Google AdMob - all code needed to get an interstitial ad working in an app

adMob.swift

import GoogleMobileAds

class FirstViewController: UIViewController, UITextFieldDelegate, GADInterstitialDelegate {
    
    // Your ad as a variable
    var interstitialAd: GADInterstitial?
  
    // Creates and loads an interstitial
    func createAndLoadInterstitial() -> GADInterstitial {
        let request = GADRequest()
        let interstitial = GADInterstitial(adUnitID: "ca-app-pub-3940256099942544/1033173712") // interstitial test
        interstitial.delegate = self
        interstitial.load(request)
        return interstitial
    }
    
    // Lets you know when it's ready
    func interstitialDidReceiveAd(_ ad: GADInterstitial) {
        if interstitialAd!.isReady {
            interstitialAd!.present(fromRootViewController: self)
        } else {
            print("Ad wasn't ready")
        }
        print("interstitialDidReceiveAd")
    }
    
    // The ad with a 1 second delay, insert this code in whatever action you want. Delay is not required.
    let when = DispatchTime.now() + 1 // change 1 to desired number of seconds
    DispatchQueue.main.asyncAfter(deadline: when) {
        self.interstitialAd = self.createAndLoadInterstitial()
    }
  
}