Swift：设置 rootViewController 不起作用？

我正在尝试启动一个新的 Swift 项目。这是我第一次尝试以编程方式创建视图。然而，我的控制器看起来根本没有被加载？我所看到的只是启动屏幕，然后当我将其加载到模拟器上时出现黑屏。

这是我的 AppDelegate：

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

  var window: UIWindow?

  func application(application: UIApplication,
      didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    NSLog("zrrrzz") // <------------------------------ Prints properly
    self.window?.rootViewController = self.rootViewController()
    return true
  }

  private func rootViewController() -> UIViewController {
    NSLog("zzz") // <---------------------------------- Does not print ????
    return MapViewController.init()
  }

}

地图视图控制器：

import UIKit

class MapViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        var label = UILabel(frame: CGRectMake(0, 0, 200, 21))
        label.center = CGPointMake(160, 284)
        label.textAlignment = NSTextAlignment.Center
        label.text = "I am a test label"
        self.view.backgroundColor = UIColor.whiteColor()
        self.view.addSubview(label)
        NSLog("heyyyy!!") //<------------------------------ Also doesn't print ??
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    /*
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        // Get the new view controller using segue.destinationViewController.
        // Pass the selected object to the new view controller.
    }
    */

}

我是不是少了一步？启动模拟器时我没有看到任何警告/错误

在行中：

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

If the window财产是nil，它不会执行你的self.rootViewController()称呼。您可以阅读更多有关调用文档中带有可选链的方法了解详情。

如果您尝试在代码中创建初始用户界面，则需要创建一个UIWindow实例并将其分配给self.window。使用情节提要时，这是自动为您完成的。

免责声明：我还没有在 iOS 的几个版本中编写过这段代码，所以这可能不完全正确，但它会让你朝着正确的方向前进：

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    let applicationFrame = UIScreen.mainScreen().applicationFrame
    let window = UIWindow(frame: applicationFrame)
    window.rootViewController = self.rootViewController()
    window.makeKeyAndVisible()
    self.window = window

    return true
}