颤振：NoSuchMethod错误：方法'connect'在 null 上调用

Question

我正在尝试使用Flutter连接到mosquitto MQTT测试代理。它一直给我nosuchmethoderror，说我试图在null上调用connect，尽管我认为我已经根据包[文档]正确设置了所有内容。

这是我的代码：

import 'dart:io';
import 'package:flutter/material.dart';
import 'converter.dart';
import 'constants.dart' as Constants;
import 'package:location/location.dart';
import 'package:mqtt_client/mqtt_client.dart';
import 'models.dart';
import 'package:mqtt_client/mqtt_server_client.dart';
class MQTTClientWrapper {
MqttClient client;
LocationToJsonConverter locationToJsonConverter = LocationToJsonConverter();
JsonToLocationConverter jsonToLocationConverter = JsonToLocationConverter();
MqttCurrentConnectionState connectionState = MqttCurrentConnectionState.IDLE;
MqttSubscriptionState subscriptionState = MqttSubscriptionState.IDLE;
final VoidCallback onConnectedCallback;
final Function(LocationData) onLocationReceivedCallback;

MQTTClientWrapper(this.onConnectedCallback, this.onLocationReceivedCallback);
void prepareMqttClient() async {
_setupMqttClient();
await _connectClient();
_subscribeToTopic(Constants.topicName);
}
void publishLocation(LocationData locationData) {
String locationJson = locationToJsonConverter.convert(locationData);
_publishMessage(locationJson);
}
Future<void> _connectClient() async {
try {
print('MQTTClientWrapper::Mosquitto client connecting....');
connectionState = MqttCurrentConnectionState.CONNECTING;
await client.connect();
} on NoConnectionException catch (e) {
print('MQTTClientWrapper::client exception - $e');
connectionState = MqttCurrentConnectionState.ERROR_WHEN_CONNECTING;
client.disconnect();
} on SocketException catch (e) {
print('EXAMPLE::socket exception - $e');
client.disconnect();
}
if (client.connectionStatus.state == MqttConnectionState.connected) {
connectionState = MqttCurrentConnectionState.CONNECTED;
print('MQTTClientWrapper::Mosquitto client connected');
} else {
print(
'MQTTClientWrapper::ERROR Mosquitto client connection failed - disconnecting, status is ${client.connectionStatus}');
connectionState = MqttCurrentConnectionState.ERROR_WHEN_CONNECTING;
client.disconnect();
exit(-1);
}
}
void _setupMqttClient() {
final client = MqttServerClient('test.mosquitto.org', '');
client.logging(on: false);
client.keepAlivePeriod = 20;
client.onDisconnected = _onDisconnected;
client.onConnected = _onConnected;
client.onSubscribed = _onSubscribed;
final connMess = MqttConnectMessage()
.withClientIdentifier('sisterClient_publishHp')
.keepAliveFor(20)
.startClean()
.withWillQos(MqttQos.atLeastOnce);
client.connectionMessage = connMess;
}
void _subscribeToTopic(String topicName) {
print('MQTTClientWrapper::Subscribing to the $topicName topic');
client.subscribe(topicName, MqttQos.atMostOnce);
client.updates.listen((List<MqttReceivedMessage<MqttMessage>> c) {
final MqttPublishMessage recMess = c[0].payload;
final String newLocationJson =
MqttPublishPayload.bytesToStringAsString(recMess.payload.message);
print("MQTTClientWrapper::GOT A NEW MESSAGE $newLocationJson");
LocationData newLocationData = _convertJsonToLocation(newLocationJson);
if (newLocationData != null) onLocationReceivedCallback(newLocationData);
});
}
LocationData _convertJsonToLocation(String newLocationJson) {
try {
return jsonToLocationConverter.convert(
newLocationJson);
} catch (exception) {
print("Json can't be formatted ${exception.toString()}");
}
return null;
}
void _publishMessage(String message) async {
client.published.listen((MqttPublishMessage message) {
print(
'EXAMPLE::Published notification:: topic is ${message.variableHeader.topicName}, with Qos ${message.header.qos}');
});

final MqttClientPayloadBuilder builder = MqttClientPayloadBuilder();
builder.addString(message);
print('EXAMPLE::Subscribing to topic');
client.subscribe(Constants.topicName, MqttQos.exactlyOnce);
print('MQTTClientWrapper::Publishing message $message to topic ${Constants.topicName}');
client.publishMessage(Constants.topicName, MqttQos.exactlyOnce, builder.payload);

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

print('EXAMPLE::Unsubscribing');
client.unsubscribe(Constants.topicName);
await MqttUtilities.asyncSleep(2);
print('EXAMPLE::Disconnecting');
client.disconnect();
}
void _onSubscribed(String topic) {
print('MQTTClientWrapper::Subscription confirmed for topic $topic');
subscriptionState = MqttSubscriptionState.SUBSCRIBED;
}
void _onDisconnected() {
print('MQTTClientWrapper::OnDisconnected client callback - Client disconnection');
if (client.connectionStatus.disconnectionOrigin == MqttDisconnectionOrigin.solicited) {
print('MQTTClientWrapper::OnDisconnected callback is solicited, this is correct');
}
connectionState = MqttCurrentConnectionState.DISCONNECTED;
exit(-1);
}
void _onConnected() {
connectionState = MqttCurrentConnectionState.CONNECTED;
print(
'MQTTClientWrapper::OnConnected client callback - Client connection was sucessful');
onConnectedCallback();
}
}

这是我得到的错误：

Launching libmain.dart on Android SDK built for x86 in debug mode...
package:publish_hp/main.dart:1
√ Built buildappoutputsflutter-apkapp-debug.apk.
Connecting to VM Service at ws://127.0.0.1:56651/pQyStOIm6Cw=/ws
D/EGL_emulation(17695): eglMakeCurrent: 0xe6fb7d40: ver 2 0 (tinfo 0xdbb4fcc0)
D/eglCodecCommon(17695): setVertexArrayObject: set vao to 0 (0) 1 0
I/flutter (17695): MQTTClientWrapper::Mosquitto client connecting....
E/flutter (17695): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: NoSuchMethodError: The method 'connect' was called on null.
E/flutter (17695): Receiver: null
E/flutter (17695): Tried calling: connect()

我的应用程序基于此：

https://medium.com/swlh/using-mqtt-with-flutter-to-build-a-location-sharing-app-24e7307b21d3

但将其更改为mqttserverclient，因为我早些时候在尝试按原样阅读文章时遇到了一个不正确的实例化错误。

关于如何解决这个问题有什么建议吗？

Answer 1

首先，您需要初始化MqttClient。现在，这是无效的，问题是。你可以用下面的例子来修复它：

MqttServerClient client = MqttServerClient.withPort('yourServer.com', 'clientIdentifier', maxConnectionAttempts);

有关更多信息，您可以查看此处。