A minimal iOS 13 app with no storyboard or xib

main.swift

import UIKit

// MARK: - ViewController
class ViewController: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .red
  }
}

// MARK: - App
class App {
  static let app = App()
  
  private var window: UIWindow?
  
  func start(window: UIWindow = UIWindow(frame: UIScreen.main.bounds)) {
    window.rootViewController = ViewController()
    window.makeKeyAndVisible()
    self.window = window
  }
}

// MARK: - AppDelegate
@UIApplicationMain
class AppDelegate: UIResponder {}

// MARK: UIApplicationDelegate
extension AppDelegate: UIApplicationDelegate {
  func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    if #available(iOS 13.0, *) {} else {
      App.app.start()
    }
    return true
  }
}

// MARK: UIWindowSceneDelegate
@available(iOS 13.0, *)
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    
  func scene(
    _ scene: UIScene,
    willConnectTo session: UISceneSession,
    options connectionOptions: UIScene.ConnectionOptions) {
    guard let windowScene = scene as? UIWindowScene else {
      return
    }
    let window = UIWindow(windowScene: windowScene)
    App.app.start(window: window)
  }
}

@available(iOS 13.0, *)
extension AppDelegate {
  func application(
    _ application: UIApplication,
    configurationForConnecting connectingSceneSession: UISceneSession,
    options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
  }
  
  func application(
    _ application: UIApplication,
    didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
  }
}