MVC中IActionFilter过滤器俄罗斯套娃的实现方式

看mvc的源码我们知道，它是在 ControllerActionInvoker 类中执行 InvokeAction 方法来实现过滤器和action方法执行的。

通过查看源码我们知道，他是通过调用 InvokeActionMethodWithFilters 方法来实现IActionFilter过滤器和action方法执行的，如图

点进去这个方法我们可以看到

 protected virtual ActionExecutedContext InvokeActionMethodWithFilters(ControllerContext controllerContext, IList<IActionFilter> filters, ActionDescriptor actionDescriptor, IDictionary<string, object> parameters)
 {
     ActionExecutingContext preContext = new ActionExecutingContext(controllerContext, actionDescriptor, parameters);
     Func<ActionExecutedContext> seed = () => new ActionExecutedContext(controllerContext, actionDescriptor, false, null)
     {
         Result = this.InvokeActionMethod(controllerContext, actionDescriptor, parameters)
     };
     return filters.Reverse<IActionFilter>().Aggregate(seed, (Func<ActionExecutedContext> next, IActionFilter filter) => () => ControllerActionInvoker.InvokeActionMethodFilter(filter, preContext, next))();
 }

看到这里我直接懵逼了，由于它委托中嵌套了委托而且还简写，还调用了扩展方法Aggregate累加器，所以很难直接看懂，这到底是怎么执行代码的呐？我来把代码整理如下

    public class Class1
     {

         protected virtual ActionExecutedContext InvokeActionMethodWithFilters(ControllerContext controllerContext, IList<IActionFilter> filters, ActionDescriptor actionDescriptor, IDictionary<string, object> parameters)
         {
             ActionExecutingContext preContext = new ActionExecutingContext(controllerContext, actionDescriptor, parameters);

             Func<ActionExecutedContext> seed = () => new ActionExecutedContext(controllerContext, actionDescriptor, false, null)
             {
                 Result = this.InvokeActionMethod(controllerContext, actionDescriptor, parameters)
             };

             Func<Func<ActionExecutedContext>, IActionFilter, Func<ActionExecutedContext>> secondParam =
                 (Func<ActionExecutedContext> next, IActionFilter filter) =>
                 {
                     Func<ActionExecutedContext> returnFunc = () =>
                     {
                         return Class1.InvokeActionMethodFilter(filter, preContext, next);
                     };

                     return returnFunc;

                     //这个是简写
                     //return () => Class1.InvokeActionMethodFilter(filter, preContext, next);
                 };

             return filters.Reverse<IActionFilter>().Aggregate(seed,
                 //(Func<ActionExecutedContext> next, IActionFilter filter) => () => Class1.InvokeActionMethodFilter(filter, preContext, next)
                 secondParam
                 )
                 .Invoke();
         }

         internal static ActionExecutedContext InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func<ActionExecutedContext> continuation)
         {
             filter.OnActionExecuting(preContext);
             if (preContext.Result != null)
             {
                 return new ActionExecutedContext(preContext, preContext.ActionDescriptor, true, null)
                 {
                     Result = preContext.Result
                 };
             }
             bool flag = false;
             ActionExecutedContext actionExecutedContext = null;
             try
             {
                 actionExecutedContext = continuation();
             }
             catch (ThreadAbortException)
             {
                 actionExecutedContext = new ActionExecutedContext(preContext, preContext.ActionDescriptor, false, null);
                 filter.OnActionExecuted(actionExecutedContext);
                 throw;
             }
             catch (Exception exception)
             {
                 flag = true;
                 actionExecutedContext = new ActionExecutedContext(preContext, preContext.ActionDescriptor, false, exception);
                 filter.OnActionExecuted(actionExecutedContext);
                 if (!actionExecutedContext.ExceptionHandled)
                 {
                     throw;
                 }
             }
             if (!flag)
             {
                 filter.OnActionExecuted(actionExecutedContext);
             }
             return actionExecutedContext;
         }

         protected virtual ActionResult InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary<string, object> parameters)
         {
             object actionReturnValue = actionDescriptor.Execute(controllerContext, parameters);
             return this.CreateActionResult(controllerContext, actionDescriptor, actionReturnValue);
         }

         protected virtual ActionResult CreateActionResult(ControllerContext controllerContext, ActionDescriptor actionDescriptor, object actionReturnValue)
         {
             if (actionReturnValue == null)
             {
                 return new EmptyResult();
             }
             ActionResult arg_29_0;
             if ((arg_29_0 = (actionReturnValue as ActionResult)) == null)
             {
                 arg_29_0 = new ContentResult();
                 //(arg_29_0 = new ContentResult()).Content = Convert.ToString(actionReturnValue, CultureInfo.InvariantCulture);
                 (arg_29_0 as ContentResult).Content = Convert.ToString(actionReturnValue, CultureInfo.InvariantCulture);
             }
             return arg_29_0;
         }

     }

咋一看，还是不知所云，一步一步来，

首先，我们先要知道 Aggregate 这个扩展方法是怎么执行的，直接看源码如下

看了源码就很容易理解了，它就是遍历数据源来循环执行传递过来的委托，并把结果当成参数，执行下一次循环的委托。

所有我整理了一个容易理解的一串代码

     public class Class2
     {

         public void Test()
         {
             var preContext = new ActionExecutingContext();

             Func<ActionExecutedContext> seed = () =>
             {
                 Console.WriteLine("执行action");
                 return new ActionExecutedContext();
             };

             int[] arr = { , , , , , , , ,  };

             Func<Func<ActionExecutedContext>, int, Func<ActionExecutedContext>> secondParam =
             (Func<ActionExecutedContext> next, int filter) =>
             {
                 return () => this.getStr(next, filter, preContext);
             };

             var reFunc2 = arr.Reverse().Aggregate<int, Func<ActionExecutedContext>>(seed, secondParam);
             reFunc2.Invoke();

         }

         public ActionExecutedContext getStr(Func<ActionExecutedContext> func, int filter, ActionExecutingContext preContext)
         {

             Console.WriteLine("before action----" + filter + "----" + preContext.ToString());

             var res = func.Invoke();

             Console.WriteLine("before action----" + filter + "----" + res.ToString());

             return res;
         }

     }

我是用一个int数组来模拟IActionFilter集合，其它的写法都和mvc框架的写法一样。

运行结果为 如图

看到这里，你是否明白了，它就是通过 委托里嵌套委托 来巧妙的实现了俄罗斯套娃的形式来实现IActionFilter过滤器和Action方法的执行。