MQTT 客戶端應用及常見問題（C#）

最近因為工作需要，需要使用C# 語言編寫一個通過MQTT協議 ，上傳資料到雲端的工具。因為之前沒有用過MQTT，所以 使用的時候遇到很多問題.下面將會把我遇到的問題一一解釋。

1.引用原始碼庫地址
https://github.com/eclipse/paho.mqtt.m2mqtt
2.說明
https://m2mqtt.wordpress.com/m2mqtt_doc/
3.使用後遇到的問題
當網路中斷後,MQTT 程式有時候不會自動重連。
解決方案 新增監控MQTT連線狀態

1.新增全域性靜態變數 uPLibrary.Networking.M2Mqtt.MQTTConfig.IsSocketRun;

class MQTTConfig{
 public static bool IsSocketRun = false;
}

2.修改MqttClient 類 的Connect 方法，在連線成功後把IsSocketRun = true.
MQTTConfig.IsSocketRun = true;

  /// <summary>
        /// Connect to broker
        /// </summary>
        /// <param name="clientId">Client identifier</param>
        /// <param name="username">Username</param>
        /// <param name="password">Password</param>
        /// <param name="willRetain">Will retain flag</param>
        /// <param name="willQosLevel">Will QOS level</param>
        /// <param name="willFlag">Will flag</param>
        /// <param name="willTopic">Will topic</param>
        /// <param name="willMessage">Will message</param>
        /// <param name="cleanSession">Clean sessione flag</param>
        /// <param name="keepAlivePeriod">Keep alive period</param>
        /// <returns>Return code of CONNACK message from broker</returns>
        public byte Connect(string clientId,
            string username,
            string password,
            bool willRetain,
            byte willQosLevel,
            bool willFlag,
            string willTopic,
            string willMessage,
            bool cleanSession,
            ushort keepAlivePeriod)
        {
            // create CONNECT message
            MqttMsgConnect connect = new MqttMsgConnect(clientId,
                username,
                password,
                willRetain,
                willQosLevel,
                willFlag,
                willTopic,
                willMessage,
                cleanSession,
                keepAlivePeriod,
                (byte)this.ProtocolVersion);

            try
            {
                // connect to the broker
                this.channel.Connect();
            }
            catch (Exception ex)
            {
                throw new MqttConnectionException("Exception connecting to the broker", ex);
            }

            this.lastCommTime = 0;
            this.isRunning = true;
            MQTTConfig.IsSocketRun = true;
            this.isConnectionClosing = false;
            // start thread for receiving messages from broker
            Fx.StartThread(this.ReceiveThread);

....

3.繼續修改 MqttClient .cs類中的Ping() 方法

/// <summary>
/// Execute ping to broker for keep alive
/// </summary>
/// <returns>PINGRESP message from broker</returns>
private MqttMsgPingResp Ping()
{
    MqttMsgPingReq pingreq = new MqttMsgPingReq();
    try
    {
        // broker must send PINGRESP within timeout equal to keep alive period
        return (MqttMsgPingResp)this.SendReceive(pingreq, this.keepAlivePeriod);
    }
    catch (Exception e)
    {
#if TRACE
        MqttUtility.Trace.WriteLine(TraceLevel.Error, "Exception occurred: {0}", e.ToString());
#endif
        MQTTConfig.IsSocketRun = false;
        // client must close connection
        this.OnConnectionClosing();
        return null;
    }
}

4.最後在我們程式集入口初始化程式的時候 新增執行緒呼叫 。當MQTT中斷後就會自動重連 ，另外提醒方法異常時一定要異常處理哦。

while (true)            
{                
	LogWriter.DebugLog(string.Format("執行次數{0}  IsSocketRun {1}", i, uPLibrary.Networking.M2Mqtt.MQTTConfig.IsSocketRun));                
	if (!uPLibrary.Networking.M2Mqtt.MQTTConfig.IsSocketRun)               
	{
		程式執行到嗎。。。
	}
		System.Threading.Thread.Sleep(10000);
}

MQTT 訂閱

// 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 
} 

MQTT 釋出

…

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

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

string strValue = Convert.ToString(value); 

// publish a message on "/home/temperature" topic with QoS 2 
client.Publish("/home/temperature", Encoding.UTF8.GetBytes(strValue), MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, false); 

...