose your initial view controller isn't set from the Storyboard. That means, you didn't set any of the view controllers from the storyboard file as Is Initial View Controller like in the image below:

---

As a result your window isn't set. You need to initialize your window property manually. Use following instead of your version:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // Override point for customization after application launch.

    do { _ = try Realm()
    } catch { 
        print("Error initializing new realm, \(error)")
    }

    let window = UIWindow(frame: UIScreen.main.bounds)
    let navController = UINavigationController(rootViewController: FirstViewController())
    window.rootViewController = navController
    window.makeKeyAndVisible()
    self.window = window //this is important because otherwise your window will be deallocated immediately
    return true
}

But if you have SceneDelegate follow iOS 13: Swift - 'Set application root view controller programmatically' does not work