我们在上面对ASP.NET Core默认提供的具有跨平台能力的KestrelServer进行了详细介绍（《》），为了让读者朋友们对管道中的Server具有更加深刻的认识，接下来我们采用实例演示的形式创建一个自定义的Server。这个自定义的Server直接利用HttpListener来完成针对请求的监听、接收和响应，我们将其命名为HttpListenerServer。在正式介绍HttpListenerServer的设计和实现之前，我们先来显示一下如何将它应用到 一个具体的Web应用中。

一、HttpListenerServer的使用

我们依然采用最简单的Hello World应用来演示针对HttpListenerServer的应用，所以我们在Startup类的Configure方法中编写如下的程序直接响应一个“Hello World”字符串。

1: public class Startup

2: {

3:     public void Configure(IApplicationBuilder app)

4:     {

5:         app.Run(context => context.Response.WriteAsync("Hello World!"));

6:     }

7: }

在作为程序入口的Main方法中，我们直接创建一个WebHostBuilder对象并调用扩展方法UseHttpListener完成针对自定义HttpListenerServer的注册。我们接下来调用UseStartup方法注册上面定义的这个启动类型，然后调用Build方法创建一个WebHost对象，最后调用Run方法运行这个作为宿主的WebHost。

1: public class Program

2: {

3:     public static void Main()

4:     {

5:         new WebHostBuilder()

6:

7:             .UseStartup
      
       ()
      

8:             .Build()

9:             .Run();

10:     }

11: }

12: 

13: public static class WebHostBuilderExtensions

14: {

15:     public static IWebHostBuilder UseHttpListener(this IWebHostBuilder builder)

16:     {

17:         builder.ConfigureServices(services => services.AddSingleton
      
       ());
      

18:         return builder;

19:     }

20: }

我们自定义的扩展方法UseHttpListener的逻辑很简单，它只是调用WebHostBuilder的ConfigureServices方法将我们自定义的HttpListenerServer类型以单例模式注册到指定的ServiceCollection上而已。我们直接运行这个程序并利用浏览器访问默认的监听地址（http://localhost:5000），服务端响应的“Hello World”字符串会按照如下图所示的形式显示在浏览器上。

 

二、总体设计

接下来我们来介绍一下HttpListenerServer的大体涉及。除了HttpListenerServer这个实现了IServer的自定义Server类型之外，我们只定义了一个名为HttpListenerServerFeature的特性类型，下图所示的UML基本上体现了HttpListenerServer的总体设计。

 

三、HttpListenerServerFeature

如果我们利用HttpListener来监听请求，它会为接收到的每次请求创建一个属于自己的上下文，具体来说这是一个类型为HttpListenerContext对象。我们可以利用这个HttpListenerContext对象获取所有与请求相关的信息，针对请求的任何响应也都是利用它完成的。上面这个HttpListenerServerFeature实际上就是对这个作为原始上下文的HttpListenerContext对象的封装，或者说它是管道使用的DefaultHttpContext与这个原始上下文之间沟通的中介。

如下所示的代码片段展示了HttpListenerServerFeature类型的完整定义。简单起见，我们并没有实现上面提到过的所有特性接口，而只是选择性地实现了IHttpRequestFeature和IHttpResponseFeature这两个最为核心的特性接口。它的构造函数除了具有一个类型为HttpListenerContext的参数之外，还具有一个字符串的参数pathBase用来指定请求URL的基地址（对应IHttpRequestFeature的PathBase属性），我们利用它来计算请求URL的相对地址（对应IHttpRequestFeature的Path属性）。IHttpRequestFeature和IHttpResponseFeature中定义的属性都可以直接利用HttpListenerContext对应的成员来实现，这方面并没有什么特别之处。

1: public class HttpListenerServerFeature : IHttpRequestFeature, IHttpResponseFeature

2: {

3:     private readonly HttpListenerContext     httpListenerContext;

4:     private string            queryString;

5:     private IHeaderDictionary         requestHeaders;

6:     private IHeaderDictionary         responseHeaders;

7:     private string            protocol;

8:     private readonly string       pathBase;

9: 

10:     public HttpListenerServerFeature(HttpListenerContext httpListenerContext, string pathBase)

11:     {

12:         this.httpListenerContext     = httpListenerContext;

13:         this.pathBase         = pathBase;

14:     }

15: 

16:     #region IHttpRequestFeature

17: 

18:     Stream IHttpRequestFeature.Body

19:     {

20:         get { return httpListenerContext.Request.InputStream; }

21:         set { throw new NotImplementedException(); }

22:     }

23: 

24:     IHeaderDictionary IHttpRequestFeature.Headers

25:     {

26:         get { return requestHeaders

27:          ?? (requestHeaders = GetHttpHeaders(httpListenerContext.Request.Headers)); }

28:         set { throw new NotImplementedException(); }

29:     }

30: 

31:     string IHttpRequestFeature.Method

32:     {

33:         get { return httpListenerContext.Request.HttpMethod; }

34:         set { throw new NotImplementedException(); }

35:     }

36: 

37:     string IHttpRequestFeature.Path

38:     {

39:         get { return httpListenerContext.Request.RawUrl.Substring(pathBase.Length);}

40:         set { throw new NotImplementedException(); }

41:     }

42: 

43:     string IHttpRequestFeature.PathBase

44:     {

45:         get { return pathBase; }

46:         set { throw new NotImplementedException(); }

47:     }

48: 

49:     string IHttpRequestFeature.Protocol

50:     {

51:         get{ return protocol ?? (protocol = this.GetProtocol());}

52:         set { throw new NotImplementedException(); }

53:     }

54: 

55:     string IHttpRequestFeature.QueryString

56:     {

57:         Get { return queryString ?? (queryString = this.ResolveQueryString());}

