ASP.NET MVC 1.0 流程分析(System.Web.Routing)【zz】

MVC 依賴 System.Web.Routing 處理請求路徑解析，也就是說整個流程的起始是由 System.Web.Routing.UrlRoutingModule 開始的。 

Web.config 
<httpModules> 
<add name="UrlRoutingModule" type="System.Web.Routing.UrlRoutingModule, System.Web.Routing, ..."/> 
</httpModules> 

那麼我們就看看這個 UrlRoutingModule 內部都做了什麼。 

public class UrlRoutingModule : IHttpModule 
{ 
protected virtual void Init(HttpApplication application) 
{ 
application.PostResolveRequestCache += new EventHandler(this.OnApplicationPostResolveRequestCache); 
application.PostMapRequestHandler += new EventHandler(this.OnApplicationPostMapRequestHandler); 
} 
} 

訂閱了兩個 HttpApplication 事件。在 ASP.NET 應用程式生命週期中 PostResolveRequestCache 會在 PostMapRequestHandler 之前觸發，最後是 IHttpHandler.ProcessRequest() 完成最終的請求處理。 

通過對請求上下文進行封裝，進一步進行處理。 

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 被執行。 

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 實際是對 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; 
} 
} 
} 

好嗎，又跳了一回。 

public class RouteTable 
{ 
private static RouteCollection _instance = new RouteCollection(); 

public static RouteCollection Routes 
{ 
get 
{ 
return _instance; 
} 
} 
} 
這時候，我們要提及 Global.asax.cs 中的代碼。 

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 中提供的擴充方法。 

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內部一個細節，其註冊的 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"); 
} 
} 
} 
} 
在擷取 RouteData 和 MvcRouteHandler 後，調用 MvcRouteHandler.GetHttpHandler() 擷取 IHttpHandler。 

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 出現了，這意味著執行流程從 System.Web.Routing 轉到 System.Web.Mvc 的契機出現了。UrlRoutingModule.PostResolveRequestCache() 的最後將相關請求參數儲存到上下文中，還有一行奇怪的代碼，看看我們從 Microsoft Symbol Server 下載的原始碼會有什麼說明。 

具體步驟： 

1. 首先添加 PostResolveRequestCache 斷點 (我們可以為沒有原始碼的方法設定斷點)。

2. 然後下載符號檔案。 

3. 開啟原始碼。 

好了，看看原作者如何說的。 

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"); 
} 
不扯閑話了，我們繼續執行流程。依照 ASP.NET HttpApplication 事件順序，接下來 UrlRoutingModule.PostMapRequestHandler() 會被執行。 

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 顯然就是 PostResolveRequestCache() 儲存的資料。通過將 HttpContext.Handler 設定為 MvcHandler，使得後續執行得以進行。對了，原作者對於這個方法的 RewritePath 也有說明。 

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，有關 System.Web.Routing 的流程分析就到這了。