ASP.NET MVC5学习笔记之Filter提供体系

　　前面我们介绍了Filter的基本使用，但各种Filter要在合适的时机运行起来，需要预先准备好，现在看看ASP.NET MVC框架是怎么做的。

一.Filter集合

　　在ControlerActionInvoker的InvokeAction方法中，只有一行代码FilterInfo filterInfo = GetFilters(controllerContext, actionDescriptor), 把收集的Filter信息放到了FilterInfo中，我们来看看

访类型的定义:

 public class FilterInfo
    {
        
        public FilterInfo();
      
        public FilterInfo(IEnumerable<Filter> filters);

       
        public IList<IActionFilter> ActionFilters { get; }
        public IList<System.Web.Mvc.Filters.IAuthenticationFilter> AuthenticationFilters { get; }
       
        public IList<IAuthorizationFilter> AuthorizationFilters { get; }
      
        public IList<IExceptionFilter> ExceptionFilters { get; }
   
        public IList<IResultFilter> ResultFilters { get; }
    }

    可以看到，这个类型定义的5种系统Filter集合信息. 来看看它带参的构造函数:

 public FilterInfo(IEnumerable<Filter> filters)
        {
            // Determine the override scope for each filter type and cache the filters list.
            OverrideFilterInfo processed = ProcessOverrideFilters(filters);
            // Split the cached filters list based on filter type and override scope.
            SplitFilters(processed);
        }

　 可以看到分两步:

    1 .ProcessOverrideFilters处理前面的提到IOverrideFilter接口

　 2. 从列表中把Filter分离到各自的Filter接口集合 

　 ProcessOverrideFilters方法的代码如下:

private static OverrideFilterInfo ProcessOverrideFilters(IEnumerable<Filter> filters)
        {
            OverrideFilterInfo result = new OverrideFilterInfo
            {
                ActionOverrideScope = FilterScope.First,
                AuthenticationOverrideScope = FilterScope.First,
                AuthorizationOverrideScope = FilterScope.First,
                ExceptionOverrideScope = FilterScope.First,
                ResultOverrideScope = FilterScope.First,
                Filters = new List<Filter>()
            };

            // Evaluate the 'filters' enumerable only once since the operation can be quite expensive.
            foreach (Filter filter in filters)
            {
                if (filter == null)
                {
                    continue;
                }
                IOverrideFilter overrideFilter = filter.Instance as IOverrideFilter;

                if (overrideFilter != null)
                {
                    if (overrideFilter.FiltersToOverride == typeof(IActionFilter)
                        && filter.Scope >= result.ActionOverrideScope)
                    {
                        result.ActionOverrideScope = filter.Scope;
                    }
                    else if (overrideFilter.FiltersToOverride == typeof(IAuthenticationFilter)
                        && filter.Scope >= result.AuthenticationOverrideScope)
                    {
                        result.AuthenticationOverrideScope = filter.Scope;
                    }
                    else if (overrideFilter.FiltersToOverride == typeof(IAuthorizationFilter)
                        && filter.Scope >= result.AuthorizationOverrideScope)
                    {
                        result.AuthorizationOverrideScope = filter.Scope;
                    }
                    else if (overrideFilter.FiltersToOverride == typeof(IExceptionFilter)
                        && filter.Scope >= result.ExceptionOverrideScope)
                    {
                        result.ExceptionOverrideScope = filter.Scope;
                    }
                    else if (overrideFilter.FiltersToOverride == typeof(IResultFilter)
                        && filter.Scope >= result.ResultOverrideScope)
                    {
                        result.ResultOverrideScope = filter.Scope;
                    }
                }

                // Cache filters to avoid having to enumerate it again (expensive). Do so here to avoid an extra loop.
                result.Filters.Add(filter);
            }

            return result;
        }

    这段代码遍历Filter列表，记录实现了IOverrideFilter的Filter的最高OverrideScope， 在下面的SplitFilters处理中，少于OverrideScope的Filter将不会添加到最终的集合中

　 SplitFilters方法代码如下:

 private void SplitFilters(OverrideFilterInfo info)
        {
            Contract.Assert(info.Filters != null);

            foreach (Filter filter in info.Filters)
            {
                Contract.Assert(filter != null);

                IActionFilter actionFilter = filter.Instance as IActionFilter;

                if (actionFilter != null && filter.Scope >= info.ActionOverrideScope)
                {
                    _actionFilters.Add(actionFilter);
                }

                IAuthenticationFilter authenticationFilter = filter.Instance as IAuthenticationFilter;

                if (authenticationFilter != null && filter.Scope >= info.AuthenticationOverrideScope)
                {
                    _authenticationFilters.Add(authenticationFilter);
                }

                IAuthorizationFilter authorizationFilter = filter.Instance as IAuthorizationFilter;

                if (authorizationFilter != null && filter.Scope >= info.AuthorizationOverrideScope)
                {
                    _authorizationFilters.Add(authorizationFilter);
                }

                IExceptionFilter exceptionFilter = filter.Instance as IExceptionFilter;

                if (exceptionFilter != null && filter.Scope >= info.ExceptionOverrideScope)
                {
                    _exceptionFilters.Add(exceptionFilter);
                }

