OIDServiceConfiguration *configuration = [GTMAuthSession configurationForGoogle];

NSURL *authorizationEndpoint =
    [NSURL URLWithString:@"https://accounts.google.com/o/oauth2/v2/auth"];
NSURL *tokenEndpoint =
    [NSURL URLWithString:@"https://www.googleapis.com/oauth2/v4/token"];

OIDServiceConfiguration *configuration =
    [[OIDServiceConfiguration alloc]
        initWithAuthorizationEndpoint:authorizationEndpoint
                        tokenEndpoint:tokenEndpoint];

// perform the auth request...

NSURL *issuer = [NSURL URLWithString:@"https://accounts.google.com"];

[OIDAuthorizationService discoverServiceConfigurationForIssuer:issuer
    completion:^(OIDServiceConfiguration *_Nullable configuration,
                 NSError *_Nullable error) {
  if (!configuration) {
    NSLog(@"Error retrieving discovery document: %@",
          [error localizedDescription]);
    return;
  }

  // perform the auth request...
}];

// property of the app's UIApplicationDelegate
@property(nonatomic, nullable)
    id<OIDExternalUserAgentSession> currentAuthorizationFlow;

// property of the containing class
@property(nonatomic, nullable) GTMAuthSession *authSession;

// builds authentication request
OIDAuthorizationRequest *request =
    [[OIDAuthorizationRequest alloc] initWithConfiguration:configuration
                                                  clientId:kClientID
                                              clientSecret:kClientSecret
                                                    scopes:@[OIDScopeOpenID, OIDScopeProfile]
                                               redirectURL:redirectURI
                                              responseType:OIDResponseTypeCode
                                      additionalParameters:nil];
// performs authentication request
self.appDelegate.currentAuthorizationFlow =
    [OIDAuthState authStateByPresentingAuthorizationRequest:request
        callback:^(OIDAuthState *_Nullable authState,
                   NSError *_Nullable error) {
  if (authState) {
    // Creates a GTMAuthSession from the OIDAuthState.
    self.authSession = [[GTMAuthSession alloc] initWithAuthState:authState];
    NSLog(@"Got authorization tokens. Access token: %@",
          authState.lastTokenResponse.accessToken);
  } else {
    NSLog(@"Authorization error: %@", [error localizedDescription]);
    self.authSession = nil;
  }
}];

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
  // Other app initialization code ...

  // Register for GetURL events.
  NSAppleEventManager *appleEventManager =
      [NSAppleEventManager sharedAppleEventManager];
  [appleEventManager setEventHandler:self
                         andSelector:@selector(handleGetURLEvent:withReplyEvent:)
                       forEventClass:kInternetEventClass
                          andEventID:kAEGetURL];
}

- (void)handleGetURLEvent:(NSAppleEventDescriptor *)event
           withReplyEvent:(NSAppleEventDescriptor *)replyEvent {
  NSString *URLString = [[event paramDescriptorForKeyword:keyDirectObject] stringValue];
  NSURL *URL = [NSURL URLWithString:URLString];
  [_currentAuthorizationFlow resumeExternalUserAgentFlowWithURL:URL];
}

- (BOOL)application:(UIApplication *)app
            openURL:(NSURL *)url
            options:(NSDictionary<NSString *, id> *)options {
  // Sends the URL to the current authorization flow (if any) which will
  // process it if it relates to an authorization response.
  if ([_currentAuthorizationFlow resumeExternalUserAgentFlowWithURL:url]) {
    _currentAuthorizationFlow = nil;
    return YES;
  }

  // Your additional URL handling (if any) goes here.

  return NO;
}

// Creates a GTMSessionFetcherService with the authorization.
// Normally you would save this service object and re-use it for all REST API calls.
GTMSessionFetcherService *fetcherService = [[GTMSessionFetcherService alloc] init];
fetcherService.authorizer = self.authSession;

