1、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。
（具体描述自行百度）
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。
2、连接服务端
客户端连接服务端，启动异步接收服务端消息。
1 //连接选项
2 var option = new MQTT.MQTTClientOption()
4 ClientId = this.txtClientId.Text,
5 IpString = this.txtServer.Text,
6 Port = Convert.ToInt32(this.txtPort.Text),
7 UserName = this.txtUserName.Text,
8 Password = this.txtPassword.Text
9 };
10 //实例客户端
11 this.mqttClient = new MQTT.MQTTClient();
12 this.mqttClient.ClientReceived += MqttClient_ClientReceived;
13 var result = await this.mqttClient.ConnectAsync(option);
14 if(result.State == false)
15 {
16 MessageBox.Show(result.Fail, "提示", MessageBoxButtons.OK, MessageBoxIcon.Warning);
17 return;
18 }
19 //开始接收
20 this.mqttClient.BeginReceive();
3、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
1 private short _PacketId = 1;
2 private async void BtnPublish_Click(object sender, EventArgs e)
4 this.btnPublish.Enabled = false;
5 this.Cursor = Cursors.WaitCursor;
6 try
8 short qos = 0;
9 if (rdbLevel1.Checked)
10 qos = 1;
11 if (rdbLevel2.Checked)
12 qos = 2;
14 var data = new MQTT.PublishEntity()
15 {
16 Topic = this.txtTopic.Text,
17 Reload = System.Text.Encoding.UTF8.GetBytes(this.txtContent.Text),
18 Qos = qos,
19 PacketId=_PacketId
20 };
21 var result = await this.mqttClient.PublishAsync(data,false);
22 if(result.State == false)
23 {
24 this.AppendText("发送失败：" + result.Fail);
25 return;
26 }
27 this._PacketId++;
28 this.AppendText("发送成功，返回结果：" + result.Result);
29 }
30 finally
31 {
32 this.btnPublish.Enabled = true;
33 this.Cursor = Cursors.Default;
34 }
4、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：
1 private async void BtnSubscribe_Click(object sender, EventArgs e)
3 this.btnSubscribe.Enabled = false;
4 this.Cursor = Cursors.WaitCursor;
6 try
8 string fail = string.Empty;
10 short qos = 0;
11 if (rdbQos1.Checked)
12 qos = 1;
13 if (rdbQos2.Checked)
14 qos = 2;
16 MQTT.SubscribeEntity entity = new MQTT.SubscribeEntity();
17 entity.Filter = this.txtFilter.Text;
18 entity.Qos = qos;
20 this.AppendText("开始订阅...");
21 //if (mqttClient.Subscribe(this.txtFilter.Text, qos, out result, out fail) == false)
22 //{
23 // this.AppendText(fail);
24 // return;
25 //}
26 var result = await mqttClient.SubscribeAsync(entity.Filter,entity.Qos);
27 if (result.State == false)
28 {
29 this.AppendText(result.Fail);
30 return;
31 }
33 this.AppendText("订阅成功！返回结果：" + result.Result);
34 MQTT.SubscribeHelper.Save(entity);
36 DataRow drData = this.dtSubscribes.NewRow();
37 drData["Id"] = entity.Id;
38 drData["Filter"] = entity.Filter;
39 drData["Qos"] = entity.Qos;
40 drData["Count"] = 0;
41 dtSubscribes.Rows.Add(drData);
42 }
43 finally
44 {
45 this.btnSubscribe.Enabled = true;
46 this.Cursor = Cursors.Default;
47 }
5、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：
1 private async void DataGridView1_CellDoubleClick(object sender, DataGridViewCellEventArgs e)
3 if (this.dataGridView1.CurrentCell == null)
4 return;
6 DataRowView drv = this.dataGridView1.CurrentRow.DataBoundItem as DataRowView;
7 if (drv == null)
8 return;
10 if (MessageBox.Show("确认取消当前选择：" + drv["Filter"].ToString() + " 订阅?", "确认", MessageBoxButtons.OKCancel, MessageBoxIcon.Question) != DialogResult.OK)
11 return;
12 try
13 {
14 this.AppendText("开始取消订阅...");
15 var result = await mqttClient.UnSubscribeAsync(drv["Filter"].ToString());
16 if (result.State == false)
17 {
18 this.AppendText(result.Fail);
19 return;
20 }
21 this.AppendText("取消订阅成功！返回结果：" + result.Result);
22 MQTT.SubscribeHelper.Delete(drv["Id"].ToString());
23 this.dataGridView1.Rows.RemoveAt(this.dataGridView1.CurrentCell.RowIndex);
24 }
25 catch (Exception ex)
26 {
27 this.AppendText("取消订阅异常：" + ex.ToString());
28 }
6、接收服务端消息
客户端接收服务端转发过来的消息。
1 private void MqttClient_ClientReceived(MQTT.MQTTClient client, MQTT.ReceiveResult receive)
3 if(receive.Data != null)
4 this.AppendText("DataReceived state="+receive.State+" topic="+ receive.Data.Topic + " content=" + System.Text.Encoding.UTF8.GetString(receive.Data.Reload));
5 else if(receive.State)
6 this.AppendText("DataReceived state=" + receive.State + " result=" + receive.Result);
7 else
8 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！
客户端下载地址（码云服务器）：https://gitee.com/ShiQuan25/full-product-management/raw/master/WinMQTTClient/WinMQTTClient.zip
作者：HengXiao
本文版权归作者和博客园共有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接。如有问题，可以邮件：[email protected] QQ：896374871 联系我，非常感谢。