                IResultFilter resultFilter = filter.Instance as IResultFilter;

                if (resultFilter != null && filter.Scope >= info.ResultOverrideScope)
                {
                    _resultFilters.Add(resultFilter);
                }
            }
        }

    实现各个Filter的分离，代码很简单，不再说明。

 二. Filter收集

　　在ASP.NET MVC5中最终通过FilterProviders.Providers.GetFilters方法得到所有的Filter列表，我们先来看看FilterProviders这个类型,定义如下：

public static class FilterProviders
    {
        static FilterProviders()
        {
            Providers = new FilterProviderCollection();
            Providers.Add(GlobalFilters.Filters);
            Providers.Add(new FilterAttributeFilterProvider());
            Providers.Add(new ControllerInstanceFilterProvider());
        }

        public static FilterProviderCollection Providers { get; private set; }
    }

　从中我们可以看到系统定义了三个FilterProvider,它们都实现了IFilterProvider接口，该接口定义如下:

public interface IFilterProvider
    {
        IEnumerable<Filter> GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor);
    }

 现在来看看这几个Provider:

    a. GlobalFilters.Filters

　　　这个故名思义是收集全局范围运行的Filter,代码如下: 

public static class GlobalFilters
    {
        static GlobalFilters()
        {
            Filters = new GlobalFilterCollection();
        }

        public static GlobalFilterCollection Filters { get; private set; }
    }

　　比如通常的项目模板在App_Start的FilterConfig中有如下代码,添加全局出错处理

public static void RegisterGlobalFilters(GlobalFilterCollection filters)
        {
            filters.Add(new HandleErrorAttribute());
        }

　　b. FilterAttributeFilterProvider

　　　　该类型是帮助收集应用在Controller和Action上的Filter，代码如下:

public class FilterAttributeFilterProvider : IFilterProvider
    {
        private readonly bool _cacheAttributeInstances;

        public FilterAttributeFilterProvider()
            : this(true)
        {
        }

        public FilterAttributeFilterProvider(bool cacheAttributeInstances)
        {
            _cacheAttributeInstances = cacheAttributeInstances;
        }

        protected virtual IEnumerable<FilterAttribute> GetActionAttributes(ControllerContext controllerContext, ActionDescriptor actionDescriptor)
        {
            return actionDescriptor.GetFilterAttributes(_cacheAttributeInstances);
        }

        protected virtual IEnumerable<FilterAttribute> GetControllerAttributes(ControllerContext controllerContext, ActionDescriptor actionDescriptor)
        {
            return actionDescriptor.ControllerDescriptor.GetFilterAttributes(_cacheAttributeInstances);
        }

        public virtual IEnumerable<Filter> GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor)
        {
            // Results are low in number in the common case so use yield return to avoid creating intermediate collections or nested enumerables
            if (controllerContext.Controller != null)
            {
                foreach (FilterAttribute attr in GetControllerAttributes(controllerContext, actionDescriptor))
                {
                    yield return new Filter(attr, FilterScope.Controller, order: null);
                }
                foreach (FilterAttribute attr in GetActionAttributes(controllerContext, actionDescriptor))
                {
                    yield return new Filter(attr, FilterScope.Action, order: null);
                }
            }             
        }
    }

　　c. ControllerInstanceFilterProvider

　　   controller本身也实现了一些Filter接口，通过该Provider加入,代码如下:

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

　　接下来看一看FilterProviderCollection的GetFilters实现:

 public IEnumerable<Filter> GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor)
        {
            //省略检查代码
            IFilterProvider[] providers = CombinedItems;
            List<Filter> filters = new List<Filter>();
            for (int i = 0; i < providers.Length; i++)
            {
                IFilterProvider provider = providers[i];
                foreach (Filter filter in provider.GetFilters(controllerContext, actionDescriptor))
                {
                    filters.Add(filter);
                }
            }

            filters.Sort(_filterComparer);

            if (filters.Count > 1)
            {
                RemoveDuplicates(filters);
            }
            return filters;
        }

   从中我们可以看到，主要分为三步:

   1. 通过FilterProviders 把Filter收集到一个Filter列表

　2. 对列表进行排序，排序规则是根据Order和Scope

　3. 列表去重

   FilterComparer是Filter排序比较器，代码如下, 从中我们可以看到Order 和 Scope是怎么影响排序顺序

private class FilterComparer : IComparer<Filter>
        {
            public int Compare(Filter x, Filter y)
            {
                // Nulls always have to be less than non-nulls
                if (x == null && y == null)
                {
                    return 0;
                }
                if (x == null)
                {
                    return -1;
                }
                if (y == null)
                {
                    return 1;
                }

                // Sort first by order...

                if (x.Order < y.Order)
                {
                    return -1;
                }
                if (x.Order > y.Order)
                {
                    return 1;
                }

                // ...then by scope

                if (x.Scope < y.Scope)
                {
                    return -1;
                }
                if (x.Scope > y.Scope)
                {
                    return 1;
                }

                return 0;
            }
        }

  最后返回列表传递给FilterInfo类型，FilterInfo内部处理见上面.