1 什么是MQTT?
mqtt （Message Queuing Telemetry Transport，消息队列遥测传输）是 IBM 开发的一个即时通讯协议，有可能成为物联网的重要组成部分。MQTT 是基于二进制消息的发布/订阅编程模式的消息协议，如今已经成为 OASIS 规范，由于规范很简单，非常适合需要低功耗和网络带宽有限的 IoT 场景。
2 MQTTnet
MQTTnet 是一个基于MQTT协议高度专业的.net库，它同时提供MQTT client和MQTT server（broke），支持v3.1.0,v3.1.1和v5.0.0的标准MQTT协议.
3 MQTTnet支持范围
.Net Standard 1.3+
.Net Core 1.1+
.Net Core App 1.1+
.Net Framework 4.5.2+(x86,x64,AnyCPU)
Mono 5.2+
Universal Windows Platform(UWP) 10.0.1024+(x86,x64,ARM,AnyCPU,Windwos 10 IoT Core)
Xamarin.Android 7.5+
Xamarin.iOS 10.14+
4 创建服务器
MQTT服务器以称为"消息代理"（Broker），可以是一个应用程序或一台设备。它是位于消息发布者和订阅者之间，它可以：
（1）接受来自客户的网络连接；
（2）接受客户发布的应用信息；
（3）处理来自客户端的订阅和退订请求；
（4）向订阅的客户转发应用程序消息。
服务器创建一个控制台应用，可选>>控制台应用(.NET Core)创建新项目MqttNetServer，代码如下：
sing MQTTnet;
using MQTTnet.Protocol;
using MQTTnet.Server;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.IO;
using System.Reflection;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Threading.Tasks; namespace MqttServerTest
{
class Program
{
public static IMqttServer mqttServer;
static void Main(string[] args)
{
mqttServer = new MQTTnet.MqttFactory().CreateMqttServer();
mqttServer.UseClientConnectedHandler(e =>
{
Console.WriteLine("***new connect:" + e.ClientId); });
mqttServer.UseClientDisconnectedHandler(e =>
{
Console.WriteLine("*** disconnect:" + e.ClientId);
}); //var options = new MqttServerOptions();
//await mqttServer.StartAsync(options); //var currentPath = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
//var certificate = new X509Certificate2(Path.Combine(currentPath, "certificate.pfx"), "yourPassword", X509KeyStorageFlags.Exportable); var optionsBuilder = new MqttServerOptionsBuilder()
.WithConnectionBacklog()
.WithDefaultEndpointPort()
.WithConnectionValidator(c=> {
//c.SessionItems.
//if (c.ClientId.Length < 10)
//{
// c.ReturnCode = MqttConnectReturnCode.ConnectionRefusedIdentifierRejected;
// //c.ReasonCode = MqttConnectReasonCode.ClientIdentifierNotValid;
// return;
//}
//if (c.Username != "mySecretUser")
//{
// c.ReturnCode = MqttConnectReturnCode.ConnectionRefusedBadUsernameOrPassword;
// return;
//} //if (c.Password != "mySecretPassword")
//{
// c.ReturnCode = MqttConnectReturnCode.ConnectionRefusedBadUsernameOrPassword;
// return;
//} c.ReturnCode = MqttConnectReturnCode.ConnectionAccepted;
Console.WriteLine("***connect validator:"+c.ClientId);
})
//.WithEncryptionCertificate(certificate.Export(X509ContentType.Pfx))
//.WithEncryptionSslProtocol(SslProtocols.Tls12)
.WithApplicationMessageInterceptor(context=> {
//if (context.ApplicationMessage.Topic == "my/custom/topic")
//{
// context.ApplicationMessage.Payload = Encoding.UTF8.GetBytes("The server injected payload.");
//}
//// It is possible to disallow the sending of messages for a certain client id like this:
//if (context.ClientId != "Someone")
//{
// context.AcceptPublish = false;
// return;
//}
// It is also possible to read the payload and extend it. For example by adding a timestamp in a JSON document.
