Access Google APIs in an iOS app

Some Google services, such as Drive, Gmail, and many others, provide public APIs that you can use to create apps that help users work with their data in these services. To access these services, apps must implement one of the OAuth 2.0 client flows to get consent from users and obtain access tokens, which grant access to the APIs.

You can use the Google Sign-In library, which implements the OAuth 2.0 flow for you, to get access tokens for the signed-in user.

Before you begin

You must complete the basic Google Sign-In integration.

1. Check which scopes have been granted

Before you make a call to a Google API, check which scopes have already been granted to your app, using the grantedScopes property of GIDGoogleUser:

let driveScope = "https://www.googleapis.com/auth/drive.readonly"
let grantedScopes = user.grantedScopes
if grantedScopes == nil || !grantedScopes!.contains(driveScope) {
  // Request additional Drive scope.
}

NSString *driveScope = @"https://www.googleapis.com/auth/drive.readonly";

// Check if the user has granted the Drive scope
if (![user.grantedScopes containsObject:driveScope]) {
  // request additional drive scope
}

Based on whether or not a certain scope has been granted by the user, you might need to make a request for an additional scope in order to support a particular interaction.

2. Request additional scopes

If you need to request additional scopes, call addScopes:presentingViewController:completion or addScopes:presentingWindow:completion to ask the user to grant your app additional access.

For example, to request read-only access to a user's Drive files:

let additionalScopes = ["https://www.googleapis.com/auth/drive.readonly"]
guard let currentUser = GIDSignIn.sharedInstance.currentUser else {
    return ;  /* Not signed in. */
}

currentUser.addScopes(additionalScopes, presenting: self) { signInResult, error in
    guard error == nil else { return }
    guard let signInResult = signInResult else { return }

    // Check if the user granted access to the scopes you requested.
}

NSArray *additionalScopes = @[ @"https://www.googleapis.com/auth/drive.readonly" ];
GIDGoogleUser *currentUser = GIDSignIn.sharedInstance.currentUser;

[currentUser addScopes:additionalScopes
           presentingViewController:self
                         completion:^(GIDSignInResult * _Nullable signInResult,
                                      NSError * _Nullable error) {
    if (error) { return; }
    if (signInResult == nil) { return; }

    // Check if the user granted access to the scopes you requested.
}];

3. Make an API call with fresh tokens

To ensure that your Google API calls always have unexpired access tokens attached, wrap the calls in a refreshTokensIfNeededWithCompletion: block:

currentUser.refreshTokensIfNeeded { user, error in
    guard error == nil else { return }
    guard let user = user else { return }

    // Get the access token to attach it to a REST or gRPC request.
    let accessToken = user.accessToken.tokenString

    // Or, get an object that conforms to GTMFetcherAuthorizationProtocol for
    // use with GTMAppAuth and the Google APIs client library.
    let authorizer = user.fetcherAuthorizer()
}

[currentUser refreshTokensIfNeededWithCompletion:^(
                              GIDGoogleUser * _Nullable user,
                              NSError * _Nullable error) {
    if (error) { return; }
    if (user == nil) { return; }

    // Get the access token to attach it to a REST or gRPC request.
    NSString *accessToken = user.accessToken.tokenString;

    // Or, get an object that conforms to GTMFetcherAuthorizationProtocol for
    // use with GTMAppAuth and the Google APIs client library.
    id<GTMFetcherAuthorizationProtocol> authorizer = [user fetcherAuthorizer];
}];

Use the access token to call the API, by either including the access token in the header of a REST or gRPC request (Authorization: Bearer ACCESS_TOKEN), or by using the fetcher authorizer with the Google APIs Client Library.