我們在《服務器在管道中的“龍頭”地位》中對ASP.NET Core默認提供的具有跨平台能力的KestrelServer進行了介紹，為了讓讀者朋友們對管道中的服務器具有更加深刻的認識，接下來我們采用實例演示的形式創建一個自定義的服務器。這個自定義的服務器直接利用HttpListener來完成針對請求的監聽、接收和響應，我們將其命名為HttpListenerServer。在正式介紹HttpListenerServer的設計和實現之前，我們先來顯示一下如何將它應用到 一個具體的Web應用中。我們依然采用最簡單的Hello World應用來演示針對HttpListenerServer的應用，所以我們在Startup類的Configure方法中編寫如下的程序直接響應一個“Hello World”字符串。[本文已經同步到《ASP.NET Core框架揭秘》之中]
1: public class Startup
2: {3: public void Configure(IApplicationBuilder app)
4: { 5: app.Run(async context => await context.Response.WriteAsync("Hello World!"));6: }
7: }
在作為程序入口的Main方法中，我們直接創建一個WebHostBuilder對象並調用擴展方法UseHttpListener完成針對自定義HttpListenerServer的注冊。我們接下來調用UseStartup方法注冊上面定義的這個啟動類型，然後調用Build方法創建一個WebHost對象，最後調用Run方法運行這個作為宿主的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: }
我們自定義的擴展方法UseHttpListener的邏輯很簡單，它只是調用WebHostBuilder的ConfigureServices方法將我們自定義的HttpListenerServer類型以單例模式注冊到指定的ServiceCollection上而已。我們直接運行這個程序並利用浏覽器訪問默認的監聽地址（http://localhost:5000），服務端響應的“Hello World”字符串會按照如下圖所示的形式顯示在浏覽器上。
接下來我們來介紹一下HttpListenerServer的大體涉及。除了HttpListenerServer這個實現了IServer的自定義Server類型之外，我們只定義了一個名為HttpListenerServerFeature的特性類型，圖7所示的UML基本上體現了HttpListenerServer的總體設計。
如果我們利用HttpListener來監聽請求，它會為接收到的每次請求創建一個屬於自己的上下文，具體來說這是一個類型為HttpListenerContext對象。我們可以利用這個HttpListenerContext對象獲取所有與請求相關的信息，針對請求的任何響應也都是利用它完成的。上面這個HttpListenerServerFeature實際上就是對這個作為原始上下文的HttpListenerContext對象的封裝，或者說它是管道使用的DefaultHttpContext與這個原始上下文之間溝通的中介。
如下所示的代碼片段展示了HttpListenerServerFeature類型的完整定義。簡單起見，我們並沒有實現上面提到過的所有特性接口，而只是選擇性地實現了IHttpRequestFeature和IHttpResponseFeature這兩個最為核心的特性接口。它的構造函數除了具有一個類型為HttpListenerContext的參數之外，還具有一個字符串的參數pathBase用來指定請求URL的基地址（對應IHttpRequestFeature的PathBase屬性），我們利用它來計算請求URL的相對地址（對應IHttpRequestFeature的Path屬性）。IHttpRequestFeature和IHttpResponseFeature中定義的屬性都可以直接利用HttpListenerContext對應的成員來實現，這方面並沒有什麼特別之處。
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 ?? (requestHeaders = GetHttpHeaders(httpListenerContext.Request.Headers)); } 27: set { throw new NotImplementedException(); }28: }
29:
30: string IHttpRequestFeature.Method
31: { 32: get { return httpListenerContext.Request.HttpMethod; } 33: set { throw new NotImplementedException(); }34: }
35:
36: string IHttpRequestFeature.Path
37: { 38: get { return httpListenerContext.Request.RawUrl.Substring(pathBase.Length);} 39: set { throw new NotImplementedException(); }40: }
41:
42: string IHttpRequestFeature.PathBase
43: { 44: get { return pathBase; } 45: set { throw new NotImplementedException(); }46: }
47:
48: string IHttpRequestFeature.Protocol
49: { 50: get{ return protocol ?? (protocol = this.GetProtocol());} 51: set { throw new NotImplementedException(); }52: }
53:
54: string IHttpRequestFeature.QueryString
55: { 56: Get { return queryString ?? (queryString = this.ResolveQueryString());} 57: set { throw new NotImplementedException(); }58: }
59:
60: string IHttpRequestFeature.Scheme
61: { 62: get { return httpListenerContext.Request.IsWebSocketRequest ? "https" : "http"; } 63: set { throw new NotImplementedException(); }64: }
65: #endregion
66:
67: #region IHttpResponseFeature
68: Stream IHttpResponseFeature.Body
69: { 70: get { return httpListenerContext.Response.OutputStream; } 71: set { throw new NotImplementedException(); }72: }
73:
74: string IHttpResponseFeature.ReasonPhrase
