clientViaApplicationDefaultCredentials function

Future<AutoRefreshingAuthClient> clientViaApplicationDefaultCredentials({ required List<String> scopes, Client? baseClient, })

Create a client using Application Default Credentials.

Looks for credentials in the following order of preference:

1. A JSON file whose path is specified by GOOGLE_APPLICATION_CREDENTIALS, this file typically contains exported service account keys.
2. A JSON file created by gcloud auth application-default login in a well-known location (%APPDATA%/gcloud/application_default_credentials.json on Windows and $HOME/.config/gcloud/application_default_credentials.json on Linux/Mac).
3. On Google Compute Engine and App Engine Flex we fetch credentials from GCE metadata service.

If baseClient is provided, all HTTP requests will be made with it. Otherwise, a new Client instance will be created.

HTTP requests made on the returned client will get an additional Authorization header with the AccessCredentials obtained. Once the AccessCredentials expire, it will use it's refresh token (if available) to obtain new credentials. See autoRefreshingClient for more information.

Implementation

Future<AutoRefreshingAuthClient> clientViaApplicationDefaultCredentials({
  required List<String> scopes,
  Client? baseClient,
}) async {
  if (baseClient == null) {
    baseClient = Client();
  } else {
    baseClient = nonClosingClient(baseClient);
  }

  // If env var specifies a file to load credentials from we'll do that.
  final credsEnv = Platform.environment['GOOGLE_APPLICATION_CREDENTIALS'];
  if (credsEnv != null && credsEnv.isNotEmpty) {
    // If env var is specific and not empty, we always try to load, even if
    // the file doesn't exist.
    return await fromApplicationsCredentialsFile(
      File(credsEnv),
      'GOOGLE_APPLICATION_CREDENTIALS',
      scopes,
      baseClient,
    );
  }

  // Attempt to use file created by `gcloud auth application-default login`
  File credFile;
  if (Platform.isWindows) {
    credFile = File.fromUri(
      Uri.directory(Platform.environment['APPDATA']!)
          .resolve('gcloud/application_default_credentials.json'),
    );
  } else {
    final homeVar = Platform.environment['HOME'];
    if (homeVar == null) {
      throw StateError('The expected environment variable HOME must be set.');
    }
    credFile = File.fromUri(
      Uri.directory(homeVar)
          .resolve('.config/gcloud/application_default_credentials.json'),
    );
  }
  // Only try to load from credFile if it exists.
  if (await credFile.exists()) {
    return await fromApplicationsCredentialsFile(
      credFile,
      '`gcloud auth application-default login`',
      scopes,
      baseClient,
    );
  }

  return await clientViaMetadataServer(baseClient: baseClient);
}