Swift 登录判断

var vc: UIViewController!
        let mainStoryboard = UIStoryboard(name: "Main", bundle: nil)
        
        //验证用户
        var nsd = NSUserDefaults.standardUserDefaults()
        nsd.synchronize()
        let username = nsd.objectForKey("user") as? String
        let password = nsd.objectForKey("pwd") as? String
        
        /*
        验证失败跳转回登录页
        */
        
        //1.如果不存在
        if username == nil || password == nil {
            vc = mainStoryboard.instantiateViewControllerWithIdentifier("ViewController1") as UIViewController
        } else {
            //2.存在发送服务器 验证用户  失败跳转到登录页
            
            vc = mainStoryboard.instantiateViewControllerWithIdentifier("ViewController2") as UIViewController
        }
        
        self.window?.rootViewController = vc
        window?.makeKeyAndVisible()
        return true