在AppDelegate中预加载adMob间隙广告，并在其他UIViewController中显示该广告

import GoogleMobileAds

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    GADMobileAds.sharedInstance().start(completionHandler: nil)
    let adRequest = GADRequest()
    // 设置其他广告请求参数，如用户标识、关键字等
    adRequest.testDevices = [kGADSimulatorID] // 在模拟器中测试广告
    GADInterstitialAd.load(withAdUnitID: "YOUR_AD_UNIT_ID", request: adRequest, completionHandler: nil)
    return true
}

import GoogleMobileAds

class YourViewController: UIViewController, GADInterstitialDelegate {
    var interstitial: GADInterstitialAd?

    override func viewDidLoad() {
        super.viewDidLoad()
        // 在视图加载完成后，加载广告
        let appDelegate = UIApplication.shared.delegate as! AppDelegate
        self.interstitial = appDelegate.interstitial
        self.interstitial?.delegate = self
    }

    // 在需要显示广告的时机调用该方法
    func showInterstitialAd() {
        if let interstitial = self.interstitial {
            if interstitial.isReady {
                interstitial.present(fromRootViewController: self)
            }
        }
    }

    // 广告加载完成的回调方法
    func interstitialDidReceiveAd(_ ad: GADInterstitialAd) {
        // 广告加载成功，可以调用showInterstitialAd方法显示广告
    }

    // 广告加载失败的回调方法
    func interstitial(_ ad: GADInterstitialAd, didFailToReceiveAdWithError error: GADRequestError) {
        // 广告加载失败，可以根据错误信息进行处理
    }
}