MVC系列（5） Http Pipeline詳細分析（下）

接上面的章節，我們這篇要講解的是Pipeline是執行的各種事件，我們知道，在自定義的HttpModule的Init方法裡，我們可以添加自己的事件，比如如下代碼：

public class Test : IHttpModule  
{  
    public void Init(HttpApplication context)  
    {  
        context.BeginRequest += new EventHandler(context_BeginRequest);  
        context.AuthenticateRequest += new EventHandler(context_AuthenticateRequest);  
    }  
      
    void context_AuthenticateRequest(object sender, EventArgs e)  
    {  
        throw new NotImplementedException();  
    }  
      
    void context_BeginRequest(object sender, EventArgs e)  
    {  
        throw new NotImplementedException();  
    }  
}

然後添加的代碼，在Pipeline裡執行的時候就會把這些事件給執行了，那麼如何執行並且按照什麼順序執行的呢？ 在了解這些之前，我們先看看這些事件是如何在HttpApplication裡暴露出來了，添加事件存放在何處的呢？閱讀HttpApplication 的源碼，我們可以看到，所有的事件都是按照如下的形式暴露的，選擇其中兩個看一下：

/// <devdoc><para>[To be supplied.]</para></devdoc>   
public event EventHandler BeginRequest {  
    add { AddSyncEventHookup(EventBeginRequest, value, RequestNotification.BeginRequest); }  
    remove { RemoveSyncEventHookup(EventBeginRequest, value, RequestNotification.BeginRequest); }  
}   
      
       
/// <devdoc><para>[To be supplied.]</para></devdoc>   
public event EventHandler AuthenticateRequest {  
    add { AddSyncEventHookup(EventAuthenticateRequest, value, RequestNotification.AuthenticateRequest); }   
    remove { RemoveSyncEventHookup(EventAuthenticateRequest, value, RequestNotification.AuthenticateRequest); }  
}

可以發現，所有的事件都是調用AddSyncEventHookup方法添加進去的，其中第一個參數是以Event+事件名稱的值，這個值是如何得來的，我們找到聲明的代碼：

private static readonly object EventDisposed = new object();  
private static readonly object EventErrorRecorded = new object();  
private static readonly object EventPreSendRequestHeaders = new object();   
private static readonly object EventPreSendRequestContent = new object();  
       
private static readonly object EventBeginRequest = new object();   
private static readonly object EventAuthenticateRequest = new object();  
private static readonly object EventDefaultAuthentication = new object();   
private static readonly object EventPostAuthenticateRequest = new object();  
private static readonly object EventAuthorizeRequest = new object();  
private static readonly object EventPostAuthorizeRequest = new object();  
private static readonly object EventResolveRequestCache = new object();   
private static readonly object EventPostResolveRequestCache = new object();  
private static readonly object EventMapRequestHandler = new object();   
private static readonly object EventPostMapRequestHandler = new object();   
private static readonly object EventAcquireRequestState = new object();  
private static readonly object EventPostAcquireRequestState = new object();   
private static readonly object EventPreRequestHandlerExecute = new object();  
private static readonly object EventPostRequestHandlerExecute = new object();  
private static readonly object EventReleaseRequestState = new object();  
private static readonly object EventPostReleaseRequestState = new object();   
private static readonly object EventUpdateRequestCache = new object();  
private static readonly object EventPostUpdateRequestCache = new object();   
private static readonly object EventLogRequest = new object();   
private static readonly object EventPostLogRequest = new object();  
private static readonly object EventEndRequest = new object();

再結合add和remove方法，可以大膽猜想，這些值應該是作為key值用的，我們先看完第2個參數，再來驗證我們的猜想，第2個參數是枚舉類型 RequestNotification，這裡我們再猜想一下，所有的事件都應該放在統一的地方，然後用這個枚舉來區分。讓我們先看看這個枚舉類的代碼：

[Flags]  
public enum RequestNotification  
{  
    BeginRequest = 1,  
    AuthenticateRequest = 2,  
    AuthorizeRequest = 4,  
    ResolveRequestCache = 8,  
    MapRequestHandler = 16,  
    AcquireRequestState = 32,  
    PreExecuteRequestHandler = 64,  
    ExecuteRequestHandler = 128,  
    ReleaseRequestState = 256,  
    UpdateRequestCache = 512,  
    LogRequest = 1024,  
    EndRequest = 2048,  
    SendResponse = 536870912,  
}

發現什麼了沒有？雖然使用了Flags標記來記錄以便進行異或查詢，但是這裡的枚舉類型好像少了一些吧，仔細對照代碼發現所有以Post開頭的事件都沒出現在這個枚舉類裡，為什麼呢？那這些事件是如何聲明的？回到HttpApplication類來繼續查看代碼，

/// <devdoc><para>[To be supplied.]</para></devdoc>  
public event EventHandler PostAuthenticateRequest {  
    add { AddSyncEventHookup(EventPostAuthenticateRequest, value, RequestNotification.AuthenticateRequest, true); }   
    remove { RemoveSyncEventHookup(EventPostAuthenticateRequest, value, RequestNotification.AuthenticateRequest, true); }  
}