$ pod init 

# Uncomment the next line to define a global platform for your project
# platform :ios, '9.0'

target 'SignIn' do
  # Comment the next line if you don't want to use dynamic frameworks
  use_frameworks!

  # Pods for SignIn
    pod 'Firebase/Auth'
    pod 'GoogleSignIn'

  target 'SignInTests' do
    inherit! :search_paths
    # Pods for testing
  end

  target 'SignInUITests' do
    # Pods for testing
  end

end

$ pod install

import UIKit
import Firebase

@main
class AppDelegate: UIResponder, UIApplicationDelegate {



    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        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.
    }


}

import UIKit
import Firebase
import FirebaseAuth
import GoogleSignIn

class ViewController: UIViewController {

    // 設置したボタンを関連づけ
    @IBOutlet weak var GoogleSignInButton: UIButton!
    override func viewDidLoad() {
        super.viewDidLoad()
        // ボタンをタップした時のアクションを指定。SignInファンクションを呼び出すよ。
        GoogleSignInButton.addTarget(self, action: #selector(SignIn), for: .touchUpInside)
    }

    // サインインを行います。
    @objc func SignIn() {
        print("サインインします。")
        guard let clientID = FirebaseApp.app()?.options.clientID else { return }

        // Create Google Sign In configuration object.
        let config = GIDConfiguration(clientID: clientID)

        // Start the sign in flow!
        GIDSignIn.sharedInstance.signIn(with: config, presenting: self) { [unowned self] user, error in
            if let error = error {
                print(error)
                return
            }
            guard
                let authentication = user?.authentication,
                let idToken = authentication.idToken
            else {
                return
            }
            let credential = GoogleAuthProvider.credential(withIDToken: idToken,
                                                           accessToken: authentication.accessToken)
            Auth.auth().signIn(with: credential) { authResult, error in
                if let error = error {
                    let authError = error as NSError
                    print(authError)
                    return
                }
                print("Googoleアカウントでサインしました。")
            }
        }
    }
}