// This is useful when the IoT device has no own clock and the creation time of the message might be important. context.AcceptPublish = true;
Console.WriteLine("***Message:" + context.ApplicationMessage.Payload);
})
.WithSubscriptionInterceptor(context=>
{
//if (context.TopicFilter.Topic.StartsWith("admin/foo/bar") && context.ClientId != "theAdmin")
//{
// context.AcceptSubscription = false;
//} //if (context.TopicFilter.Topic.StartsWith("the/secret/stuff") && context.ClientId != "Imperator")
//{
// context.AcceptSubscription = false;
// context.CloseConnection = true;
//} context.AcceptSubscription = true;
Console.WriteLine("***Subscript:" + context.TopicFilter);
})
//.WithStorage(new RetainedMessageHandler())
;
var options = optionsBuilder.Build(); //// Setting the options
//options.Storage=new RetainedMessageHandler(); StartServer(options); Console.WriteLine("Press any key to exit.");
Console.ReadLine(); //await mqttServer.StopAsync();
} public static async void StartServer(IMqttServerOptions options)
{
await mqttServer.StartAsync(options);
} } // The implementation of the storage:
// This code uses the JSON library "Newtonsoft.Json".
public class RetainedMessageHandler : IMqttServerStorage
{
private const string Filename = "C:\\MQTT\\RetainedMessages.json"; public Task SaveRetainedMessagesAsync(IList<MqttApplicationMessage> messages)
{
File.WriteAllText(Filename, JsonConvert.SerializeObject(messages));
return Task.FromResult();
} public Task<IList<MqttApplicationMessage>> LoadRetainedMessagesAsync()
{
IList<MqttApplicationMessage> retainedMessages;
if (File.Exists(Filename))
{
var json = File.ReadAllText(Filename);
retainedMessages = JsonConvert.DeserializeObject<List<MqttApplicationMessage>>(json);
}
else
{
retainedMessages = new List<MqttApplicationMessage>();
} return Task.FromResult(retainedMessages);
}
} }
代码直接运行起来，就是一个简单的Mqtt server。
5 创建xamarin APP
一个使用MQTT协议的应用程序或者设备，它总是建立到服务器的网络连接。客户端可以：
（1）发布其他客户端可能会订阅的信息；
（2）订阅其它客户端发布的消息；
（3）退订或删除应用程序的消息；
（4）断开与服务器连接。
在VS中新建一个xamarin.Forms的移动应用，创建好后在Nuget上搜索mqttnet，添加对MQTTnet包的引用。更改代码如下：
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:CatShell"
x:Class="CatShell.MainPage"> <StackLayout>
<!-- Place new controls here -->
<Label Text="SubscribeTopic"/>
<Entry x:Name="txtSubTopic" Placeholder="Subscribe Topic" />
<Button Text="BtnSubscribe" Clicked="SubButton_Clicked"/>
<Entry x:Name="txtReceiveMessage"/>
<Label Text="PublishTopic"/>
<Entry x:Name="txtPubTopic"/>
<Entry x:Name="txtSendMessage" />
<Button Text="Publish" Clicked="PubButton_Clicked"/>
<Editor> </Editor> </StackLayout> </ContentPage>
using MQTTnet;
using MQTTnet.Client;
using MQTTnet.Client.Options;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Xamarin.Forms; namespace CatShell
{
public partial class MainPage : ContentPage
{
public IMqttClient mqttClient;
public IMqttClientOptions options;
public MainPage()
{
InitializeComponent();
InitMqttClient();
ConnectMqttServer();
} public void InitMqttClient()
{
// Create a new MQTT client.
var factory = new MqttFactory();
mqttClient = factory.CreateMqttClient(); mqttClient.UseConnectedHandler(e => { Device.BeginInvokeOnMainThread(() =>
{
txtReceiveMessage.Text = txtReceiveMessage.Text + $">> connect success." + Environment.NewLine;
});
});
mqttClient.UseDisconnectedHandler(e =>
{
Device.BeginInvokeOnMainThread(() =>
{
txtReceiveMessage.Text = txtReceiveMessage.Text + $">> Disconnect." + Environment.NewLine;
});
});
mqttClient.UseApplicationMessageReceivedHandler(e =>
{
Device.BeginInvokeOnMainThread(() =>
{
txtReceiveMessage.Text = $">> {Encoding.UTF8.GetString(e.ApplicationMessage.Payload)}" + Environment.NewLine;
});
}); // Create TCP based options using the builder.
options = new MqttClientOptionsBuilder()
.WithClientId("Client4")
.WithTcpServer("10.100.1.247", ) // Use TCP connection, Port is opptinal
//.WithWebSocketServer("broker.hivemq.com:8000/mqtt") // Use WebSocket connection.
