计时器xcode / Swift上的Google Admob非页内广告

Question

我有一个WebKit应用，并且一直在尝试让AdMob在计时器上展示插页式广告，以便每隔x秒显示一次。我在这里搜索，发现了一个旧的解决方案，但是它似乎不再起作用。广告只会展示一次，而不会再次展示。使它每隔x秒显示一次的任何帮助都会很棒。

这就是我得到的：

var interstital: GADInterstitial!

override func viewDidLoad() {
    super.viewDidLoad()
    interstital = GADInterstitial(adUnitID: 
"ca-app-pub-3940256099942544/1033173712")

    let request = GADRequest()
    interstital.load(request)

    let randomTime = Double(arc4random() + 15).truncatingRemainder(dividingBy: 20.0)
    Timer.scheduledTimer(timeInterval: randomTime, target: self, selector: #selector(ViewController.CreateAd), userInfo: nil, repeats: true)


}
@objc func CreateAd() -> GADInterstitial {

    if (interstital.isReady) {

        interstital.present(fromRootViewController: self)
        let randomTime = Double(arc4random() + 180).truncatingRemainder(dividingBy: 200.0)
        Timer.scheduledTimer(timeInterval: randomTime, target: self, selector: #selector(ViewController.CreateAd), userInfo: nil, repeats: true)

    }
    return interstital

}