ios之当以编程方式设置 Root View Controller 时，导航和选项卡栏丢失

应用委托(delegate)代码:

let storyboard = UIStoryboard(name: "Main", bundle: nil) 
 
        let myViewController = storyboard.instantiateViewControllerWithIdentifier("MyViewController") as UIViewController 
if self.window != nil { 
                self.window!.rootViewController = myViewController 
 
            } 

当我在没有此代码的情况下加载应用程序时，一切都很好。
但是，如果我使用此代码运行， View 已加载但导航和标签栏丢失。
我在这里错过了什么？
我正在使用 Storyboard。

请您参考如下方法:

这是有效的代码:

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool { 
let storyboard = UIStoryboard(name: "Main", bundle: nil) 
let mainRootController = storyboard.instantiateViewControllerWithIdentifier("MainViewController") as UIViewController 
if self.window != nil { 
 
                let navigationController:UINavigationController = storyboard.instantiateInitialViewController() as UINavigationController 
                navigationController.viewControllers = [mainRootController] 
 
                self.window!.rootViewController = navigationController 
} 
return true