Publish a pubsub message in flutter/dart

Question

I'm trying to publish a pub/sub message in my flutter application like this:

import 'package:flutter/material.dart';
import 'package:googleapis/pubsub//v1.dart';
import 'package:googleapis_auth/auth_io.dart';

const _SCOPES = const [PubsubApi.PubsubScope];

class Activities extends StatefulWidget {
  Activities();

  @override
  _Activities createState() => _Activities();
}

final _credentials = new ServiceAccountCredentials.fromJson(r'''
{
  "type": "service_account",
  ...
}
''');

class _Activities extends State<Activities> with SingleTickerProviderStateMixin {

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: Center(
        child: Container(
          child: new MaterialButton(
            onPressed: () {
              debugPrint("trying to publish a message...");
              clientViaServiceAccount(_credentials, _SCOPES)
                .then((http_client) {
                  var pubSubClient = new PubsubApi(http_client);
                  Map<String, dynamic> message = new Map<String, dynamic>();
                  message["data"] = Map<String, dynamic>();
                  message["data"]["type"] = "foo";
                  message["data"]["senderId"] = "bar";

                  pubSubClient.projects.topics
                      .publish(new PublishRequest.fromJson(message), "projects/myProject/topics/events")
                      .then((publishResponse) {
                    debugPrint(publishResponse.toString());
                  }).catchError((e,m){
                    debugPrint(e.toString());
                  });
                })
                .catchError((e,m) {
                  debugPrint(e.toString());
                });
            },
            child: new Text("Publish message"),
          ),
        ),
      )
    );
  }
}

But in the logs I get the following error message:

I/flutter ( 5281): DetailedApiRequestError(status: 400, message: The value for 0 is too small. You passed message_count in the request, but the minimum value is 1.)

I googled this message but did not found anything. I think my message structure is maybe wrong, but maybe it's something else ? Don't wanna lose too much time on it...

Solution

Ok, for those who wonder, here is what I have come up with:

var messages = {
    'messages': [
                  {
                    'data': base64Encode(utf8.encode('{"foo": "bar"}')),
                  },
                ]
              };

pubSubClient.projects.topics
                  .publish(new PublishRequest.fromJson(messages), "your topic")
                  .then((publishResponse) {
                debugPrint(publishResponse.toString());
              }).catchError((e,m){
                debugPrint(e.toString());
              });

Maybe someone should write an article on it, this was not very clear and I had to try different things and also read the source from the pubsub client library...