C# 接受MQTT服務器推送的消息

前言：

 MQTT是IBM開發的一個即時通信協議。MQTT是面向M2M和物聯網的鏈接協議，採用輕量級發佈和訂閱消息傳輸機制。

 你們能夠直接上GitHub下載MQQT服務的源碼，源碼地址：https://github.com/mqtt/mqtt.github.io/wiki/libraries

主要內容：

官方文檔翻譯：

M2Mqtt庫提供了一個主類MqttClient，表明鏈接到代理的MQTT客戶端。您能夠鏈接到提供其IP地址或主機名的代理，以及可選的與MQTT協議相關的一些參數。

鏈接到代理後，您能夠使用Publish（）方法向主題和Subscribe（）方法發佈消息以訂閱主題並接收其上發佈的消息。

MqttClient類是基於事件，以便您在郵件發佈到您訂閱的主題時收到一個事件。消息發佈完成後，您能夠收到事件，您已訂閱或取消訂閱主題。

以客戶端爲主題的例子：

...

// 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
}

通常C#客戶端都是應用單例模式，下面的是我封裝的類：

 1   public class MqttClientService
 2     {
 3 
 4         private static volatile MqttClientService _instance = null;
 5 
 6         private static readonly object LockHelper = new object();
 7 
 8         /// <summary>
 9         /// 建立單例模式
10         /// </summary>
11         /// <param name="ipAddress"></param>
12         /// <returns></returns>
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         /// <summary>
27         /// 實例化訂閱客戶端
28         /// </summary>
29         public MqttClient SubscribeClient { get; set; }
30 
31 
32         public Action<Object, MqttMsgPublishEventArgs> 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     }

用戶只須要把訂閱的路徑寫到Subscribe便可。