MVC生命周期

浏览数：21 / 时间：2015年06月09日

对于Asp.net MVC,我对它的生命周期还是兴趣很浓，于是提出两个问题：

一个HTTP请求从IIS移交到Asp.net运行时,Asp.net MVC是在什么时机获得了控制权并对请求进行处理呢?处理过程又是怎样的?

以IIS7中asp.net应用程序生命周期为例，下图是来自MSDN的一张HTTP请求处理过程发生事件的简图，后面我列出了一个完整的事件列表。既然Asp.net Mvc还是以Asp.net运行时为基础那么它必然要在Asp.net应用程序的生命周期中对请求进行截获。第一反应当然是去web.config里面去翻翻，我们可以看到UrlRoutingModule的配置节：

下面要做的就顺理成章了，用Reflector打开这个程序集,可以看到以下代码：

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

看到这里我们的第一个问题实际上已经有了答案：时机是在PostResolveRequestCache和PostMapRequestHandler.

ResolveRequestCache event
Occurs when ASP.NET finishes an authorization event to let the caching modules serve requests from the cache, bypassing execution of the event handler (for example, a page or an XML Web service).

源文档 <http://msdn.microsoft.com/en-us/library/system.web.httpapplication.resolverequestcache.aspx>

PostMapRequestHandler event
Occurs when ASP.NET has mapped the current request to the appropriate event handler.

源文档 <http://msdn.microsoft.com/en-us/library/system.web.httpapplication.postmaprequesthandler.aspx>

我们使用VS2008中Asp.net Mvc模板创建一个Demo完成后续的讨论，当我们访问/Home的时候发生了什么呢？

Request 请求到来
IIS 根据请求特征将处理权移交给 ASP.NET
UrlRoutingModule将当前请求在 Route Table中进行匹配
UrlRoutingModule在RouteCollection中查找Request匹配的RouteHandler,默认是MvcRouteHandler MvcRouteHandler 创建 MvcHandler实例.
MvcHandler执行 ProcessRequest.
MvcHandler 使用 IControllerFactory 获得实现了IController接口的实例,找到对应的HomeController
根据Request触发HomeController的Index方法
Index将执行结果存放在ViewData
HomeController的Index方法返回 ActionResult
Views/Home/Index.aspx将 ViewData呈现在页面上
Index.aspx执行ProcessRequest方法
Index.aspx执行Render方法输出到客户端

通过阅读Asp.net Mvc的源码,我们可以得到更为详细的处理过程,我尽可能的忽略掉枝节，强调请求处理的流程.我们从Global.asax.cs文件切入,下面是一段样例代码,这里初始化了路由表,请特别特别注意注释部分:

     public class MvcApplication : System.Web.HttpApplication
        {
            public static void RegisterRoutes(RouteCollection routes)
            {
                routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

                //The controller route value is a special value that the System.Web.Mvc.MvcHandler class uses to call into the IControllerFactory interface.
                //The basic route handler is an instance of IRouteHandler named MvcRouteHandler.
                //We have complete control and could provide our own implementation of IRouteHandler if we wished.
                routes.MapRoute(
                    "Default",                                              // Route name
                    "{controller}/{action}/{id}",                           // URL with parameters
                    new { controller = "Home", action = "Index", id = "" }  // Parameter defaults
                );


            }

            protected void Application_Start()
            {
                RegisterRoutes(RouteTable.Routes);
            }

UrlRoutingMoudule在PostResolveRequestCache阶段从RouteCollection中获取当前请求的RouteData.RouteData包含了一个请求处理对应的Controller和Action,RouteData这个作用贯穿请求的处理过程.RouteData中提取RouteHandler,这里默认是MvcRouteHandler,MvcRouteHandler获取HttpHandler,这里默认的是MvcHandler.

public virtual void PostResolveRequestCache(HttpContextBase context)

{

RouteData routeData = this.RouteCollection.GetRouteData(context);

if (routeData != null)

{

IRouteHandler routeHandler = routeData.RouteHandler;

if (routeHandler == null)

{

throw new InvalidOperationException(string.Format(CultureInfo.CurrentUICulture, RoutingResources.UrlRoutingModule_NoRouteHandler, new object[0]));

}

if (!(routeHandler is StopRoutingHandler))

{

RequestContext requestContext = new RequestContext(context, routeData);

IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);

if (httpHandler == null)

{

throw new InvalidOperationException(string.Format(CultureInfo.CurrentUICulture, RoutingResources.UrlRoutingModule_NoHttpHandler, new object[] { routeHandler.GetType() }));

}

RequestData data2 = new RequestData();

data2.OriginalPath = context.Request.Path;

data2.HttpHandler = httpHandler;

context.Items[_requestDataKey] = data2;

context.RewritePath("~/UrlRouting.axd");

}

MvcHandler.ProcessRequest()中首先使用HttpContextWrapper对HttpContext进行封装,封装的目的是为了解耦以获得可测试性.然后从RequestContext.RouteData中提取Controller名称.
ControllerBuilder.GetControllerFactory --> ControllerFactory.CreateController --> IController.Execute

ControllerBase实现了IController接口,在Initialize时将RequestContext封装成为ControllerContext,Controller继承自ControllerBase并实现抽象方法ExecuteCore()

在ExecuteCore中,Controller首先从RouteData中获得ActionName,然后执行ActionInvoker.InvokeAction.

在ActionInvoker中我们可以看到各种Filter,这是一种AOP实践:在Action方法执行的前后执行若干方法.这里有四种Filter:ActionFilters,ResultFilters,AuthorizationFilters,ExceptionFilters.这四种Filter并不是封闭的,都有对应的接口,这四个只是默认实现.Filter的执行顺序是:AuthorizationFilter--->Action Filter.OnActionExecuting--->Action Method--->ActionFilter.OnActionExecuted.InvokeActionMethodWithFilters返回的结果是ActionExecutedContext,接下来将Controller执行OnResultExecuting 方法.ActionResult执行的结果可以是ViewResult,JsonResult,RedirectResult,ContentResult,或者是自定义的Result类型.

如果返回的类型是ViewResult,我们先看一下ViewReuslt的继承关系:ViewResult-->ViewResultBase-->ActionResult,ViewResult包含两个属性View和ViewEngineCollection,实际上是包含了两个接口的实现:IViewEngine定义了怎么定位View/Partial View.IView定义了如何RenderView.默认的实现时WebFormView和WebFormViewEngine.