75: { 76: get { return httpListenerContext.Response.StatusDescription; } 77: set { httpListenerContext.Response.StatusDescription = value; }78: }
79:
80: bool IHttpResponseFeature.HasStarted
81: { 82: get { return httpListenerContext.Response.SendChunked; }83: }
84:
85: IHeaderDictionary IHttpResponseFeature.Headers
86: { 87: get { return responseHeaders ?? (responseHeaders = GetHttpHeaders(httpListenerContext.Response.Headers)); } 88: set { throw new NotImplementedException(); }89: }
90: int IHttpResponseFeature.StatusCode
91: { 92: get { return httpListenerContext.Response.StatusCode; } 93: set { httpListenerContext.Response.StatusCode = value; }94: }
95:
96: void IHttpResponseFeature.OnCompleted(Func<object, Task> callback, object state)
97: {98: throw new NotImplementedException();
99: }
100:
101: void IHttpResponseFeature.OnStarting(Func<object, Task> callback, object state)
102: {103: throw new NotImplementedException();
104: }
105: #endregion
106:
107: private string ResolveQueryString()
108: {109: string queryString = "";
110: var collection = httpListenerContext.Request.QueryString;
111: for (int i = 0; i < collection.Count; i++)
112: { 113: queryString += $"{collection.GetKey(i)}={collection.Get(i)}&";114: }
115: return queryString.TrimEnd('&');116: }
117:
118: private IHeaderDictionary GetHttpHeaders(NameValueCollection headers)
119: {120: HeaderDictionary dictionary = new HeaderDictionary();
121: foreach (string name in headers.Keys)
122: {123: dictionary[name] = new StringValues(headers.GetValues(name));
124: }
125: return dictionary;
126: }
127:
128: private string GetProtocol()
129: {130: HttpListenerRequest request = httpListenerContext.Request;
131: Version version = request.ProtocolVersion;
132: return string.Format("{0}/{1}.{2}", request.IsWebSocketRequest ? "HTTPS" : "HTTP", version.Major, version.Minor);133: }
134: }
接下來我們來看看HttpListenerServer的定義。如下面的代碼片段所示，用來監聽請求的HttpListener在構造函數中被創建，與此同時，我們會創建一個用於獲取監聽地址的ServerAddressesFeature對象並將其添加到屬於自己的特性列表中。當HttpListenerServer隨著Start方法的調用而被啟動後，它將這個ServerAddressesFeature對象提取出來，然後利用它得到所有的地址並添加到HttpListener的Prefixes屬性表示的監聽地址列表中。接下來，HttpListener的Start方法被調用，並在一個無限循環中開啟請求的監聽與接收。
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
31: .First(address => httpListenerContext.Request.Url.IsBaseOf(new Uri(address)));
32: string pathBase = new Uri(listenUrl).LocalPath.TrimEnd('/') ;33: HttpListenerServerFeature feature = new HttpListenerServerFeature(httpListenerContext, pathBase);
34:
35: FeatureCollection features = new FeatureCollection();
36: features.Set<IHttpRequestFeature>(feature);
37: features.Set<IHttpResponseFeature>(feature);
38: TContext context = application.CreateContext(features);
39:
40: application.ProcessRequestAsync(context).ContinueWith(task =>
41: {42: httpListenerContext.Response.Close();
43: application.DisposeContext(context, task.Exception);
44: });
45: }
46: }
47: }
HttpListener的GetContext方法以同步的方式監聽請求，並利用接收到的請求創建返回的HttpListenerContext對象。我們利用它解析出當前請求的基地址，並進一步創建出描述當前原始上下文的HttpListenerServerFeature。接下來我們將這個對象分別采用特性接口IHttpRequestFeature和IHttpResponseFeature添加到創建的FeatureCollection對象中。然後我們將這個FeatureCollection作為參數調用HttpApplication的CreateContext創建出上下文對象，並將其作為參數調用HttpApplication的ProcessContext方法讓注冊的中間件來逐個地對請求進行處理。
