C# 接受MQTT服務器推送的消息

前言：

 MQTT是IBM開發的一個即時通訊協議。MQTT是面向M2M和物聯網的連接協議，采用輕量級發布和訂閱消息傳輸機制。

 大家可以直接上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，代表連接到代理的MQTT客戶端。您可以連接到提供其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。

連接到代理后，您可以使用Publish（）方法向主題和Subscribe（）方法發布消息以訂閱主題並接收其上發布的消息。

MqttClient類是基於事件，以便您在郵件發布到您訂閱的主題時收到一個事件。消息發布完成后，您可以收到事件，您已訂閱或取消訂閱主題。

以客戶端為主題的例子：

...

// 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
}

一般C#客戶端都是應用單例模式，下面的是我封裝的類：

  public class MqttClientService
    {

        private static volatile MqttClientService _instance = null;

        private static readonly object LockHelper = new object();

        /// <summary>
        /// 創建單例模式
        /// </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>
        /// 實例化訂閱客戶端
        /// </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);
        }
    }

用戶只需要把訂閱的路徑寫到Subscribe即可。