为什么parentViewController，presentViewController和PresentedViewController始终为nil

Question

我有一个带有几个ViewControllers的简单应用程序，我需要知道当我在VC中时，我来自哪个VC（第一个除外）。所有VC都在故事板中创建，我使用以下代码浏览VC：

class FirstVC: UIViewController
{    
    func ShowStartVC()
    {
        if gFirstStart
        {
            let vc = self.storyboard!.instantiateViewControllerWithIdentifier("EvalUserLevelVC") as EvalUserLevelVC
            self.presentViewController(vc, animated: false, completion: nil)
        }
        else
        {
            let vc = self.storyboard!.instantiateViewControllerWithIdentifier("ChoixLevelVC") as ChoixLevelVC
            self.presentViewController(vc, animated: true, completion: nil)
        }
    }

    func ShowPrefsVC()
    {
        let vc = self.storyboard!.instantiateViewControllerWithIdentifier("PrefsVC") as PrefsVC
        self.presentViewController(vc, animated: true, completion: nil)
    }
}


class FirstVCSceneClass: SKScene
{
    var VCFirst = FirstVC?()

    override func touchesBegan(touches: NSSet, withEvent event: UIEvent)
    {    
        for touch: AnyObject in touches
        {
            let location = touch.locationInNode(self)

            switch self.nodeAtPoint(location).name!
            {
            case "Start":
                VCFirst?.ShowStartVC()
                self.paused = true

            case "Preferences":
                VCFirst?.ShowPrefsVC()
                self.paused = true

            default:
                break
            }
        }
    }
}

在＆＃34; ChoixLevelVC＆＃34;上面的代码中可以从这里和另一个VC（完全相同的方式）调用，我需要知道从哪个VC调用它。我的理解是属性＆＃34; presentViewController＆＃34;当我从＆＃34; ChoixLevelVC＆＃34;访问它时，应该有这些信息。 UIViewController类

但如果我将以下代码放在＆＃34; ChoixLevelVC＆＃34;一切都是零，虽然我希望找到＆＃34; FirstVC＆＃34;在属性＆＃34; presentsViewController＆＃34;

class ChoixLevelVC: UIViewController, UICollectionViewDelegateFlowLayout, UICollectionViewDataSource
{
    override func viewDidLoad()
    {
        println("ParentViewController : \(self.parentViewController)")
        println("presentingViewController : \(self.presentingViewController)")
        println("presentedViewController : \(self.presentedViewController)")
    }
}

每个VC都是一样的。为了正确设置这些属性，我需要做些什么吗？