//.WithCredentials("bud", "%spencer%")
//.WithTls()
//.WithTls(new MqttClientOptionsBuilderTlsParameters
//{
// UseTls = true,
// CertificateValidationCallback = (X509Certificate x, X509Chain y, SslPolicyErrors z, IMqttClientOptions o) =>
// {
// // TODO: Check conditions of certificate by using above parameters.
// return true;
// }
//})
.WithCleanSession()
.Build(); } public async void ConnectMqttServer()
{
await mqttClient.ConnectAsync(options, CancellationToken.None); // Since 3.0.5 with CancellationToken } private async void SubButton_Clicked(object sender, EventArgs e)
{ string topic = txtSubTopic.Text.Trim(); if (string.IsNullOrEmpty(topic))
{
//MessageBox.Show("订阅主题不能为空！");
return;
} if (!mqttClient.IsConnected)
{
//MessageBox.Show("MQTT客户端尚未连接！");
return;
} // Subscribe to a topic
await mqttClient.SubscribeAsync(new TopicFilterBuilder().WithTopic(topic).Build()); txtReceiveMessage.Text = txtReceiveMessage.Text + $"已订阅[{topic}]主题" + Environment.NewLine;
txtSubTopic.IsReadOnly = false;
//BtnSubscribe.Enabled = false;
} private async void PubButton_Clicked(object sender, EventArgs e)
{
string topic = txtPubTopic.Text.Trim(); if (string.IsNullOrEmpty(topic))
{
//MessageBox.Show("发布主题不能为空！");
return;
} string inputString = txtSendMessage.Text.Trim(); PublishMessages(topic, inputString);
} public async void PublishMessages(string topicMsg, string payloadMsg)
{
var message = new MqttApplicationMessageBuilder()
.WithTopic(topicMsg)
.WithPayload(payloadMsg)
.WithExactlyOnceQoS()
.WithRetainFlag()
.Build(); await mqttClient.PublishAsync(message);
} }
}
代码运行起来，在APP上可以直接发信息。
6 创建winForm client（可选）
可以创建一个winForm来相互互动，在VS上新建一个windows窗体应用(.NET Framework)，界面设计如下
后台代码如下：
using MQTTnet;
using MQTTnet.Client.Options;
using MQTTnet.Client;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Net.Security;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms; namespace MqttClientWin
{
public partial class Form1 : Form
{
public IMqttClient mqttClient;
public IMqttClientOptions options;
public Form1()
{
InitializeComponent();
InitMqttClient();
ConnectMqttServer();
} public void InitMqttClient()
{
// Create a new MQTT client.
var factory = new MqttFactory();
mqttClient = factory.CreateMqttClient(); mqttClient.UseApplicationMessageReceivedHandler(e =>
{
Console.WriteLine("### RECEIVED APPLICATION MESSAGE ###");
Console.WriteLine($"+ Topic = {e.ApplicationMessage.Topic}");
Console.WriteLine($"+ Payload = {Encoding.UTF8.GetString(e.ApplicationMessage.Payload)}");
Console.WriteLine($"+ QoS = {e.ApplicationMessage.QualityOfServiceLevel}");
Console.WriteLine($"+ Retain = {e.ApplicationMessage.Retain}");
Console.WriteLine(); this.Invoke(new Action(() =>
{
txtReceiveMessage.AppendText($">> {Encoding.UTF8.GetString(e.ApplicationMessage.Payload)}{Environment.NewLine}");
})); //Task.Run(() => mqttClient.PublishAsync("hello/world"));
}); mqttClient.UseConnectedHandler(async e =>
{
Console.WriteLine("### CONNECTED WITH SERVER ###"); //// Subscribe to a topic
//await mqttClient.SubscribeAsync(new TopicFilterBuilder().WithTopic("my/topic").Build()); //Console.WriteLine("### SUBSCRIBED ###"); this.Invoke(new Action(() =>
{
txtReceiveMessage.AppendText($">> connect success.{Environment.NewLine}");
}));
}); mqttClient.UseDisconnectedHandler(e =>
{
this.Invoke(new Action(() =>
{
txtReceiveMessage.AppendText($">> Disconnect .{Environment.NewLine}");
}));
}); // Create TCP based options using the builder.
options = new MqttClientOptionsBuilder()
.WithClientId("Client5")
.WithTcpServer("10.100.1.247", ) // Use TCP connection, Port is opptinal
//.WithWebSocketServer("broker.hivemq.com:8000/mqtt") // Use WebSocket connection.
