【譯】asp.net Web API ：Web宿主

上一節描述了API的整個運行框架，即分為三層hosting、message handler pipeline 和 controller handling。此節講其中一個宿主，WebHost 寄宿在asp.net 傳統管道上。

Routing（路由）

在asp.net平臺，路由是一般由RouteTables.Routes靜態屬性添加的，類型是RouteCollection，例如下面的MVC模板自帶的添加路由的代碼。

PRotected void application_Start(){    RegisterRoutes(RouteTable.Routes);} public static void RegisterRoutes(RouteCollection routes){    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");     routes.MapRoute(        "Default", // Route name        "{controller}/{action}/{id}", // URL with parameters        new { controller = "Home", action = "Index", id = UrlParameter.Optional }     );}

大部分路由邏輯都是在UrlRoutingModule里，屬于PostResolveRequestCache asp.net管道事件。每一次請求，這個module都是重新匹配一次這個路由集合，并獲取一個RouteData實例，如果匹配，則：

1、實例化RouteData同時獲取一個route handler（HttpControllerRouteHandler）。

2、從routehandler獲取一個http handle 繼承自IHttphandler接口。IRouteHandler接口方法：IHttpHandler GetHttpHandler(RequestContext requestContext)

3、最后，當前的請求上下文(RequestContext）被映射到上面的這個httphandler。

因此，最后結果是asp.net管道請求被此handler處理。

Web API 集成

當寄宿在asp.net上，Web API特定的配置被定義在一個單例模式的HttpConfiguration對象里，通過靜態屬性GlobalCnfiguration.Configuration訪問。

Web API 也定義了一對新的RouteCollection擴展方法MapHttpRoute，來注冊Web API特殊的路由，下面是配置的例子代碼：

HttpConfiguration config = GlobalConfiguration.Configuration;config.Routes.MapHttpRoute("default", "{controller}/{id}",  new {id = UrlParameter.Optional});// other configuration settings

注意：

1. 靜態屬性GlobalConfiguration.Configuration是用于獲取配置的引用，內部這個對象指向的是全局的RouteTables.Routes這個集合。
2. 當新增路由時使用的是MapHttpRoute這個擴展方法。

當一個route通過MapHttpRoute增加后匹配到一個Request，，HttpControllerRouteHandler會創建一個新的HttpControllerHandler，其繼承自IAsyncHttpHandler，此handler通過RouteData(包含了路由的信息）初始化。

當被調用，HttpControllerhandler 在他的BeginReocessRequest方法里有以下行為：

• 為當前上下文創建HttpRequestMessage實例
• 使用GlobalConfiguration.Configuration獲取配置并創建一個HttpServer，并且把HttpRequestMessage送入服務管道。

當這個請求被HttpServer接受后，進入宿主的獨立處理階段（Web API的新管道）

下面的類圖是路由解決過程的摘要，并且分配給HttpServer（消息處理管道）

原文地址：

ASP.NET Web API: webhosting

看文字描述比較晦澀難懂，需要根據源碼理解，上圖容易理解。

HttpApplication -> UrlRoutingModule -> RouteCollection -> RouteData -> HttpControllerRouteHandler -> HttpControllerHandler -> HttpRequestMessage -> HttpServer

附加一些源碼：1、UrlRoutingModule獲取RouteData

public virtual void PostResolveRequestCache(HttpContextBase context){    RouteData routeData = this.RouteCollection.GetRouteData(context);    if (routeData == null)    {        return;    }    IRouteHandler routeHandler = routeData.RouteHandler;    if (routeHandler == null)    {        throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, SR.GetString("UrlRoutingModule_NoRouteHandler"), new object[0]));    }    if (routeHandler is StopRoutingHandler)    {        return;    }    RequestContext requestContext = new RequestContext(context, routeData);    context.Request.RequestContext = requestContext;    IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);    if (httpHandler == null)    {        throw new InvalidOperationException(string.Format(CultureInfo.CurrentUICulture, SR.GetString("UrlRoutingModule_NoHttpHandler"), new object[]        {            routeHandler.GetType()        }));    }    if (!(httpHandler is UrlAuthFailureHandler))    {        context.RemapHandler(httpHandler);        return;    }    if (FormsAuthenticationModule.FormsAuthRequired)    {        UrlAuthorizationModule.ReportUrlAuthorizationFailure(HttpContext.Current, this);        return;    }    throw new HttpException(401, SR.GetString("Assess_Denied_Description3"));}

2、MapHttpRoute（RouteCollectionExtension）

public static Route MapHttpRoute(this RouteCollection routes, string name, string routeTemplate, object defaults, object constraints, HttpMessageHandler handler){    if (routes == null)    {        throw Error.ArgumentNull("routes");    }    HttpRouteValueDictionary defaultsDictionary = new HttpRouteValueDictionary(defaults);    HttpRouteValueDictionary constraintsDictionary = new HttpRouteValueDictionary(constraints);    HostedHttpRoute httpRoute = (HostedHttpRoute)GlobalConfiguration.Configuration.Routes.CreateRoute(routeTemplate, defaultsDictionary, constraintsDictionary, dataTokens: null, handler: handler);    Route route = httpRoute.OriginalRoute;    routes.Add(name, route);    return route;}

3、HttpControllerRouteHandler.GetHandler

protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext){    return new HttpControllerHandler(requestContext.RouteData);}

4、HttpControllerHandler.ProcessRequest

internal async Task ProcessRequestAsyncCore(HttpContextBase contextBase){    HttpRequestMessage request = contextBase.GetHttpRequestMessage() ?? ConvertRequest(contextBase);    // Add route data    request.SetRouteData(_routeData);    CancellationToken cancellationToken = contextBase.Response.GetClientDisconnectedTokenWhenFixed();    HttpResponseMessage response = null;    try    {        response = await _server.SendAsync(request, cancellationToken);        await CopyResponseAsync(contextBase, request, response, _exceptionLogger.Value, _exceptionHandler.Value,            cancellationToken);    }    finally    {        // The other HttpTaskAsyncHandler is HttpRouteExceptionHandler; it has similar cleanup logic.        request.DisposeRequestResources();        request.Dispose();        if (response != null)        {            response.Dispose();        }    }}

5、HttpApplication -> HttpControllerHandler

public HttpControllerHandler(RouteData routeData)    : this(routeData, GlobalConfiguration.DefaultServer){}public HttpControllerHandler(RouteData routeData, HttpMessageHandler handler){    if (routeData == null)    {        throw Error.ArgumentNull("routeData");    }    if (handler == null)    {