Follow this procedure if you want your servers to be able to make
Google API calls on behalf of users or while they are offline.

## Before you begin

You must complete the [basic Google Sign-In integration](https://developers.google.com/identity/sign-in/ios/sign-in).

## Enable server-side API access for your app

On the [Access Google APIs in an iOS app](https://developers.google.com/identity/sign-in/ios/api-access)
page, your app authenticates the user on the client side only; in that case,
your app is able to access Google APIs only while the user is actively using
your app.

With the procedure described on this page, your servers can make Google API
calls on behalf of users while they are offline. For example, a photo app could
enhance a photo in a user's Google Photos album by processing it on a backend
server and uploading the result to another album. To do this, your server
requires an access token and a refresh token.

To obtain an access token and refresh token for your server, you can
request a one-time authorization code that your server exchanges for
these two tokens. After a successful sign-in, you will find the one-time code as
the `serverAuthCode` property of `GIDSignInResult`.

1. If you haven't already, [get a server client ID](https://developers.google.com/identity/sign-in/ios/start-integrating#server_client_id)
   and specify it in your app's `Info.plist` file, [below your OAuth client ID](https://developers.google.com/identity/sign-in/ios/start-integrating#add_client_id).

   <br />

   ```
   <key>GIDServerClientID</key>
   <string>YOUR_SERVER_CLIENT_ID</string>
   ```

   <br />

2. In your sign-in callback, retrieve the one-time authorization code:

   ### Swift

       GIDSignIn.sharedInstance.signIn(withPresenting: self) { signInResult, error in
           guard error == nil else { return }
           guard let signInResult = signInResult else { return }

           let authCode = signInResult.serverAuthCode
       }

   ### Objective-C

       [GIDSignIn.sharedInstance
                 signInWithPresentingViewController:self
                                         completion:^(GIDSignInResult * _Nullable signInResult,
                                                      NSError * _Nullable error) {
             if (error) { return; }
             if (signInResult == nil) { return; }

             NSString *authCode = signInResult.serverAuthCode;
       }];

3. Securely pass the `serverAuthCode` string to your server using HTTPS POST.

4. On your app's backend server, exchange the auth code for access and refresh
   tokens. Use the access token to call Google APIs on behalf of the user and,
   optionally, store the refresh token to acquire a new access token when the
   access token expires.

   For example:

   ##### Java

   ```java
   // (Receive authCode via HTTPS POST)


   if (request.getHeader("X-Requested-With") == null) {
     // Without the `X-Requested-With` header, this request could be forged. Aborts.
   }

   // Set path to the Web application client_secret_*.json file you downloaded from the
   // Google API Console: https://console.cloud.google.com/apis/credentials
   // You can also find your Web application client ID and client secret from the
   // console and specify them directly when you create the GoogleAuthorizationCodeTokenRequest
   // object.
   String CLIENT_SECRET_FILE = "/path/to/client_secret.json";

   // Exchange auth code for access token
   GoogleClientSecrets clientSecrets =
       GoogleClientSecrets.load(
           JacksonFactory.getDefaultInstance(), new FileReader(CLIENT_SECRET_FILE));
   GoogleTokenResponse tokenResponse =
             new GoogleAuthorizationCodeTokenRequest(
                 new NetHttpTransport(),
                 JacksonFactory.getDefaultInstance(),
                 "https://oauth2.googleapis.com/token",
                 clientSecrets.getDetails().getClientId(),
                 clientSecrets.getDetails().getClientSecret(),
                 authCode,
                 REDIRECT_URI)  // Specify the same redirect URI that you use with your web
                                // app. If you don't have a web version of your app, you can
                                // specify an empty string.
                 .execute();

   String accessToken = tokenResponse.getAccessToken();

   // Use access token to call API
   GoogleCredential credential = new GoogleCredential().setAccessToken(accessToken);
   Drive drive =
       new Drive.Builder(new NetHttpTransport(), JacksonFactory.getDefaultInstance(), credential)
           .setApplicationName("Auth Code Exchange Demo")
           .build();
   File file = drive.files().get("appfolder").execute();

   // Get profile info from ID token
   GoogleIdToken idToken = tokenResponse.parseIdToken();
   GoogleIdToken.Payload payload = idToken.getPayload();
   String userId = payload.getSubject();  // Use this value as a key to identify a user.
   String email = payload.getEmail();
   boolean emailVerified = Boolean.valueOf(payload.getEmailVerified());
   String name = (String) payload.get("name");
   String pictureUrl = (String) payload.get("picture");
   String locale = (String) payload.get("locale");
   String familyName = (String) payload.get("family_name");
   String givenName = (String) payload.get("given_name");
   ```

   ##### Python

   ```python
   from apiclient import discovery
   import httplib2
   from oauth2client import client

   # (Receive auth_code by HTTPS POST)


   # If this request does not have `X-Requested-With` header, this could be a CSRF
   if not request.headers.get('X-Requested-With'):
       abort(403)

   # Set path to the Web application client_secret_*.json file you downloaded from the
   # Google API Console: https://console.cloud.google.com/apis/credentials
   CLIENT_SECRET_FILE = '/path/to/client_secret.json'

   # Exchange auth code for access token, refresh token, and ID token
   credentials = client.credentials_from_clientsecrets_and_code(
       CLIENT_SECRET_FILE,
       ['https://www.googleapis.com/auth/drive.appdata', 'profile', 'email'],
       auth_code)

   # Call Google API
   http_auth = credentials.authorize(httplib2.Http())
   drive_service = discovery.build('drive', 'v3', http=http_auth)
   appfolder = drive_service.files().get(fileId='appfolder').execute()

   # Get profile info from ID token
   userid = credentials.id_token['sub']
   email = credentials.id_token['email']
   ```

   <br />