xamarin Mqtt

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。