iOS 13: Swift - 'Set Application Root View Controller Programmatically' Does Not Work

Set rootViewController iOS 13

This is because AppDelegate doesn't have window property anymore.
Now you must use SceneDelegate's scene(_:willConnectTo:options:) method to change root view controller.
Like shown in this example:

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let scene = (scene as? UIWindowScene) else { return }

        // Instantiate UIWindow with scene
        let window = UIWindow(windowScene: scene)
        // Assign window to SceneDelegate window property
        self.window = window
        // Set initial view controller from Main storyboard as root view controller of UIWindow
        self.window?.rootViewController = UIStoryboard(name: "Main", bundle: nil).instantiateInitialViewController()
        // Present window to screen
        self.window?.makeKeyAndVisible()
    }

Set rootViewController programmatically not working, picking initial view controller from storyboard only in Xcode 11

Got the answer by doing some more google, i mean stackoverflow.

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

var window: UIWindow?
let storyboard = UIStoryboard(name: "Main", bundle: nil)

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
  guard let windowScene = scene as? UIWindowScene else { return }
  let vc = storyboard.instantiateViewController (withIdentifier: "Primary") as! ViewController
  window = UIWindow(windowScene: windowScene)
  window?.rootViewController = vc
  window?.makeKeyAndVisible()
}

SceneDelegate is used for this purpose since iOS 13 not AppDelegate. Here is the link SceneDelegate example

Why is manually setup root view controller showing black screen?

To ensure you see your root view controller in iOS 13 when everything is done programmatically, you must do the following:

In the scene delegate, you must create the window instance and the root view controller:

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let winScene = (scene as? UIWindowScene) else { return }

        // Create the root view controller as needed
        let vc = ViewController()
        let nc = UINavigationController(rootViewController: vc)

        // Create the window. Be sure to use this initializer and not the frame one.
        let win = UIWindow(windowScene: winScene) 
        win.rootViewController = nc
        win.makeKeyAndVisible()
        window = win
    }
}

Your Info.plist has to have the "Application Scene Manifest" entry. Below it should be the "Enable Multiple Windows" entry. Set to YES or NO as appropriate to your app. Optionally you should also have the "Scene Configuration" entry.

All of these entries are added by Xcode when you check the "Supports multiple windows" setting on the General tab of your target. This will default the "Enable Multiple Windows" entry to YES so you can change that to NO if you want scenes but not multiple windows.

How to set root view controller programmatically in Xcode 13 with SpriteKit?

If you want to delete the Main.Storyboard, you need to add the following to your AppDelegate

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    
     window = UIWindow(frame: UIScreen.main.bounds)
     let viewController = GameViewController()
     window?.rootViewController = viewController
     window?.makeKeyAndVisible()

    return true
}

And in GameViewController.swift you need to add the following:

    override func loadView() {
       self.view = SKView(frame: UIScreen.main.bounds)
    }

set initial viewcontroller in appdelegate - swift

Xcode11 and SceneDelegate note:

Starting from Xcode11, because of SceneDelegates, it's likely that you shouldn't do it inside AppDelegate. Instead do it from SceneDelegate. For more on that see this other answer

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Old answer:

I used this thread to help me convert the objective C to swift, and its working perfectly.

Instantiate and Present a viewController in Swift

Swift 2 code:

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    self.window = UIWindow(frame: UIScreen.mainScreen().bounds)
    
    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    
    let initialViewController = storyboard.instantiateViewControllerWithIdentifier("LoginSignupVC")
    
    self.window?.rootViewController = initialViewController
    self.window?.makeKeyAndVisible()
    
    return true
}

Swift 3 code:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    self.window = UIWindow(frame: UIScreen.main.bounds)

    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    
    let initialViewController = storyboard.instantiateViewController(withIdentifier: "LoginSignupVC")

    self.window?.rootViewController = initialViewController
    self.window?.makeKeyAndVisible()

    return true
}

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