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：

 1   public class MqttClientService
 2     {
 3 
 4         private static volatile MqttClientService _instance = null;
 5 
 6         private static readonly object LockHelper = new object();
 7 
 8         /// 

 9         /// 创建单例模式
10         /// 
11         /// 
12         /// 
13         public static MqttClientService CreateInstance(string ipAddress)
14         {
15             if (_instance == null)
16             {
17                 lock (LockHelper)
18                 {
19                     if (_instance == null)
20                         _instance = new MqttClientService(ipAddress);
21                 }
22             }
23             return _instance;
24         }
25 
26         /// 

27         /// 实例化订阅客户端
28         /// 
29         public MqttClient SubscribeClient { get; set; }
30 
31 
32         public Action ClientPublishReceivedAction { get; set; }
33 
34         public MqttClientService(string ipAddress)
35         {
36             // create client instance 
37             SubscribeClient = new MqttClient(IPAddress.Parse(ipAddress));
38 
39             // register to message received 
40             SubscribeClient.MqttMsgPublishReceived += client_MqttMsgPublishReceived;
41 
42             string clientId = Guid.NewGuid().ToString();
43 
44             SubscribeClient.Connect(clientId);
45 
46             // subscribe to the topic "/home/temperature" with QoS 2 
47             SubscribeClient.Subscribe(new string[] { "avatar/uploaded" }, new byte[] { MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE });
48         }
49 
50         void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
51         {
52             // handle message received 
53             ClientPublishReceivedAction.Invoke(sender, e);
54         }
55 
56         public void client_MqttMsgPublish(string publishString)
57         {
58             SubscribeClient.Publish("avatar/signed", Encoding.UTF8.GetBytes(publishString), MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, false);
59         }
60     }

用户只需要把订阅的路径写到Subscribe即可。