How to show AdMob app open ads using Swift language?

OC code here: https://developers.google.com/admob/ios/app-open-ads

All code below goes to AppDelegate.swift

Import GoogleMobileAds:

import GoogleMobileAds

In class definition add GADFullScreenContentDelegate:

class AppDelegate: UIResponder, UIApplicationDelegate, GADFullScreenContentDelegate {

Add two variables:

var appOpenAd: GADAppOpenAd?
var loadTime = Date()

Add these three functions:

func requestAppOpenAd() {
    let request = GADRequest()
    GADAppOpenAd.load(withAdUnitID: "YOUR_ADUNIT_ID",
                      request: request,
                      orientation: UIInterfaceOrientation.portrait,
                      completionHandler: { (appOpenAdIn, _) in
                        self.appOpenAd = appOpenAdIn
                        self.appOpenAd?.fullScreenContentDelegate = self
                        self.loadTime = Date()
                        print("Ad is ready")
                      })
}

func tryToPresentAd() {
    if let gOpenAd = self.appOpenAd, let rwc = self.window?.rootViewController, wasLoadTimeLessThanNHoursAgo(thresholdN: 4) {
        gOpenAd.present(fromRootViewController: rwc)
    } 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)
}

Finally call tryToPresentAd() from applicationDidBecomeActive():

func applicationDidBecomeActive(_ application: UIApplication) {
    self.tryToPresentAd()
}

That's it.

Edit: Added the need to import GoogleMobileAds

Along with Mikrasya and TripleTroop answers and if you need to show open ads from SceneDelegate you can use instruction below:

All code below goes to SceneDelegate.swift

Import GoogleMobileAds:

import GoogleMobileAds

In class definition add GADFullScreenContentDelegate:

class SceneDelegate: UIResponder, UIWindowSceneDelegate, GADFullScreenContentDelegate {

Add variable:

var appOpenAd: GADAppOpenAd?

Add these functions:

    func requestAppOpenAd() {
    let request = GADRequest()
    GADAppOpenAd.load(withAdUnitID: "YOUR_ID",
                      request: request,
                      orientation: UIInterfaceOrientation.portrait,
                      completionHandler: { (appOpenAdIn, _) in
                        self.appOpenAd = appOpenAdIn
                        self.appOpenAd?.fullScreenContentDelegate = self
                        print("Ad is ready")
                      
                      })
    
}

func tryToPresentAd() {
    if let gOpenAd = self.appOpenAd, let rwc = UIApplication.shared.windows.last?.rootViewController {
        gOpenAd.present(fromRootViewController: rwc)
    } else {
        self.requestAppOpenAd()
    }
}

func ad(_ ad: GADFullScreenPresentingAd, didFailToPresentFullScreenContentWithError error: Error) {
    requestAppOpenAd()
}

func adDidDismissFullScreenContent(_ ad: GADFullScreenPresentingAd) {
    requestAppOpenAd()
}

func adDidPresentFullScreenContent(_ ad: GADFullScreenPresentingAd) {
    print("Ad did present")
}

And now we can change sceneDidBecomeActive function to call open ads:

   func sceneDidBecomeActive(_ scene: UIScene) {
    self.tryToPresentAd()
    // Called when the scene has moved from an inactive state to an active state.
    // Use this method to restart any tasks that were paused (or not yet started) when the scene was inactive.
}

Along with Mikrasya answer, in order to load another ad in case of ad fail or dimiss you gona need to call below functions as well

func ad(_ ad: GADFullScreenPresentingAd, didFailToPresentFullScreenContentWithError error: Error) {
    requestAppOpenAd()
}

func adDidDismissFullScreenContent(_ ad: GADFullScreenPresentingAd) {
    requestAppOpenAd()
}

func adDidPresentFullScreenContent(_ ad: GADFullScreenPresentingAd) {
    print("Ad did present")
}