58:         set { throw new NotImplementedException(); }

59:     }

60: 

61:     string IHttpRequestFeature.Scheme

62:     {

63:         get { return httpListenerContext.Request.IsWebSocketRequest ? "https" : "http"; }

64:         set { throw new NotImplementedException(); }

65:     }

66:     #endregion

67: 

68:     #region IHttpResponseFeature

69:     Stream IHttpResponseFeature.Body

70:     {

71:         get { return httpListenerContext.Response.OutputStream; }

72:         set { throw new NotImplementedException(); }

73:     }

74: 

75:     string IHttpResponseFeature.ReasonPhrase

76:     {

77:         get { return httpListenerContext.Response.StatusDescription; }

78:         set { httpListenerContext.Response.StatusDescription = value; }

79:     }

80: 

81:     bool IHttpResponseFeature.HasStarted

82:     {

83:         get { return httpListenerContext.Response.SendChunked; }

84:     }

85: 

86:     IHeaderDictionary IHttpResponseFeature.Headers

87:     {

88:         get { return responseHeaders

89:             ?? (responseHeaders = GetHttpHeaders(httpListenerContext.Response.Headers)); }

90:         set { throw new NotImplementedException(); }

91:     }

92:     int IHttpResponseFeature.StatusCode

93:     {

94:         get { return httpListenerContext.Response.StatusCode; }

95:         set { httpListenerContext.Response.StatusCode = value; }

96:     }

97: 

98:     void IHttpResponseFeature.OnCompleted(Func
      
        callback, object state)
      

99:     {

100:         throw new NotImplementedException();

101:     }

102: 

103:     void IHttpResponseFeature.OnStarting(Func
      
        callback, object state)
      

104:     {

105:         throw new NotImplementedException();

106:     }

107:     #endregion

108: 

109:     private string ResolveQueryString()

110:     {

111:         string queryString = "";

112:         var collection = httpListenerContext.Request.QueryString;

113:         for (int i = 0; i < collection.Count; i++)

114:         {

115:             queryString += $"{collection.GetKey(i)}={collection.Get(i)}&";

116:         }

117:         return queryString.TrimEnd('&');

118:     }

119: 

120:     private IHeaderDictionary GetHttpHeaders(NameValueCollection headers)

121:     {

122:         HeaderDictionary dictionary = new HeaderDictionary();

123:         foreach (string name in headers.Keys)

124:         {

125:             dictionary[name] = new StringValues(headers.GetValues(name));

126:         }

127:         return dictionary;

128:     }

129: 

130:     private string GetProtocol()

131:     {

132:         HttpListenerRequest request = httpListenerContext.Request;

133:         Version version = request.ProtocolVersion;

134:         return string.Format("{0}/{1}.{2}", request.IsWebSocketRequest ? "HTTPS" : "HTTP", version.Major, version.Minor);

135:     }

136: }

四、HttpListenerServer

接下来我们来看看HttpListenerServer的定义。如下面的代码片段所示，用来监听请求的HttpListener在构造函数中被创建，与此同时，我们会创建一个用于获取监听地址的ServerAddressesFeature对象并将其添加到属于自己的特性列表中。当HttpListenerServer随着Start方法的调用而被启动后，它将这个ServerAddressesFeature对象提取出来，然后利用它得到所有的地址并添加到HttpListener的Prefixes属性表示的监听地址列表中。接下来，HttpListener的Start方法被调用，并在一个无限循环中开启请求的监听与接收。

1: public class HttpListenerServer : IServer

2: {

3:     private readonly HttpListener listener;

4: 

5:     public IFeatureCollection Features { get; } = new FeatureCollection();

6:

7:     public HttpListenerServer()

8:     {

9:         listener = new HttpListener();

10:         this.Features.Set
      
       (new ServerAddressesFeature());
      

11:     }

12: 

13:     public void Dispose()

14:     {

15:         listener.Stop();

16:     }

17: 

18:     public void Start
      
       (IHttpApplication
       
         application)
       
      

19:     {

20:         foreach (string address in this.Features.Get
      
       ().Addresses)
      

21:         {

22:             listener.Prefixes.Add(address.TrimEnd('/') + "/");

23:         }

24: 

25:         listener.Start();

26:         while (true)

27:         {

28:             HttpListenerContext httpListenerContext = listener.GetContext();

29: 

30:             string listenUrl = this.Features.Get
      
       ().Addresses.First(address => httpListenerContext.Request.Url.IsBaseOf(new Uri(address)));
      

31:             string pathBase = new Uri(listenUrl).LocalPath.TrimEnd('/') ;

32:             HttpListenerServerFeature feature = new HttpListenerServerFeature(httpListenerContext, pathBase);

33: 

34:             FeatureCollection features = new FeatureCollection();

35:             features.Set
      
       (feature);
      

36:             features.Set
      
       (feature);
      

37:             TContext context = application.CreateContext(features);

38: 

39:             application.ProcessRequestAsync(context).ContinueWith(task =>

40:             {

41:                 httpListenerContext.Response.Close();

42:                 application.DisposeContext(context, task.Exception);

43:             });

44:         }

45:     }

46: }

HttpListener的GetContext方法以同步的方式监听请求，并利用接收到的请求创建返回的HttpListenerContext对象。我们利用它解析出当前请求的基地址，并进一步创建出描述当前原始上下文的HttpListenerServerFeature。接下来我们将这个对象分别采用特性接口IHttpRequestFeature和IHttpResponseFeature添加到创建的FeatureCollection对象中。然后我们将这个FeatureCollection作为参数调用HttpApplication的CreateContext创建出上下文对象，并将其作为参数调用HttpApplication的ProcessContext方法让注册的中间件来逐个地对请求进行处理。