swift – 在App Delegate中管理两个故事板

我一直在关注本教程：http://pinkstone.co.uk/how-to-load-a-different-storyboard-depending-on-screen-size-in-ios/

我在尝试将Objective C代码转换为Swift时遇到问题.

这是我目前在AppDelegate中的代码：

@UIApplicationMain
class AppDelegate: UIResponder,UIApplicationDelegate {

var window: UIWindow?

func grabStoryboard() -> UIStoryboard {
    var storyboard = UIStoryboard()
    var height = UIScreen.mainScreen().bounds.size.height

    if height == 480 {
        storyboard = UIStoryboard(name: "Main3.5",bundle: nil)
    } else {
        storyboard = UIStoryboard(name: "Main",bundle: nil)
    }
    return storyboard
}

func application(application: UIApplication!,didFinishLaunchingWithOptions launchOptions: NSDictionary!) -> Bool {
    // Override point for customization after application launch.   

    var storyboard: UIStoryboard = self.grabStoryboard()

    self.window?.rootViewController.storyboard.instantiateInitialViewController()
    self.window?.makeKeyAndVisible()

    return true
}

该应用程序运行,我没有错误,但无论我是在3.5英寸设备或4英寸设备上运行应用程序,我只是得到4英寸的故事板.

我哪里错了？