Asp.net MVC源碼分析–Action Filter的鏈式調用

上一篇中我們介紹了asp.net MVC 的Filter的種類，以及調用的時點．今天我們來看一下ActionFilter/ResultFilter 調用的細節以及源碼中令人叫絕的代碼實現．首先我們看到在Contoller這個類中已經實現了IActionFilter/IResultFilter，並且它們的介面實現是調用兩個虛函數來實現的，這就為我們提供了便利,可以在我們的Controller中重寫這些虛函數來截獲並實現我們自己的邏輯．

Controller.cs

View Code

 1   protected virtual void OnActionExecuting(ActionExecutingContext filterContext) {
 2         }
 3 
 4         protected virtual void OnActionExecuted(ActionExecutedContext filterContext) {
 5         }
 6 
 7         protected virtual void OnAuthorization(AuthorizationContext filterContext) {
 8         }
 9 
10         protected virtual void OnException(ExceptionContext filterContext) {
11         }
12 
13         protected virtual void OnResultExecuted(ResultExecutedContext filterContext) {
14         }
15 
16         protected virtual void OnResultExecuting(ResultExecutingContext filterContext) {
17         }
18 
19 
20  #region IActionFilter Members
21         void IActionFilter.OnActionExecuting(ActionExecutingContext filterContext) {
22             OnActionExecuting(filterContext);
23         }
24 
25         void IActionFilter.OnActionExecuted(ActionExecutedContext filterContext) {
26             OnActionExecuted(filterContext);
27         }
28         #endregion
29 
30    #region IResultFilter Members
31         void IResultFilter.OnResultExecuting(ResultExecutingContext filterContext) {
32             OnResultExecuting(filterContext);
33         }
34 
35         void IResultFilter.OnResultExecuted(ResultExecutedContext filterContext) {
36             OnResultExecuted(filterContext);
37         }
38         #endregion

但是我們如何來擷取這個由Controller類實現的Filter介面呢？請看ControllerInstanceFilterProvider,這個已經預設註冊的Filter Provider可以得到這些Filter的介面實現.

ControllerInstanceFilterProvider.cs

View Code

1  public class ControllerInstanceFilterProvider : IFilterProvider {
2         public IEnumerable<Filter> GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor) {
3             if (controllerContext.Controller != null) {
4                 // Use FilterScope.First and Order of Int32.MinValue to ensure controller instance methods always run first
5                 yield return new Filter(controllerContext.Controller, FilterScope.First, Int32.MinValue);
6             }
7         }
8     }

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但是請大家考慮一個問題,如果我們在Action 上也同時標記了Action Filter的時候會產生一次調用需要調用兩個或多個Action filter的時候MVC 怎麼處理這樣的情況呢? 請看在ControllerActionInvoker.InvokeAction方法中的InvokeActionMethodWithFilters 方法.  我們今天的主角登場了,嘿嘿.

ControllerActionInvoker.cs

View Code

 1    protected virtual ActionExecutedContext InvokeActionMethodWithFilters(ControllerContext controllerContext, IList<IActionFilter> filters, ActionDescriptor actionDescriptor, IDictionary<string, object> parameters) {
 2             ActionExecutingContext preContext = new ActionExecutingContext(controllerContext, actionDescriptor, parameters);
 3             Func<ActionExecutedContext> continuation = () =>
 4                 new ActionExecutedContext(controllerContext, actionDescriptor, false /* canceled */, null /* exception */) {
 5                     Result = InvokeActionMethod(controllerContext, actionDescriptor, parameters)
 6                 };
 7 
 8             // need to reverse the filter list because the continuations are built up backward
 9             Func<ActionExecutedContext> thunk = filters.Reverse().Aggregate(continuation,
10                 (next, filter) => () => InvokeActionMethodFilter(filter, preContext, next));
11             return thunk();
12         }

首先我們看到代碼中聲明了Func<ActionExecutedContext> continuation, 的這樣一個委託, 注意這裡只是聲明還沒有調用哦.這個委託是用來調用Action方法的. 接下來又聲明了 Func<ActionExecutedContext> thunk 這樣一個委託, 這個委託的內容是用filters.Aggregate 方法來合并我們的Filter執行鏈,(這裡不理解的話,我們需要看Aggregate 方法的說明). InvokeActionMethodFilter的方法是執行IActionFilter中的介面實現.

