MVC 依赖 System.Web.Routing 处理请求路径解析，也就是说整个流程的起始是由 System.Web.Routing.UrlRoutingModule 开始的。
Web.config
<httpModules>
<add name="UrlRoutingModule" type="System.Web.Routing.UrlRoutingModule, System.Web.Routing, ..."/>
</httpModules>
那么我们就看看这个 UrlRoutingModule 内部都做了什么。
public class UrlRoutingModule : IHttpModule
{
protected virtual void Init(HttpApplication application)
{
application.PostResolveRequestCache += new EventHandler(this.OnApplicationPostResolveRequestCache);
application.PostMapRequestHandler += new EventHandler(this.OnApplicationPostMapRequestHandler);
}
}
订阅了两个 HttpApplication 事件。在 ASP.NET 应用程序生命周期中 PostResolveRequestCache 会在 PostMapRequestHandler 之前触发，最后是 IHttpHandler.ProcessRequest() 完成最终的请求处理。
通过对请求上下文进行包装，进一步进行处理。
public class UrlRoutingModule : IHttpModule
{
private void OnApplicationPostMapRequestHandler(object sender, EventArgs e)
{
HttpContextBase context = new HttpContextWrapper(((HttpApplication) sender).Context);
this.PostMapRequestHandler(context);
}
private void OnApplicationPostResolveRequestCache(object sender, EventArgs e)
{
HttpContextBase context = new HttpContextWrapper(((HttpApplication) sender).Context);
this.PostResolveRequestCache(context);
}
}
首先， PostResolveRequestCache 被执行。
public class UrlRoutingModule : IHttpModule
{
public virtual void PostResolveRequestCache(HttpContextBase context)
{
RouteData routeData = this.RouteCollection.GetRouteData(context);
if (routeData != null)
{
IRouteHandler routeHandler = routeData.RouteHandler;
...
if (!(routeHandler is StopRoutingHandler))
{
RequestContext requestContext = new RequestContext(context, routeData);
IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);
...
RequestData data2 = new RequestData();
data2.OriginalPath = context.Request.Path;
data2.HttpHandler = httpHandler;
context.Items[_requestDataKey] = data2;
context.RewritePath("~/UrlRouting.axd");
}
}
}
}
RouteCollection 实际是对 RouteTable.Routes 的引用。
public class UrlRoutingModule : IHttpModule
{
public RouteCollection RouteCollection
{
get
{
if (this._routeCollection == null)
{
this._routeCollection = RouteTable.Routes;
}
return this._routeCollection;
}
set
{
this._routeCollection = value;
}
}
}
好吗，又跳了一回。
public class RouteTable
{
private static RouteCollection _instance = new RouteCollection();
public static RouteCollection Routes
{
get
{
return _instance;
}
}
}
public class MvcApplication : System.Web.HttpApplication
{
public static void RegisterRoutes(RouteCollection routes)
{
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
routes.MapRoute(
"Default",
"{controller}/{action}/{id}",
new { controller = "Home", action = "Index", id = "" }
);
}
protected void Application_Start()
{
RegisterRoutes(RouteTable.Routes);
}
}
IgnoreRoute 和 MapRoute 都是 System.Web.Mvc.dll 中提供的扩展方法。
public static class RouteCollectionExtensions
{
public static void IgnoreRoute(this RouteCollection routes, string url);
...
public static Route MapRoute(this RouteCollection routes, string name, string url);
...
}
唯一需要注意的是方法内部一个细节，其注册的 IHttpHandler 是 MvcRouteHandler。
public static class RouteCollectionExtensions
{
public static Route MapRoute(this RouteCollection routes, string name, ...)
