Action Filter

为了让用户更简单的创建一个自定义Action filter，ASP.NET MVC Framework提供了一个基类ActionFilterAttribute，这个类实现了IActionFilter和IResultFilter接口，并且继承了FilterAttribute类。从广义上来说，在ASP.NET MVC Framework中，任何实现filter的类型都是action filter。

ActionFilter成员：

1. OnActionExecuting – 在controller action执行之前调用
2. OnActionExecuted – 在controller action执行之后调用
3. OnResultExecuting – 在controller action result执行之前调用
4. OnResultExecuted – 在controller action result执行之后调用

执行顺序如下图：

下面是个小例子：

新建一个类MyActionFilter继承ActionFilterAttribute

public class MyActionFilter : ActionFilterAttribute
    {
        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            //记录时间
            filterContext.HttpContext.Items["StartTime"] = DateTime.Now;

            //验证是否登录
            var service = (IFormsAuthorizeService)filterContext.Controller;

            //子控件不做权限验证
            if (filterContext.IsChildAction)
                return;

            //源请求页面url地址
            string url = filterContext.HttpContext.Request.RawUrl;

            if (!service.SignIn())
            {
                if (filterContext.HttpContext.Request.IsAjaxRequest())
                {

                    JsonResult jr = new JsonResult();
                    jr.Data = new { url = "/passport/login" };
                    jr.JsonRequestBehavior = JsonRequestBehavior.AllowGet;
                    filterContext.Result = jr;
                }
                else
                    filterContext.Result = new RedirectResult("/passport/login?redirectUrl=" + url);
            }
        }

        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            //处理异常
            if (filterContext.Exception != null)
            {
                filterContext.ExceptionHandled = true;
                filterContext.Result = new RedirectToRouteResult("Default"
                    , new System.Web.Routing.RouteValueDictionary(new { action = "OnError" }));
            }
        }

        public override void OnResultExecuting(ResultExecutingContext filterContext)
        {
            //压缩输出结果
            string acceptEncoding = filterContext.HttpContext.Request.Headers["Accept-Encoding"];

            if (string.IsNullOrEmpty(acceptEncoding)) return;

            var response = filterContext.HttpContext.Response;
            acceptEncoding = acceptEncoding.ToUpperInvariant();

            if (acceptEncoding.Contains("GZIP"))
            {
                response.AppendHeader("Content-encoding", "gzip");
                response.Filter = new GZipStream(response.Filter, CompressionMode.Compress);
            }
            else if (acceptEncoding.Contains("DEFLATE"))
            {
                response.AppendHeader("Content-encoding", "deflate");
                response.Filter = new DeflateStream(response.Filter, CompressionMode.Compress);
            }
        }

        public override void OnResultExecuted(ResultExecutedContext filterContext)
        {
            //计算出页面相应时间
            DateTime statTime = (DateTime)filterContext.HttpContext.Items["StartTime"];
            TimeSpan diff = DateTime.Now - statTime;

            filterContext.HttpContext.Response.Write("<hr />" + diff.ToString());
        }
    }