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（《聊聊ASP.NET Core默认提供的这个跨平台的服务器——KestrelServer》），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: .UseHttpListener()
7: .UseStartup<Startup>()
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<IServer, HttpListenerServer>());
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<object, Task> callback, object state)
99: {
100: throw new NotImplementedException();
101: }
102:
103: void IHttpResponseFeature.OnStarting(Func<object, Task> 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<IServerAddressesFeature>(new ServerAddressesFeature());
11: }
12:
13: public void Dispose()
14: {
15: listener.Stop();
16: }
17:
18: public void Start<TContext>(IHttpApplication<TContext> application)
19: {
20: foreach (string address in this.Features.Get<IServerAddressesFeature>().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<IServerAddressesFeature>().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<IHttpRequestFeature>(feature);
36: features.Set<IHttpResponseFeature>(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。