我正在开发使用Facebook SDK登录的ios应用程序。我LogInViewController在情节提要中将a设置为初始View
Controller，用户从中使用FB帐户登录。

我有另一个ViewController，一旦用户登录，它将正确加载。

我正在检查AppDelegate文件currentAccessToken，如果不是nil，则直接加载第二个ViewController，因为用户已经登录。

但是，currentAccessToken如果我退出该应用程序并重新启动它，则始终为零。仅当我按下主页按钮并在后台运行该应用程序时重新打开该应用程序时，它才有效。

以下是代码中的详细信息：

AppDelegate.swift

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    self.customNavigationBar()
    if (!isIcloudAvailable()) {
        self.displayAlertWithTitle("iCloud", message: "iCloud is not available." +
           " Please sign into your iCloud account and restart this app")
        return true
    }

    if (FBSDKAccessToken.currentAccessToken() != nil) {
        self.instantiateViewController("MapViewController", storyboardIdentifier: "Main")
    }

    return FBSDKApplicationDelegate.sharedInstance().application(application, didFinishLaunchingWithOptions: launchOptions)
}

func application(application: UIApplication, openURL url: NSURL, sourceApplication: String?, annotation: AnyObject) -> Bool {
    return FBSDKApplicationDelegate.sharedInstance().application(
            application,
            openURL: url,
            sourceApplication: sourceApplication,
            annotation: annotation)
}

func applicationWillResignActive(application: UIApplication) {
        FBSDKAppEvents.activateApp()
}

func applicationDidBecomeActive(application: UIApplication) {
        FBSDKAppEvents.activateApp()
}

LogInViewController.swift

override func viewDidLoad() {
    super.viewDidLoad()    
    // Listen to the Facebook notification and when received, execute func handleFBSessionStateChangeWithNotification
NSNotificationCenter.defaultCenter().addObserver(self, selector:"handleFBSessionStateChangeWithNotification:", name: "SessionStateChangeNotification", object: nil)
}

func handleFBSessionStateChangeWithNotification(notification: NSNotification) {
    // Switch to MapViewController when logged in
    if ((FBSDKAccessToken.currentAccessToken()) != nil) {
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let mapViewController = storyboard.instantiateViewControllerWithIdentifier("MapViewController") as! MapViewController
        self.presentViewController(mapViewController, animated: false, completion: nil)
    }
}

我不知道它是否相关，但是我也收到警告，MapViewController因为情节提要中没有针对它的警告：

警告：尝试显示视图不在窗口层次结构中的MapViewController！