在Swift SpriteKit中的场景之间显示Admob非页内广告

当我展示GameOverScene时，我想知道如何设置Admob非页内广告。我应该怎么做才能仅在游戏结束后才显示广告？以及我该如何在Swift中实现呢？

I would like to know how to set up Admob Interstitial ads when I present my GameOverScene. What should I do to show the ads only sometimes when the game gets over? And how do I implement this in Swift?

我指的是这篇文章但我想知道如何在两者之间调用广告场景。

Im referring to this post How to call admob interstitial ad using swift, spritekit and xcode? but i'd like to know how to call the ads in between scenes.

编辑
这是我用来展示广告的代码

EDIT Here is the code I used to present the ad

class GameViewController: UIViewController, GADInterstitialDelegate {

var interstitial = GADInterstitial()
var intersitialRecievedAd = false
let defaults = NSUserDefaults.standardUserDefaults()

override func viewDidLoad() {
    super.viewDidLoad()

    interstitial.delegate = self
     self.interstitial = createAndLoadAd()

    let timer = NSTimer.scheduledTimerWithTimeInterval(0.5, target: self, selector: "checkIfAdIsToBeDisplayed:", userInfo: nil, repeats: true)

   //Scene implementation, boring stuff, got nothing to do with the ads...
    ...
}

func checkIfAdIsToBeDisplayed(timer:NSTimer) {



    if defaults.boolForKey("adToBeShown") == true && intersitialRecievedAd == true {

        showInterstitial()
        defaults.setBool(false, forKey: "adToBeShown")
        intersitialRecievedAd = false


    } else {


    }

}

func interstitialDidReceiveAd(ad: GADInterstitial!) {


    intersitialRecievedAd = true

}


func createAndLoadAd() -> GADInterstitial {
    var ad = GADInterstitial(adUnitID: "...")
    ad.delegate = self
    let request = GADRequest()
    request.testDevices = ["..."]
    ad.loadRequest(request)
    return ad
}

func showInterstitial(){
    if self.interstitial.isReady {
        self.interstitial.presentFromRootViewController(self)
        self.interstitial = createAndLoadAd()
    } 
}






override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Release any cached data, images, etc that aren't in use.
}

override func prefersStatusBarHidden() -> Bool {
    return true
}
}

此代码使用一个计时器，用于不断检查是否显示了gameOverScene。当显示gameOverScene时，我将 true 分配给 adToBeShown 布尔值。
这不是最好的方法，所以有人可以告诉我如何在呈现场景时直接调用广告吗？

This code uses a timer to constantly check whether the gameOverScene has been presented. When the gameOverScene IS presented, I assign true to the "adToBeShown" bool. This is not the best method, so could anyone tell me how to directly call the ad when a scene is presented?