public void DidComplete(ASAuthorizationController controller, ASAuthorization authorization)
        {
            var credential = authorization.GetCredential <ASAuthorizationAppleIdCredential>();

            if (credential != null && !string.IsNullOrEmpty(currentNonce))
            {
                appleToken = credential.IdentityToken.ToString();
                var firebaseCredential = OAuthProvider.GetCredentialWithRawNonce("apple.com", credential.IdentityToken.ToString(), currentNonce);
                Auth.DefaultInstance.SignInWithCredential(firebaseCredential, SignInOnCompletion);
            }
            else
            {
                appleToken = string.Empty;
                SetVerificationStatus(VerificationStatus.Failed, "Sign in failed");
            }
        }