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我无法在订阅时收到消息。我究竟做错了什么？

我能够注册与经纪人的连接。我只是不能发出任何东西client.updates.listen。

MqttPublishMessage和 和有什么区别MqttSubscribeMessage？

import 'dart:async';
import 'dart:io';
import 'package:mqtt_client/mqtt_client.dart';
import 'package:mqtt_client/mqtt_server_client.dart';

final client = MqttServerClient.withPort('ws://192.168.43.56', 'cliendId', 8080,);

Future<int> main() async {
  /// Set logging on if needed, defaults to off
  client.logging(on: false);
  client.keepAlivePeriod = 20;
  client.port = 8080;
  client.useWebSocket = true;
  client.onDisconnected = onDisconnected;
  client.onConnected = onConnected;
  client.onSubscribed = onSubscribed;
  
  client.pongCallback = pong;
  final connMess = MqttConnectMessage()
      .withClientIdentifier('client_id')
      .keepAliveFor(60) // Must agree with the keep alive set above or not set
      .startClean() // Non persistent session for testing
      .withWillQos(MqttQos.atLeastOnce);
  print('EXAMPLE::Mosquitto client connecting....');
  client.connectionMessage = connMess;

  try {
    await client.connect();
  } on Exception catch (e) {
    print('EXAMPLE::client exception - $e');
    client.disconnect();
  }

  /// Check we are connected
  if (client.connectionStatus.state == MqttConnectionState.connected) {
    print('EXAMPLE::Mosquitto client connected');
  } else {
    /// Use status here rather than state if you also want the broker return code.
    print(
        'EXAMPLE::ERROR Mosquitto client connection failed - disconnecting, status is ${client.connectionStatus}');
    client.disconnect();
    return -1;
  }

  /// Ok, lets try a subscription
  print('EXAMPLE::Subscribing to the test/lol topic');
  const topic = '/busline/201'; // Not a wildcard topic
  client.subscribe(topic, MqttQos.exactlyOnce);

  client.updates.listen((List<MqttReceivedMessage<MqttMessage>> c) {
    final MqttPublishMessage recMess = c[0].payload;
    final pt = MqttPublishPayload.bytesToStringAsString(recMess.payload.message);

    print(
        'EXAMPLE::Change notification:: topic is <${c[0].topic}>, payload is <-- $pt -->');
    print('');
  });

  

  print('EXAMPLE::Sleeping....');
  await MqttUtilities.asyncSleep(120);

}

/// The subscribed callback
void onSubscribed(String topic) {
  print('EXAMPLE::Subscription confirmed for topic $topic');
}

/// The unsolicited disconnect callback
void onDisconnected() {
  print('EXAMPLE::OnDisconnected client callback - Client disconnection');
  if (client.connectionStatus.disconnectionOrigin ==
      MqttDisconnectionOrigin.solicited) {
    print('EXAMPLE::OnDisconnected callback is solicited, this is correct');
  }
}

/// The successful connect callback
void onConnected() {
  print(
      'EXAMPLE::OnConnected client callback - Client connection was sucessful');
}

/// Pong callback
void pong() {
  print('EXAMPLE::Ping response client callback invoked');
}
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