问由于缺少conack包，MqttBrowserClient无法连接。

EN

MqttBrowserClient mq = MqttBrowserClient(
    'wss://broker.mqttdashboard.com:8000', '',
    maxConnectionAttempts: 1);

/*

MqttBrowserClient mq = MqttBrowserClient('ws://test.mosquitto.org', 'client-1',
    maxConnectionAttempts: 1);

 */
class mqttService {
  Future<MqttBrowserClient?> connectToServer() async {
    try {
      final connMess = MqttConnectMessage()
          .withClientIdentifier('clientz5tWzoydVL')
          .authenticateAs('a14guguliye', 'z5tWzoydVL')
          .withWillTopic('willtopic')
          .withWillMessage('My Will message')
          .startClean() // Non persistent session for testing
          .withWillQos(MqttQos.atLeastOnce);

      mq.port = 1883;
      mq.keepAlivePeriod = 50;
      mq.connectionMessage = connMess;
      mq.websocketProtocols = MqttClientConstants.protocolsSingleDefault;
      mq.onConnected = onConnected;

      var status = await mq.connect();
      return mq;
    } catch (e) {
      print("error is " + e.toString());
      mq.disconnect();
      return null;
    }
  }
}