.Net Core MVC中过滤器简介

在.Net Framework MVC 中有四种过滤器，授权过滤器（Authorize）、Action 过滤器、结果过滤器（Result）、异常过滤器（Exception）四种过滤器。在.Net Core MVC中，有五种过滤器，授权过滤器、Action过滤器、异常过滤器、结果过滤器、资源过滤器，新增了资源过滤器。

.Net Core MVC和.Net Framework MVC在基本使用上差别不大，主要的还是框架的差别。其中路由是个很重要的东西，参考文章：https://www.cnblogs.com/durow/p/5992926.html

Action过滤器、结果过滤器、Exception，三个特性，没有什么变化。在ExceptionFilter多了个Order参数，就是用来排序的。

结果过滤器，包装了单个Action Result的执行，当且晋档Action方法成功执行完毕后才运行。它们是理想的围绕视图执行或格式处理的逻辑（所在之处）。继承Attribute，IResultFilter

 /// <summary>
 /// Result的Filter
 /// </summary>
 public class CustomResultFilterAttribute : Attribute, IResultFilter
 {
     //private Logger logger = Logger.CreateLogger(typeof(CustomResultFilterAttribute));
     public void OnResultExecuted(ResultExecutedContext context)
     {
         Console.WriteLine("OnResultExecuted Executed!");
         //logger.Info("OnResultExecuted Executed!");
     }

     public void OnResultExecuting(ResultExecutingContext context)
     {
         Console.WriteLine("OnResultExecuting Executing!");
         //logger.Info("OnResultExecuting Executing!");
     }
 }

异常过滤器，为MVC隐藏程序为处理异常应用全局策略。继承ExceptionFilterAttribute

 /// <summary>
 /// 异常处理的Filter
 /// </summary>
 public class CustomExceptionFilterAttribute : ExceptionFilterAttribute
 {
     private readonly IHostingEnvironment _hostingEnvironment;
     private readonly IModelMetadataProvider _modelMetadataProvider;
     //private Logger logger = Logger.CreateLogger(typeof(CustomExceptionFilterAttribute));

     /// <summary>
     /// IOC来的
     /// </summary>
     /// <param name="hostingEnvironment"></param>
     /// <param name="modelMetadataProvider"></param>
     public CustomExceptionFilterAttribute(
         IHostingEnvironment hostingEnvironment,
         IModelMetadataProvider modelMetadataProvider)
     {
         _hostingEnvironment = hostingEnvironment;
         _modelMetadataProvider = modelMetadataProvider;
     }

     /// <summary>
     /// 没有处理的异常，就会进来
     /// </summary>
     /// <param name="filterContext"></param>
     public override void OnException(ExceptionContext filterContext)
     {
         if (!filterContext.ExceptionHandled)//异常有没有被处理过
         {
             string controllerName = (string)filterContext.RouteData.Values["controller"];
             string actionName = (string)filterContext.RouteData.Values["action"];
             string msgTemplate = "在执行 controller[{0}] 的 action[{1}] 时产生异常";
             //logger.Error(string.Format(msgTemplate, controllerName, actionName), filterContext.Exception);
             if (this.IsAjaxRequest(filterContext.HttpContext.Request))//检查请求头
             {
                 filterContext.Result = new JsonResult(
                      new
                      {
                          Result = false,
                          PromptMsg = "系统出现异常，请联系管理员",
                          DebugMessage = filterContext.Exception.Message
                      }//这个就是返回的结果
                 );
             }
             else
             {
                 var result = new ViewResult { ViewName = "~/Views/Shared/Error.cshtml" };
                 result.ViewData = new ViewDataDictionary(_modelMetadataProvider, filterContext.ModelState);
                 result.ViewData.Add("Exception", filterContext.Exception);
                 filterContext.Result = result;
             }
             filterContext.ExceptionHandled = true;
         }
     }

     private bool IsAjaxRequest(HttpRequest request)
     {
         string header = request.Headers["X-Requested-With"];
         return "XMLHttpRequest".Equals(header);
     }
 }

定义完ExceptionFilter，该怎么注册到全局呢？在Stratup.cs中ConfigureServer中，进行注册

services.AddMvc(o =>
{
    o.Filters.Add(typeof(CustomExceptionFilterAttribute));// 这里就是全局注册Filter

