C# 接受MQTT服务器推送的消息

前言：

MQTT是IBM开发的一个即时通讯协议。MQTT是面向M2M和物联网的连接协议，采用轻量级发布和订阅消息传输机制。

大家可以直接上GitHub下载MQQT服务的源码，源码地址：https://github.com/mqtt/mqtt.github.io/wiki/libraries

主要内容：

官方文档翻译：

M2Mqtt库提供了一个主类MqttClient，代表连接到代理的MQTT客户端。您可以连接到提供其IP地址或主机名的代理，以及可选的与MQTT协议相关的一些参数。

连接到代理后，您可以使用Publish（）方法向主题和Subscribe（）方法发布消息以订阅主题并接收其上发布的消息。

MqttClient类是基于事件，以便您在邮件发布到您订阅的主题时收到一个事件。消息发布完成后，您可以收到事件，您已订阅或取消订阅主题。

以客户端为主题的例子：

...

// create client instance
MqttClient client = new MqttClient(IPAddress.Parse(MQTT_BROKER_ADDRESS));

// register to message received
client.MqttMsgPublishReceived += client_MqttMsgPublishReceived;

string clientId = Guid.NewGuid().ToString();
client.Connect(clientId);

// subscribe to the topic "/home/temperature" with QoS 2
client.Subscribe(new string[] { "/home/temperature" }, new byte[] { MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE });

...

static void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
{
// handle message received
}

一般C#客户端都是应用单例模式，下面的是我封装的类：

   public class MqttClientService
     {

         private static volatile MqttClientService _instance = null;

         private static readonly object LockHelper = new object();

         /// <summary>
         /// 创建单例模式
         /// </summary>
         /// <param name="ipAddress"></param>
         /// <returns></returns>
         public static MqttClientService CreateInstance(string ipAddress)
         {
             if (_instance == null)
             {
                 lock (LockHelper)
                 {
                     if (_instance == null)
                         _instance = new MqttClientService(ipAddress);
                 }
             }
             return _instance;
         }

         /// <summary>
         /// 实例化订阅客户端
         /// </summary>
         public MqttClient SubscribeClient { get; set; }

         public Action<Object, MqttMsgPublishEventArgs> ClientPublishReceivedAction { get; set; }

         public MqttClientService(string ipAddress)
         {
             // create client instance
             SubscribeClient = new MqttClient(IPAddress.Parse(ipAddress));

             // register to message received
             SubscribeClient.MqttMsgPublishReceived += client_MqttMsgPublishReceived;

             string clientId = Guid.NewGuid().ToString();

             SubscribeClient.Connect(clientId);

             // subscribe to the topic "/home/temperature" with QoS 2
             SubscribeClient.Subscribe(new string[] { "avatar/uploaded" }, new byte[] { MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE });
         }

         void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
         {
             // handle message received
             ClientPublishReceivedAction.Invoke(sender, e);
         }

         public void client_MqttMsgPublish(string publishString)
         {
             SubscribeClient.Publish("avatar/signed", Encoding.UTF8.GetBytes(publishString), MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, false);
         }
     }

用户只需要把订阅的路径写到Subscribe即可。