Creating a navigationController programmatically (Swift)

Swift 1, 2:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
   self.window = UIWindow(frame: UIScreen.mainScreen().bounds)
   var nav1 = UINavigationController()
   var mainView = ViewController(nibName: nil, bundle: nil) //ViewController = Name of your controller
   nav1.viewControllers = [mainView]
   self.window!.rootViewController = nav1
   self.window?.makeKeyAndVisible()
}

Swift 4+: and Swift 5+

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
   self.window = UIWindow(frame: UIScreen.main.bounds)
   let nav1 = UINavigationController()
   let mainView = ViewController(nibName: nil, bundle: nil) //ViewController = Name of your controller
   nav1.viewControllers = [mainView]
   self.window!.rootViewController = nav1
   self.window?.makeKeyAndVisible()
}

I would recommend starting your AppDelegate with this skeleton:

1) use let wherever you can!

2) UINavigationController has the rootViewController property.

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    let viewController = ViewController(nibName: nil, bundle: nil) //ViewController = Name of your controller
    let navigationController = UINavigationController(rootViewController: viewController)

    self.window = UIWindow(frame: UIScreen.mainScreen().bounds)
    self.window?.rootViewController = navigationController
    self.window?.makeKeyAndVisible()

    return true
}