How to check if metadata header has been set?

[Pante]

Hey there, how do I check if a metadata header has been set? I currently have a ClientInterceptor that asynchronously attaches a authentication token to each request. At the moment, the request is simply sent without an authentication token if the application is not currently authenticated which isn't ideal since it's needlessly invoking the gRPC server. Ideally, the request would be cancelled by the client without having to make the round trip.

class AuthenticationInterceptor extends ClientInterceptor {

  @override
  ResponseFuture<R> interceptUnary<Q, R>(ClientMethod<Q, R> method, Q request, CallOptions options, ClientUnaryInvoker<Q, R> invoker) => super.interceptUnary(
      method,
      request,
      options.mergedWith(CallOptions(providers: [attach])),
      invoker
  );

    Future<void> attach(Map<String, String> metadata, String _) async {
      final token = await someOtherclass.getToken();
      if (token != null) {
        metadata['Authorization'] = 'Bearer $token';
      }
    }
  }
}

[mraleph]

You can throw an error from the MetadataProvider if you don't have a token that would terminate the call with error.