.NET+Socket 开发MQTT 客户端

一、什么是MQTT？

MQTT（Message Queuing Telemetry Transport，消息队列遥测传输协议），是一种基于发布/订阅（publish/subscribe）模式的"轻量级"通讯协议，该协议构建于TCP/IP协议上，由IBM在1999年发布。

MQTT最大优点在于，可以以极少的代码和有限的带宽，为连接远程设备提供实时可靠的消息服务。作为一种低开销、低带宽占用的即时通讯协议，使其在物联网、小型设备、移动应用等方面有较广泛的应用。

MQTT是一个基于客户端-服务器的消息发布/订阅传输协议。MQTT协议是轻量、简单、开放和易于实现的，这些特点使它适用范围非常广泛。在很多情况下，包括受限的环境中，如：机器与机器（M2M）通信和物联网（IoT）。.

其在，通过卫星链路通信传感器、偶尔拨号的医疗设备、智能家居、及一些小型化设备中已广泛使用。

本文将基于.NET C# Socket WinForm MQTT 客户端开发，实现服务端连接，消息发布，消息订阅，取消订阅及接收服务端返回消息功能。

二、连接服务端

客户端连接服务端，启动异步接收服务端消息。

//连接选项
var option = new MQTT.MQTTClientOption()
{
    ClientId = this.txtClientId.Text,
    IpString = this.txtServer.Text,
    Port = Convert.ToInt32(this.txtPort.Text),
    UserName = this.txtUserName.Text,
    Password = this.txtPassword.Text
};
//实例客户端
this.mqttClient = new MQTT.MQTTClient();
this.mqttClient.ClientReceived += MqttClient_ClientReceived;
var result = await this.mqttClient.ConnectAsync(option);
if(result.State == false)
{
    MessageBox.Show(result.Fail, "提示", MessageBoxButtons.OK, MessageBoxIcon.Warning);
    return;
}
//开始接收
this.mqttClient.BeginReceive();

三、发布消息

客户端向服务端发送消息，主要包括主题，内容及消息质量。

发送消息程序

private short _PacketId = 1;
private async void BtnPublish_Click(object sender, EventArgs e)
{
    this.btnPublish.Enabled = false;
    this.Cursor = Cursors.WaitCursor;
    try
    {
        short qos = 0;
        if (rdbLevel1.Checked)
            qos = 1;
        if (rdbLevel2.Checked)
            qos = 2;

        var data = new MQTT.PublishEntity()
        {
            Topic = this.txtTopic.Text,
            Reload = System.Text.Encoding.UTF8.GetBytes(this.txtContent.Text),
            Qos = qos,
            PacketId=_PacketId
        };
        var result = await this.mqttClient.PublishAsync(data,false);
        if(result.State == false)
        {
            this.AppendText("发送失败：" + result.Fail);
            return;
        }
        this._PacketId++;
        this.AppendText("发送成功，返回结果：" + result.Result);
    }
    finally
    {
        this.btnPublish.Enabled = true;
        this.Cursor = Cursors.Default;
    }
}

四、订阅消息

客户端订阅消息，主要包括过滤主题及消息质量。

订阅消息方法：

private async void BtnSubscribe_Click(object sender, EventArgs e)
{
    this.btnSubscribe.Enabled = false;
    this.Cursor = Cursors.WaitCursor;

    try
    {
        string fail = string.Empty;

        short qos = 0;
        if (rdbQos1.Checked)
            qos = 1;
        if (rdbQos2.Checked)
            qos = 2;

        MQTT.SubscribeEntity entity = new MQTT.SubscribeEntity();
        entity.Filter = this.txtFilter.Text;
        entity.Qos = qos;

        this.AppendText("开始订阅...");
        //if (mqttClient.Subscribe(this.txtFilter.Text, qos, out result, out fail) == false)
        //{
        //    this.AppendText(fail);
        //    return;
        //}
        var result = await mqttClient.SubscribeAsync(entity.Filter,entity.Qos);
        if (result.State == false)
        {
            this.AppendText(result.Fail);
            return;
        }

        this.AppendText("订阅成功！返回结果：" + result.Result);
        MQTT.SubscribeHelper.Save(entity);

        DataRow drData = this.dtSubscribes.NewRow();
        drData["Id"] = entity.Id;
        drData["Filter"] = entity.Filter;
        drData["Qos"] = entity.Qos;
        drData["Count"] = 0;
        dtSubscribes.Rows.Add(drData);
    }
    finally
    {
        this.btnSubscribe.Enabled = true;
        this.Cursor = Cursors.Default;
    }
}

五、取消订阅

客户端取消某一订阅消息。双击订阅消息行，取消订阅消息。

取消订阅方法：

private async void DataGridView1_CellDoubleClick(object sender, DataGridViewCellEventArgs e)
{
    if (this.dataGridView1.CurrentCell == null)
        return;

    DataRowView drv = this.dataGridView1.CurrentRow.DataBoundItem as DataRowView;
    if (drv == null)
        return;

    if (MessageBox.Show("确认取消当前选择：" + drv["Filter"].ToString() + " 订阅?", "确认", MessageBoxButtons.OKCancel, MessageBoxIcon.Question) != DialogResult.OK)
        return;
    try
    {
        this.AppendText("开始取消订阅...");
        var result = await mqttClient.UnSubscribeAsync(drv["Filter"].ToString());
        if (result.State == false)
        {
            this.AppendText(result.Fail);
            return;
        }
        this.AppendText("取消订阅成功！返回结果：" + result.Result);
        MQTT.SubscribeHelper.Delete(drv["Id"].ToString());
        this.dataGridView1.Rows.RemoveAt(this.dataGridView1.CurrentCell.RowIndex);
    }
    catch (Exception ex)
    {
        this.AppendText("取消订阅异常：" + ex.ToString());
    }

}

六、接收服务端消息

客户端接收服务端转发过来的消息。

private void MqttClient_ClientReceived(MQTT.MQTTClient client, MQTT.ReceiveResult receive)
{
    if(receive.Data != null)
        this.AppendText("DataReceived state="+receive.State+" topic="+ receive.Data.Topic + " content=" + System.Text.Encoding.UTF8.GetString(receive.Data.Reload));
    else if(receive.State)
        this.AppendText("DataReceived state=" + receive.State + " result=" + receive.Result);
    else
        this.AppendText("DataReceived state=" + receive.State + " fail=" + receive.Fail);
}

至此，客户端工具主要功能简介完毕，不当之处，欢迎指正！