{
Route <>g__initLocal1 = new Route(url, new MvcRouteHandler());
<>g__initLocal1.Defaults = new RouteValueDictionary(defaults);
<>g__initLocal1.Constraints = new RouteValueDictionary(constraints);
Route route = <>g__initLocal1;
if ((namespaces != null) && (namespaces.Length > 0))
{
route.DataTokens = new RouteValueDictionary();
route.DataTokens["Namespaces"] = namespaces;
}
routes.Add(name, route);
return route;
}
public static void IgnoreRoute(this RouteCollection routes, string url, object constraints)
{
IgnoreRouteInternal <>g__initLocal0 = new IgnoreRouteInternal(url);
<>g__initLocal0.Constraints = new RouteValueDictionary(constraints);
IgnoreRouteInternal route = <>g__initLocal0;
routes.Add(route);
}
private sealed class IgnoreRouteInternal : Route
{
public IgnoreRouteInternal(string url) : base(url, new StopRoutingHandler()) { }
}
}
public class MvcRouteHandler : IRouteHandler
{
protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext);
IHttpHandler IRouteHandler.GetHttpHandler(RequestContext requestContext);
}
好了，回到 UrlRoutingModule.PostResolveRequestCache。
public class UrlRoutingModule : IHttpModule
{
public virtual void PostResolveRequestCache(HttpContextBase context)
{
RouteData routeData = this.RouteCollection.GetRouteData(context);
if (routeData != null)
{
IRouteHandler routeHandler = routeData.RouteHandler;
...
if (!(routeHandler is StopRoutingHandler))
{
RequestContext requestContext = new RequestContext(context, routeData);
IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);
...
RequestData data2 = new RequestData();
data2.OriginalPath = context.Request.Path;
data2.HttpHandler = httpHandler;
context.Items[_requestDataKey] = data2;
context.RewritePath("~/UrlRouting.axd");
}
}
}
}
public class MvcRouteHandler : IRouteHandler
{
protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext)
{
return new MvcHandler(requestContext);
}
IHttpHandler IRouteHandler.GetHttpHandler(RequestContext requestContext)
{
return this.GetHttpHandler(requestContext);
}
}
好！MvcHandler 出现了，这意味着执行流程从 System.Web.Routing 转到 System.Web.Mvc 的契机出现了。UrlRoutingModule.PostResolveRequestCache() 的最后将相关请求参数保存到上下文中，还有一行奇怪的代码，看看我们从 Microsoft Symbol Server 下载的源代码会有什么说明。
具体步骤：
1. 首先添加 PostResolveRequestCache 断点 (我们可以为没有源代码的方法设置断点)。
2. 然后下载符号文件。 
3. 打开源代码。 
好了，看看原作者如何说的。
public virtual void PostResolveRequestCache(HttpContextBase context)
{
// Save data to be used later in pipeline
context.Items[_requestDataKey] = new RequestData()
{
OriginalPath = context.Request.Path,
HttpHandler = httpHandler
};
// Rewrite path to something registered as a managed handler in IIS. This is necessary so IIS7 will
// execute our managed handler (instead of say the static file handler).
context.RewritePath("~/UrlRouting.axd");
}
不扯闲话了，我们继续执行流程。依照 ASP.NET HttpApplication 事件顺序，接下来 UrlRoutingModule.PostMapRequestHandler() 会被执行。
public class UrlRoutingModule : IHttpModule
{
public virtual void PostMapRequestHandler(HttpContextBase context)
{
RequestData data = (RequestData) context.Items[_requestDataKey];
if (data != null)
{
context.RewritePath(data.OriginalPath);
context.Handler = data.HttpHandler;
}
}
}
RequestData data 显然就是 PostResolveRequestCache() 保存的数据。通过将 HttpContext.Handler 设置为 MvcHandler，使得后续执行得以进行。对了，原作者对于这个方法的 RewritePath 也有说明。
public virtual void PostMapRequestHandler(HttpContextBase context)
{
RequestData requestData = (RequestData)context.Items[_requestDataKey];
if (requestData != null)
{
// Rewrite the path back to its original value, so the request handler only sees the original path.
context.RewritePath(requestData.OriginalPath);
// Set Context.Handler to the IHttpHandler determined earlier in the pipeline.
context.Handler = requestData.HttpHandler;
}
}
好了，有关 System.Web.Routing 的流程分析就到这了。