}).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

Action 过滤器，包装了对单个Action方法的调用，可以将参数传递给Action并从中获得Action Result。继承IActionFilter

 /// <summary>
 /// Action的Filter`
 /// </summary>
 public class CustomActionFilterAttribute : Attribute, IActionFilter
 {
     private ILogger<CustomActionFilterAttribute> _logger = null;
     public CustomActionFilterAttribute(ILogger<CustomActionFilterAttribute> logger)
     {
         this._logger = logger;
     }
     public void OnActionExecuted(ActionExecutedContext context)
     {
         //context.HttpContext.Response.WriteAsync("ActionFilter Executed!");
         Console.WriteLine("ActionFilter Executed!");
         //this._logger.LogDebug("ActionFilter Executed!");
     }
     public void OnActionExecuting(ActionExecutingContext context)
     {
         //context.HttpContext.Response.WriteAsync("ActionFilter Executing!");
         Console.WriteLine("ActionFilter Executing!");
         //this._logger.LogDebug("ActionFilter Executing!");
     }
 }

在Startup.cs中ConfigureServices进行注册

 //允许使用ServiceFilter 标记特性
 services.AddScoped<CustomActionFilterAttribute>();

标记到Controller的ActionFilter

/// <summary>
/// 标记到Controller
/// </summary>
public class CustomControllerActionFilterAttribute : Attribute, IActionFilter
{
    private ILogger<CustomControllerActionFilterAttribute> _logger = null;
    public CustomControllerActionFilterAttribute(ILogger<CustomControllerActionFilterAttribute> logger)
    {
        this._logger = logger;
    }
    public void OnActionExecuted(ActionExecutedContext context)
    {
        //context.HttpContext.Response.WriteAsync("ActionFilter Executed!");
        Console.WriteLine("ActionFilter Executed!");
        //this._logger.LogDebug("ActionFilter Executed!");
    }
    public void OnActionExecuting(ActionExecutingContext context)
    {
        //context.HttpContext.Response.WriteAsync("ActionFilter Executing!");
        Console.WriteLine("ActionFilter Executing!");
        //this._logger.LogDebug("ActionFilter Executing!");
    }
}

 [TypeFilter(typeof(CustomControllerActionFilterAttribute),Order =-)]
 public class ThirdController : Controller
 {
}

注册到全局的ActionFilter

public class CustomGlobalActionFilterAttribute : Attribute, IActionFilter
{
    private ILogger<CustomGlobalActionFilterAttribute> _logger = null;
    public CustomGlobalActionFilterAttribute(ILogger<CustomGlobalActionFilterAttribute> logger)
    {
        this._logger = logger;
    }
    public void OnActionExecuted(ActionExecutedContext context)
    {
        //context.HttpContext.Response.WriteAsync("ActionFilter Executed!");
        Console.WriteLine("ActionFilter Executed!");
        //this._logger.LogDebug("ActionFilter Executed!");
    }
    public void OnActionExecuting(ActionExecutingContext context)
    {
        //context.HttpContext.Response.WriteAsync("ActionFilter Executing!");
        Console.WriteLine("ActionFilter Executing!");
        //this._logger.LogDebug("ActionFilter Executing!");
    }
}

还是要在Startup中进行全局的注册

services.AddMvc(o =>
{
    o.Filters.Add(typeof(CustomGlobalActionFilterAttribute));// 这里就是全局注册Filter

}).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

分别对全局、控制器、Action注册了ActionFilter默认执行顺序：

　　全局OnActionExecuting===》控制器的OnActionExecuting====》Action的OnActionExecuting====》执行Action ====》Action的OnActionExecuted====》控制器的OnActionExecuted====》全局的OnActionExecuted。类似于一个俄罗斯套娃，也可以说是一个洋葱模型。

资源过滤器，是授权之后第一个用来处理请求的过滤器，也是最后一个接触到请求的过滤器（因为之后就会离开过滤器管道）。在性能方面，资源过滤器在实现缓存或短路过滤器管道尤其有用。继承IResourceFilter

 /// <summary>
 /// 自定义的资源Filter
 /// </summary>
 public class CustomResourceFilterAttribute : Attribute, IResourceFilter
 {
     private static readonly Dictionary<string, object> _Cache = new Dictionary<string, object>();
     private string _cacheKey;
     /// <summary>
     /// 控制器实例化之前
     /// </summary>
     /// <param name="context"></param>
     public void OnResourceExecuting(ResourceExecutingContext context)
     {
         _cacheKey = context.HttpContext.Request.Path.ToString();
         if (_Cache.ContainsKey(_cacheKey))
         {
             var cachedValue = _Cache[_cacheKey] as ViewResult;
             if (cachedValue != null)
             {
                 context.Result = cachedValue;
             }
         }
     }
     /// <summary>
     /// 把请求都处理完的
     /// </summary>
     /// <param name="context"></param>
     public void OnResourceExecuted(ResourceExecutedContext context)
     {
         if (!String.IsNullOrEmpty(_cacheKey) &&
             !_Cache.ContainsKey(_cacheKey))
         {
             var result = context.Result as ViewResult;
             if (result != null)
             {
                 _Cache.Add(_cacheKey, result);
             }
         }
     }
 }