1、新建一个OpenAd类：（记得导入GoogleMobileAds）

final class OpenAd: NSObject, GADFullScreenContentDelegate {
   var appOpenAd: GADAppOpenAd?
   var loadTime = Date()
   
   func requestAppOpenAd() {
       let request = GADRequest()
       GADAppOpenAd.load(withAdUnitID: "ca-app-pub-3940256099942544/5662855259",
                         request: request,
                         orientation: UIInterfaceOrientation.portrait,
                         completionHandler: { (appOpenAdIn, _) in
                           self.appOpenAd = appOpenAdIn
                           self.appOpenAd?.fullScreenContentDelegate = self
                           self.loadTime = Date()
                           print("[OPEN AD] Ad is ready")
                         })
   }
   
   func tryToPresentAd() {
       if let gOpenAd = self.appOpenAd, wasLoadTimeLessThanNHoursAgo(thresholdN: 4) {
           gOpenAd.present(fromRootViewController: (UIApplication.shared.windows.first?.rootViewController)!)
       } else {
           self.requestAppOpenAd()
       }
   }
   
   func wasLoadTimeLessThanNHoursAgo(thresholdN: Int) -> Bool {
       let now = Date()
       let timeIntervalBetweenNowAndLoadTime = now.timeIntervalSince(self.loadTime)
       let secondsPerHour = 3600.0
       let intervalInHours = timeIntervalBetweenNowAndLoadTime / secondsPerHour
       return intervalInHours < Double(thresholdN)
   }
   
   func ad(_ ad: GADFullScreenPresentingAd, didFailToPresentFullScreenContentWithError error: Error) {
       print("[OPEN AD] Failed: \(error)")
       requestAppOpenAd()
   }
   
   func adDidDismissFullScreenContent(_ ad: GADFullScreenPresentingAd) {
       requestAppOpenAd()
       print("[OPEN AD] Ad dismissed")
   }
 
}

2、在App struct中的OpenAd类创建一个广告对象：

var ad = OpenAd()

3、当应用激活时展示广告：

ad.tryToPresentAd()

4、启动main函数代码：

@main
struct MyApp: App {
    @Environment(\.scenePhase) private var scenePhase
    var ad = OpenAd()
    
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .onChange(of: scenePhase) { phase in
            if phase == .active {
                ad.tryToPresentAd()
            }
        }
    }
}