【Xcode/Swift】Firebase Authenticationを使ってGoogleのサインイン機能を実装する方法

import UIKit
import Firebase
import GoogleSignIn

@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    
    
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        FirebaseApp.configure()
        return true
    }
    
    // MARK: UISceneSession Lifecycle
    
    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        // Called when a new scene session is being created.
        // Use this method to select a configuration to create the new scene with.
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }
    
    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
        // Called when the user discards a scene session.
        // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
        // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
    }
    
    func application(_ application: UIApplication, open url: URL, options: [UIApplication.OpenURLOptionsKey: Any]) -> Bool {
        return GIDSignIn.sharedInstance.handle(url)
    }
    
}

import UIKit
import Firebase
import GoogleSignIn

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }
    
    @IBAction func didTappSignInButton(_ sender: Any) {
        auth()
    }
    
    private func auth() {
        guard let clientID = FirebaseApp.app()?.options.clientID else { return }
        let config = GIDConfiguration(clientID: clientID)
        
        GIDSignIn.sharedInstance.signIn(with: config, presenting: self) { [unowned self] user, error in
            if let error = error {
                print("GIDSignInError: \(error.localizedDescription)")
                return
            }
            
            guard let authentication = user?.authentication,
                  let idToken = authentication.idToken else { return }
            
            let credential = GoogleAuthProvider.credential(withIDToken: idToken, accessToken: authentication.accessToken)
            self.login(credential: credential)
        }
    }
    
    private func login(credential: AuthCredential) {
        print("ログイン完了")
    }
    
}