Firebase AuthenticationでSign In With Appleを実装する

import AuthenticationServices

overide func viewDidLoad() {
    super.viewDidLoad()


    if #available(iOS 13.0, *) {
        let appleLoginButton = ASAuthorizationAppleIDButton(
                authorizationButtonType: .default,
                authorizationButtonStyle: .whiteOutline
        )

        appleLoginButton.addTarget(
            self,
            action: #selector(handleTappedAppleLoginButton(_:)),
            for: .touchUpInside
        )

        // 制約などの定義やaddSubViewなので省略
    }
}

@available(iOS 13.0, *)
    @objc func handleTappedAppleLoginButton(_ sender: ASAuthorizationAppleIDButton) {
        requestSignInWithAppleFlow()
}

    func requestSignInAppleFlow() {
        let nonce = randomNonceString()
        currentNonce = nonce
        let request = ASAuthorizationAppleIDProvider().createRequest()
        request.requestedScopes = [.fullName, .email]
        request.nonce = sha256(nonce)
        let controller = ASAuthorizationController(authorizationRequests: [request])
        controller.delegate = self
        controller.presentationContextProvider = self
        controller.performRequests()
    }

    private func randomNonceString(length: Int = 32) -> String {
        // 実装は自由
        https://auth0.com/docs/api-auth/tutorials/nonce#generate-a-cryptographically-random-nonce を参照して実装するか
        https://firebase.google.com/docs/auth/ios/apple#sign_in_with_apple_and_authenticate_with_firebase のサンプルコードを参考にして実装するか

    }

extension ASAuthorization.Scope {
    public static let fullName: ASAuthorization.Scope
    public static let email: ASAuthorization.Scope
}

<br />@available(iOS 13.0, *)
extension LoginViewController: CredentialWithAppleMakeable & ASAuthorizationControllerDelegate & ASAuthorizationControllerPresentationContextProviding {

    func authorizationController(controller: ASAuthorizationController, didCompleteWithAuthorization authorization: ASAuthorization) {
        // 認証完了時
        handle(authorization.credential)
    }

    func authorizationController(controller: ASAuthorizationController, didCompleteWithError error: Error) {
        // 認証失敗時
    }

    func presentationAnchor(for controller: ASAuthorizationController) -> ASPresentationAnchor {
        // ASAuthorizationControllerPresentationContextProvidingのメソッド
        return view.window!
    }

    func handle(_ credential: ASAuthorizationCredential) {
        // 認証完了後ASAuthorizationCredentialから必要な情報を取り出す
        // エラー時のハンドリングはアプリごとに違うので省略
        guard let appleIDCredential = credential as? ASAuthorizationAppleIDCredential else { 
            // キャスト失敗。ASPasswordCredential（パスワードは今回要求していないので起きないはず）だと失敗する
            return
        }
        guard let nonce = currentNonce else {
            // ログインリクエスト失敗
            return
        }

        guard let appleIDToken = appleIDCredential.identityToken else {
            print("Unable to fetch identity token")
            // ユーザーに関する情報をアプリに伝えるためのJSON Web Tokenの取得に失敗
            return
        }

        guard let idTokenString = String(data: appleIDToken, encoding: .utf8) else {
            // JWTからトークン(文字列)へのシリアライズに失敗
            return
        }

        // Sign In With Appleの認証情報を元にFirebase Authenticationの認証
        let oAuthCredential = OAuthProvider.credential(
            withProviderID: "apple.com",
            idToken: idTokenString,
            rawNonce: nonce)

        Auth.auth().signIn(with: oAuthCredential) { (authResult, error) in
            if (error != nil) {
                return
            }
            self.completeSigningInWithApple()
            // 今回のアプリでは認証完了
            // FireStore側に初期データを保存する処理を呼んだ後に前画面に戻る
            self.navigationController?.popViewController(animated: true)
        }
    }
}

@available(iOS 13.0, *)
private func handleASCredential() {
    let appleIDProvider = ASAuthorizationAppleIDProvider()
    appleIDProvider.getCredentialState(forUserID: currentIdentifier) { (credentialState, error) in
        switch credentialState {
        case .authorized:
            break
        case .revoked, .notFound:
            // ログアウト処理
            break
        default:
            break
        }
    }
}

@available(iOS 13.0, *)
private func addCredentialObserver() {
    let center = NotificationCenter.default
    let name = ASAuthorizationAppleIDProvider.credentialRevokedNotification
    let _ = center.addObserver(forName: name, object: nil, queue: nil) { (Notification) in
        // サインアウトして、再度サインインフローを表示するなど
        self.handleASCredential()
    }
}