InvokeActionMethodFilter方法

View Code

 1  internal static ActionExecutedContext InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func<ActionExecutedContext> continuation) {
 2             filter.OnActionExecuting(preContext);
 3             if (preContext.Result != null) {
 4                 return new ActionExecutedContext(preContext, preContext.ActionDescriptor, true /* canceled */, null /* exception */) {
 5                     Result = preContext.Result
 6                 };
 7             }
 8 
 9             bool wasError = false;
10             ActionExecutedContext postContext = null;
11             try {
12                 postContext = continuation();
13             }
14             catch (ThreadAbortException) {
15                 // This type of exception occurs as a result of Response.Redirect(), but we special-case so that
16 // the filters don't see this as an error.
17                 postContext = new ActionExecutedContext(preContext, preContext.ActionDescriptor, false /* canceled */, null /* exception */);
18                 filter.OnActionExecuted(postContext);
19                 throw;
20             }
21             catch (Exception ex) {
22                 wasError = true;
23                 postContext = new ActionExecutedContext(preContext, preContext.ActionDescriptor, false /* canceled */, ex);
24                 filter.OnActionExecuted(postContext);
25                 if (!postContext.ExceptionHandled) {
26                     throw;
27                 }
28             }
29             if (!wasError) {
30                 filter.OnActionExecuted(postContext);
31             }
32             return postContext;
33         }

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讓我們來嘗試理解上面的代碼,首先請看Aggregate 方法的調用說明以及Demo:

View Code

 1 public static TAccumulate Aggregate<TSource, TAccumulate>(
 2     this IEnumerable<TSource> source,
 3     TAccumulate seed,
 4     Func<TAccumulate, TSource, TAccumulate> func
 5 )
 6 //demo
 7 
 8   int[] ints = { 4, 8, 8, 3, 9, 0, 7, 8, 2 };
 9  // Count the even numbers in the array, using a seed value of 0.
10             int numEven = ints.Aggregate(0, (total, next) =>
11                                                 next % 2 == 0 ? total + 1 : total);
12 
13             Console.WriteLine("The number of even integers is: {0}", numEven);
14 
15             // This code produces the following output:
16 //
17 // The number of even integers is: 6 

我們看到Aggregate 第一個參數是初始值同時也決定了Aggregate 方法的傳回型別, 第二個參數是一個委託,委託的第一個參數是上一次調用的返回,這裡返回的類型也是一個委託.

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這時我們再看我們的InvokeActionMethodWithFilters 方法中 thunk 變數的聲明:

第一個參數是continuation,它的類型是Func<ActionExecutedContext>,這就決定了我們Aggregate 方法的傳回型別也是Func<ActionExecutedContext>, 再看第二個參數也是一個委託(注意這裡這個方法也沒有調用哦.)它的實現是調用InvokeActionMethodFilter方法. 這個委託會做為第三次迭代(如果有的話)Next參數.

所以最後在InvokeActionMethodFilter 方法的Next參數第一次是continuation變數,第二次是() => InvokeActionMethodFilter(filter, preContext, next));

.如果有第三次或第N次(有三個或N個Filter的情況)會同樣第二次的操作, 最後返回給thunk變數的是最後一次產生的調用委託.

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哎這裡好饒哦,對不起,我實在找不到其它詞彙了.只能靠最後的一張圖來協助大家理解了.

聲明時的虛擬碼:

var fun1 = (next, filter) => () => InvokeActionMethodFilter(filter, preContext, continuation);
var fun2 = (next, filter) => () => InvokeActionMethodFilter(filter, preContext, fun1)
var fun3 = (next, filter) => () => InvokeActionMethodFilter(filter, preContext, fun2)

最終thunk = fun3;

當thunk()調用的時候它的執行順序是這樣的.

調用時的虛擬碼:

invoke (next, filter) => () => InvokeActionMethodFilter(filter, preContext, fun3)

invoke  (next, filter) => () => InvokeActionMethodFilter(filter, preContext, fun2)

invoke  (next, filter) => () => InvokeActionMethodFilter(filter, preContext, continuation);

 

最後ResultFilter的調用和以上的分析是一樣的,理解了這個就理解的ResultFilter的調用.

 

後記;

相關文章:

1.巧用Aggregate和委託構造遞迴鏈

 

轉載請註明出處:http://www.cnblogs.com/RobbinHan/archive/2011/12/05/2270707.html 

本文作者: 十一月的雨 http://www.cnblogs.com/RobbinHan