// Creates a fetcher for the API call.
NSURL *userinfoEndpoint = [NSURL URLWithString:@"https://www.googleapis.com/oauth2/v3/userinfo"];
GTMSessionFetcher *fetcher = [fetcherService fetcherWithURL:userinfoEndpoint];
[fetcher beginFetchWithCompletionHandler:^(NSData *data, NSError *error) {
  // Checks for an error.
  if (error) {
    // OIDOAuthTokenErrorDomain indicates an issue with the authorization.
    if ([error.domain isEqual:OIDOAuthTokenErrorDomain]) {
      self.authSession = nil;
      NSLog(@"Authorization error during token refresh, clearing state. %@",
            error);
    // Other errors are assumed transient.
    } else {
      NSLog(@"Transient error during token refresh. %@", error);
    }
    return;
  }

  // Parses the JSON response.
  NSError *jsonError = nil;
  id jsonDictionaryOrArray =
      [NSJSONSerialization JSONObjectWithData:data options:0 error:&jsonError];

  // JSON error.
  if (jsonError) {
    NSLog(@"JSON decoding error %@", jsonError);
    return;
  }

  // Success response!
  NSLog(@"Success: %@", jsonDictionaryOrArray);
}];

// Create a GIDKeychainStore instance, intializing it with the Keychain item name `kKeychainItemName`
// which will be used when saving, retrieving, and removing `GTMAuthSession` instances.
GIDKeychainStore *keychainStore = [[GIDKeychainStore alloc] initWithItemName:kKeychainItemName];
    
NSError *error;

// Save to the Keychain
[keychainStore saveAuthSession:self.authSession error:&error];
if (error) {
  // Handle error
}

// Retrieve from the Keychain
self.authSession = [keychainStore retrieveAuthSessionWithError:&error];
if (error) {
  // Handle error
}

// Remove from the Keychain
[keychainStore removeAuthSessionWithError:&error];
if (error) {
  // Handle error
}

GTMKeychainStore keychainStore = [[GTMKeychainStore alloc] initWithItemName:kKeychainItemName];

// Retrieve from the Keychain
NSError *error;
GTMAuthSession *authSession =
    [keychainStore retrieveAuthSessionForGoogleInGTMOAuth2FormatWithClientID:clientID
                                                                clientSecret:clientSecret
                                                                       error:&error];

// Remove from the Keychain
[keychainStore removeAuthSessionWithError:&error];

// Save to the Keychain
[keychainStore saveWithGTMOAuth2FormatForAuthSession:authSession error:&error];

// Create an auth session from AppAuth's auth state object
GTMAuthSession *authSession = [[GTMAuthSession alloc] initWithAuthState:authState];

// Create a keychain store
GTMKeychainStore keychainStore = [[GTMKeychainStore alloc] initWithItemName:kNewKeychainName];

// Serialize to Keychain
NSError *error;
[keychainStore saveAuthSession:authSession error:&error];

// Create a keychain store
GTMKeychainStore keychainStore = [[GTMKeychainStore alloc] initWithItemName:kNewKeychainName];

// Attempt to deserialize from Keychain in GTMAppAuth format.
NSError *error;
GTMAuthSesion *authSession =
    [keychainStore retrieveAuthSessionWithError:&error];

// If no data found in the new format, try to deserialize data from GTMOAuth2
if (!authSession) {
  // Tries to load the data serialized by GTMOAuth2 using old keychain name.
  // If you created a new client id, be sure to use the *previous* client id and secret here.
  GTMKeychainStore oldKeychainStore = [[GTMKeychainStore alloc] initWithItemName:kPreviousKeychainName];
  authSession =
      [oldKeychainStore retrieveAuthSessionInGTMOAuth2FormatWithClientID:kPreviousClientID
                                                            clientSecret:kPreviousClientSecret
                                                                   error:&error];
  if (authSession) {
    // Remove previously stored GTMOAuth2-formatted data.
    [oldKeychainStore removeAuthSessionWithError:&error];
    // Serialize to Keychain in GTMAppAuth format.
    [keychainStore saveAuthSession:authSession error:&error];
  }
}