[翻译] 使用ASP.NET MVC操作过滤器记录日志

原文地址：http://www.singingeels.com/Articles/Logging_with_ASPNET_MVC_Action_Filters.aspx

翻译：Anders Liu

摘要：日志记录是一种常见的交错关注点(Cross-Cutting Concern)，很多ASP.NET开发者会在Global.asax文件中处理它。由于MVC是构建在ASP.NET之上的，所以你可以使用同样的解 决方式，但还有更好的方法。这篇文章向你展示了使用ASP.NET MVC的操作过滤器来向Web应用程序中添加日志是多么简单。

Logging is a common Cross-Cutting Concern that many ASP.NET developers solve in the Global.asax file. Because MVC is built on top of ASP.NET you could tap into the same solution, but there is a better way. This article will show how easy it is to add logging to your web app using ASP.NET MVC Action Filters.

日志记录是一种常见的交错关注点，很多ASP.NET开发者会在Global.asax文件中处理它。由于MVC是构建在ASP.NET之上的，所以你可以使用同样的解决方式，但还有更好的方法。这篇文章向你展示了使用ASP.NET MVC的操作过滤器来向Web应用程序中添加日志是多么简单。

Action Filters give you the ability to run custom code before or after an action (or page) is hit. Applying action filters to your MVC app is simple because they are implemented as attributes that can be placed on a method (an individual Action), or a class (the entire Controller).

操作过滤器使得你可以在操作（或页面）执行之前和之后运行自定义代码。在MVC应用程序中使用操作过滤器很简单，因为它们是通过特性实现的，可以放置在方法（一个单独的操作）或类（整个控制器）前面。

To show how easy this is, we‘re going to take the out-of-the-box ASP.NET MVC template, and very slightly tweak it to begin logging. We‘ll add one class (our custom Action Filter), and salt the existing pages with our new "LogRequest" attribute.

为了看到这有多简单，我们使用了开箱即用的ASP.NET MVC模板，对其进行少许调整就可以开始记录日志了。我们将会添加一个类（自定义的操作过滤器），并用这个新的"LogRequest"特性来“调制”现有的页面。

Creating a Custom Action Filter
创建自定义操作过滤器

To create your own action filter, you simply have to
inherit from the base "ActionFilterAttribute" class that‘s already a
part of the MVC framework. To make this easier on myself, I‘ve also
implemented the IActionFilter interface so that Visual Studio can
auto-generate my two methods for me.

要创建自己的操作过滤器，只需要简单地继承ActionFilterAttribute基类，该类是MVC框架的一部分。我为了更方便一些，还实现了IActionFilter接口，这样Visual Studio就会自动为我生成两个方法。

At this point, my custom action filter looks like this:

这时，我的自定义操作过滤器看起来是这样的：

 1 public class LogsRequestsAttribute : ActionFilterAttribute, IActionFilter
 2 {
 3 void IActionFilter.OnActionExecuted(ActionExecutedContext filterContext)
 4 {
 5 // I need to do something here...
 6 }
 7 void IActionFilter.OnActionExecuting(ActionExecutingContext filterContext)
 8 {
 9 // I need to do something here...
10 }
11 }

The ActionFilterAttribute and IActionFilter interface comes from the System.Web.Mvc namespace.

ActionFilterAttribute和IActionFilter接口来自System.Web.Mvc命名空间。

It‘s important to remember that this article was
written (and the sample app was compiled) for ASP.NET MVC Preview 4.
When the beta and release is eventually out, specifics of the article
may not be 100% relevant. However, this capability should remain the
same.

要记得本文（和文中的示例程序）是针对ASP.NET MVC Preview 4编写的。当beta和release版发布后，文章的细节可能不是100%有效的。不过，这一功能还是相同的。

Applying Our Custom Action Filter
应用自定义操作过滤器

Now that we have created our action filter, we need to
apply it to our Controllers or optionally, to our Actions. Because
logging is something that you would likely want on all of your "pages",
we‘ll simply add it at the controller level. Here is what we‘ve added to
the two existing controllers that were supplied in the ASP.NET MVC
template.

现在我们已经创建好操作过滤器了，我们需要将其应用到控制器上，或者有选择地应用在操作上。因为你可能希望对所有的“页面”进行日志记录，我们简单地将其添加到控制器级别上。在这里我们将其添加到ASP.NET MVC模板提供的两个控制器上。

 1 // HandleError was already there...
 2 [HandleError]
 3 [LogRequest]
 4 public class HomeController : Controller
 5 {
 6 ...
 7 }
 8 // HandleError was already there...
 9 [HandleError]
10 [LogRequest]
11 public class AccountController : Controller
12 {
13 ...
14 }   

That‘s it! The ASP.NET MVC framework will
automatically call our methods (OnActionExecuting and then
OnActionExecuted) when a request comes in to any of those two
controllers.

就是这样！当一个请求进入这两个控制器时，ASP.NET MVC框架会自动调用我们的方法（先是OnActionExecuting，然后是OnActionExecuted）。

At this point, all we have to do is actually implement
our logging code. Because I want to be able to easily report against my
site‘s activity, I‘m going to log to a SQL database. Now, it‘s
important to note that action filters are executed synchronously (for
obvious reasons), but I don‘t want the user to have to wait for my
logging to happen before he can enjoy my great site. So, I‘m going to
use the fire and forget design pattern.

此时，我们必须要真正实现日志记录代码了。由于我希望能简单地报告站点的活动，所以我将日志记录在SQL数据库中。要注意，操作过滤器是同步执行的（原因很明显），但我可不想让用户在访问我的牛逼的站点之前还要等着记录日志。因此，我将使用fire and forget设计模式。

When we‘re all done, this is what our logger will look like:

搞定所有这些之后，我们的日志记录看起来就是这样了：

 1 // By the way, I‘m using the Entity Framework for fun.
 2 public class LogRequestAttribute : ActionFilterAttribute, IActionFilter
 3 {
 4 private static LoggerDataStoreEntities DataStore = new LoggerDataStoreEntities();
 5 void IActionFilter.OnActionExecuting(ActionExecutingContext filterContext)
 6 {
 7 ThreadPool.QueueUserWorkItem(delegate
 8 {
 9 DataStore.AddToSiteLog(new SiteLog
10 {
11 Action = filterContext.ActionMethod.Name,
12 Controller = filterContext.Controller.ToString(),
13 TimeStamp = filterContext.HttpContext.Timestamp,
14 IPAddress = filterContext.HttpContext.Request.UserHostAddress,
15 });
16 DataStore.SaveChanges();
17 });
18 }
19 } 

Conclusion
小结

There are a lot of features in MVC that could each
merrit their own articles, but Action Filters are definately one feature
that shows off the advantages of the MVC design pattern. Action Filters
make perfect sense for cross-cutting concerns like logging, but you can
get creative with how and why you use them.

MVC中有太多的特性，每种都能单独写成文章，但操作过滤器最能炫耀MVC设计模式的特性。操作过滤器对交错关注点有着异同寻常的意义，但如何以及为什么使用它们，就需要你发挥创造力了。

This article isn‘t here to show you the best way to do
logging, but rather how and why you would use ASP.NET MVC Action
Filters. Here‘s the source code, play around with it: MVC_CustomActionFilter_Logging.zip

这篇文章并没有介绍记录日志最好的方法，而是给出了如何以及为什么使用ASP.NET MVC操作过滤器。这里是源代码，玩得愉快：MVC_CustomActionFilter_Logging.zip。