//.WithCredentials("bud", "%spencer%")
//.WithTls()
//.WithTls(new MqttClientOptionsBuilderTlsParameters
//{
// UseTls = true,
// CertificateValidationCallback = (X509Certificate x, X509Chain y, SslPolicyErrors z, IMqttClientOptions o) =>
// {
// // TODO: Check conditions of certificate by using above parameters.
// return true;
// }
//})
.WithCleanSession()
.Build(); } public async void ConnectMqttServer()
{
await mqttClient.ConnectAsync(options, CancellationToken.None); // Since 3.0.5 with CancellationToken } public void ReconnectMqttServer()
{
mqttClient.UseDisconnectedHandler(async e =>
{
Console.WriteLine("### DISCONNECTED FROM SERVER ###");
await Task.Delay(TimeSpan.FromSeconds()); try
{
await mqttClient.ConnectAsync(options, CancellationToken.None); // Since 3.0.5 with CancellationToken
}
catch
{
Console.WriteLine("### RECONNECTING FAILED ###");
}
});
} public async void PublishMessages(string topicMsg,string payloadMsg)
{
var message = new MqttApplicationMessageBuilder()
.WithTopic(topicMsg)
.WithPayload(payloadMsg)
.WithExactlyOnceQoS()
.WithRetainFlag()
.Build(); await mqttClient.PublishAsync(message);
} private async void BtnSubscribe_Click(object sender, EventArgs e)
{
string topic = txtSubTopic.Text.Trim(); if (string.IsNullOrEmpty(topic))
{
MessageBox.Show("订阅主题不能为空！");
return;
} if (!mqttClient.IsConnected)
{
MessageBox.Show("MQTT客户端尚未连接！");
return;
} // Subscribe to a topic
await mqttClient.SubscribeAsync(new TopicFilterBuilder().WithTopic(topic).Build()); txtReceiveMessage.AppendText($"已订阅[{topic}]主题" + Environment.NewLine);
txtSubTopic.Enabled = false;
BtnSubscribe.Enabled = false;
} private void BtnPublish_Click(object sender, EventArgs e)
{
string topic = txtPubTopic.Text.Trim(); if (string.IsNullOrEmpty(topic))
{
MessageBox.Show("发布主题不能为空！");
return;
} string inputString = txtSendMessage.Text.Trim(); PublishMessages(topic, inputString); }
}
}
7 MQTT协议中的订阅、主题、会话
一、订阅（Subscription）
订阅包含主题筛选器（Topic Filter）和最大服务质量（QoS）。订阅会与一个会话（Session）关联。一个会话可以包含多个订阅。每一个会话中的每个订阅都有一个不同的主题筛选器。
二、会话（Session）
每个客户端与服务器建立连接后就是一个会话，客户端和服务器之间有状态交互。会话存在于一个网络之间，也可能在客户端和服务器之间跨越多个连续的网络连接。
三、主题名（Topic Name）
连接到一个应用程序消息的标签，该标签与服务器的订阅相匹配。服务器会将消息发送给订阅所匹配标签的每个客户端。
四、主题筛选器（Topic Filter）
一个对主题名通配符筛选器，在订阅表达式中使用，表示订阅所匹配到的多个主题。
五、负载（Payload）
消息订阅者所具体接收的内容。
8 MQTT协议中的方法
MQTT协议中定义了一些方法（也被称为动作），来于表示对确定资源所进行操作。这个资源可以代表预先存在的数据或动态生成数据，这取决于服务器的实现。通常来说，资源指服务器上的文件或输出。主要方法有：
（1）Connect。等待与服务器建立连接。
（2）Disconnect。等待MQTT客户端完成所做的工作，并与服务器断开TCP/IP会话。
（3）Subscribe。等待完成订阅。
（4）UnSubscribe。等待服务器取消客户端的一个或多个topics订阅。
（5）Publish。MQTT客户端发送消息请求，发送完成后返回应用程序线程。