Question

我有此代码：

import 'package:flutter/foundation.dart';
import 'package:mqtt_client/mqtt_server_client.dart';
import 'package:mqtt_client/mqtt_client.dart';
import 'package:finalprojectapp/Providers/Message_provider.dart';
import 'package:finalprojectapp/Providers/Client_provider.dart';


class MQTTManager with ChangeNotifier{
  //Properties
  MqttServerClient client;
  MQTTMessageProvider messageProvider = MQTTMessageProvider();
  String _identifier;
  String _topic;
  String _host;

  //Getters

  //Setters

  Future initialize({String host, String identifier}) async {
    MqttServerClient _client = MqttServerClient(host, identifier);
    this._identifier = _client.clientIdentifier;
    this._host = host;
    _client.port = 1883;
    _client.keepAlivePeriod = 20;
    _client.onDisconnected = onDisconnected;
    _client.onConnected = onConnected;
    _client.onSubscribed = onSubscribed;
    _client.logging(on: false);

    final conMess = MqttConnectMessage()
        .withClientIdentifier(identifier)
        .keepAliveFor(20)
        .withWillTopic('willtopic')
        .withWillMessage('willmessage')
        .startClean()
        .withWillQos(MqttQos.atLeastOnce);

    _client.connectionMessage = conMess;

    try {
      MqttClientConnectionStatus result = await _client.connect('BBFF-qkHkFkvJ6oFUw9m6Pa9bzQTCbVCddH','');
      this.client = _client;
      notifyListeners();
      return result.state;
    } on Exception catch (e) {
      print('Something went wrong $e');
      disconnect();
      return null;
    }
  }

  void subscription({String topic}) {
    this._topic = topic;
    print('EXAMPLE::Subscribing to the $_topic topic');
    this.client.subscribe(this._topic, MqttQos.atMostOnce);
  }

  void unsubscribe({String topic}) {
    print('unsubscribing from $topic');
    this.client.unsubscribe(topic);
    print('Unsubscribbed!');
  }

  void publish({String topic, String message}) async {
    final builder = MqttClientPayloadBuilder();
    builder.addString(message);
    this.client.publishMessage(topic, MqttQos.atMostOnce, builder.payload);
  }

  void disconnect() async {
    await MqttUtilities.asyncSleep(2);
    print('EXAMPLE::Disconnecting');
    this.client.disconnect();
  }

  /// The subscribed callback
  void onSubscribed(String topic) {
    this._topic = topic;
    print('EXAMPLE::Subscription confirmed for topic $this._topic');
    this.client.updates.listen((List<MqttReceivedMessage<MqttMessage>> c) {
      final MqttPublishMessage _recMess = c[0].payload;
      final String _message =
      MqttPublishPayload.bytesToStringAsString(_recMess.payload.message);
      messageProvider.setMessage(_message);
    });
  }

  /// The unsolicited disconnect callback
  void onDisconnected() {
    print('EXAMPLE::OnDisconnected client callback - Client disconnection');
    if (this.client.connectionStatus.returnCode == MqttConnectReturnCode.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');
  }
}

哪个人负责在ive从Mqtt经纪人处获得客户时进行通知，然后将其传递给此类的client属性。发生这种情况时，notifyListeners应该通知此提供者：

class AppWrapper extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        ChangeNotifierProvider<MQTTMessageProvider>(
          create: (_) => MQTTMessageProvider(),
        ),
        ChangeNotifierProvider<MQTTManager>(
          create: (_) => MQTTManager(),
        ),
      ],
      child: MQTTInitialize()
    );
  }
}

，然后在此类（它是MQTTInitialize（）的子类）中检索它：

 final MQTTManager managerProvider = Provider.of<MQTTManager>(context);

    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.blueGrey,
        title: Text('Subscribe'),
        actions: <Widget>[
          FlatButton.icon(
            onPressed: (){
              managerProvider.disconnect();
            },
            icon: Icon(Icons.arrow_back),
            label: Text('return'),
          ),
        ],
      ),
... (it continues)

问题在于，当我按下按钮时，它会引发以下错误：

[ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'disconnect' was called on null.
E/flutter (30793): Receiver: null
E/flutter (30793): Tried calling: disconnect()
E/flutter (30793): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)

我想是因为managerProvider中的final MQTTManager managerProvider = Provider.of<MQTTManager>(context);为空，所以我无法调用managerProvider.disconnect();

如何获取ChangeNotifierProvider以提供正确的管理器实例？

链接到项目：https://github.com/TacoMariachi/Mqtt_flutter_app.git

无法从ChangeNotifierProvider接收对象实例

0 个答案: