.NET Core开发日志——Filter

ASP.NET Core MVC中的Filter作用是在请求处理管道的某些阶段之前或之后可以运行特定的代码。

Filter特性在之前的ASP.NET MVC中已经出现，但过去只有Authorization，Exception，Action，Result四种类型，现在又增加了一种Resource类型。所以共计五种。

Resource类型Filter在Authorization类型Filter之后执行，但又在其它类型的Filter之前。且执行顺序也在Model Binding之前，所以可以对Model Binding产生影响。

ASP.NET Core MVC框架中可以看到有ConsumesAttribute及FormatFilter两种实现IResourceFilter接口的类。

ConsumesAttribute会按请求中的Content-Type(内容类型)进行过滤，而FormatFilter能对路由或路径中设置了format值的请求作过滤。

一旦不符合要求，就对ResourceExecutingContext的Result属性设置，这样可以达到短路效果，阻止进行下面的处理。

ConsumesAttribute类的例子：

public void OnResourceExecuting(ResourceExecutingContext context){ ???... ???// Only execute if the current filter is the one which is closest to the action. ???// Ignore all other filters. This is to ensure we have a overriding behavior. ???if (IsApplicable(context.ActionDescriptor)) ???{ ???????var requestContentType = context.HttpContext.Request.ContentType; ???????// Confirm the request‘s content type is more specific than a media type this action supports e.g. OK ???????// if client sent "text/plain" data and this action supports "text/*". ???????if (requestContentType != null && !IsSubsetOfAnyContentType(requestContentType)) ???????{ ???????????context.Result = new UnsupportedMediaTypeResult(); ???????} ???}}

Filter在ASP.NET Core MVC里除了保留原有的包含同步方法的接口，现在又增加了包含异步方法的接口。

同步

• IActionFilter
• IAuthorizationFilter
• IExceptionFilter
• IResourceFilter
• IResultFilter

异步

• IAsyncActionFilter
• IAsyncAuthorizationFilter
• IAsyncExceptionFilter
• IAsyncResourceFilter
• IAsyncResultFilter

新的接口不像旧有的接口包含两个同步方法，它们只有一个异步方法。但可以实现同样的功能。

public class SampleAsyncActionFilter : IAsyncActionFilter{ ???public async Task OnActionExecutionAsync( ???????ActionExecutingContext context, ???????ActionExecutionDelegate next) ???{ ???????// 在方法处理前执行一些操作 ???????var resultContext = await next(); ???????// 在方法处理后再执行一些操作。 ???}}

Attribute形式的Filter，其构造方法里只能传入一些基本类型的值，例如字符串：

public class AddHeaderAttribute : ResultFilterAttribute{ ???private readonly string _name; ???private readonly string _value; ???public AddHeaderAttribute(string name, string value) ???{ ???????_name = name; ???????_value = value; ???} ???public override void OnResultExecuting(ResultExecutingContext context) ???{ ???????context.HttpContext.Response.Headers.Add( ???????????_name, new string[] { _value }); ???????base.OnResultExecuting(context); ???}}[AddHeader("Author", "Steve Smith @ardalis")]public class SampleController : Controller

如果想要在其构造方法里引入其它类型的依赖，现在可以使用ServiceFilterAttribute，TypeFilterAttribute或者IFilterFactory方式。

ServiceFilterAttribute需要在DI容器中注册：

public class GreetingServiceFilter : IActionFilter{ ???private readonly IGreetingService greetingService; ???public GreetingServiceFilter(IGreetingService greetingService) ???{ ???????this.greetingService = greetingService; ???} ???public void OnActionExecuting(ActionExecutingContext context) ???{ ???????context.ActionArguments["param"] = ????????????this.greetingService.Greet("James Bond"); ???} ???public void OnActionExecuted(ActionExecutedContext context) ???{ }}services.AddScoped<GreetingServiceFilter>();[ServiceFilter(typeof(GreetingServiceFilter))]public IActionResult GreetService(string param)

TypeFilterAttribute则没有必要：

public class GreetingTypeFilter : IActionFilter{ ???private readonly IGreetingService greetingService; ???public GreetingTypeFilter(IGreetingService greetingService) ???{ ???????this.greetingService = greetingService; ???} ???public void OnActionExecuting(ActionExecutingContext context) ???{ ???????context.ActionArguments["param"] = this.greetingService.Greet("Dr. No"); ???} ???public void OnActionExecuted(ActionExecutedContext context) ???{ }}[TypeFilter(typeof(GreetingTypeFilter))]public IActionResult GreetType1(string param)

IFilterFactory也是不需要的：

public class GreetingFilterFactoryAttribute : Attribute, IFilterFactory{ ???public bool IsReusable => false; ???public IFilterMetadata CreateInstance(IServiceProvider serviceProvider) ???{ ???????var logger = (IGreetingService)serviceProvider.GetService(typeof(IGreetingService)); ???????return new GreetingFilter(logger); ???} ???private class GreetingFilter : IActionFilter ???{ ???????private IGreetingService _greetingService; ???????public GreetingFilter(IGreetingService greetingService) ???????{ ???????????_greetingService = greetingService; ???????} ???????public void OnActionExecuted(ActionExecutedContext context) ???????{ ???????} ???????public void OnActionExecuting(ActionExecutingContext context) ???????{ ???????????context.ActionArguments["param"] = _greetingService.Greet("Dr. No"); ???????} ???} ???}[GreetingFilterFactory]public IActionResult GreetType1(string param)

Filter有三种范围：

• Global
• Controller
• Action

后两种可以通过Attribute的方式附加到特定Action方法或者Controller类之上。对于Global，则要在ConfigureServices方法内部添加。

public void ConfigureServices(IServiceCollection services){ ???services.AddMvc(options => ???{ ???????// by instance ???????options.Filters.Add(new AddDeveloperResultFilter("Tahir Naushad")); ???????// by type ???????options.Filters.Add(typeof(GreetDeveloperResultFilter)); ????});}

顾名思义，Global将对所有Controller及Action产生影响。所以务必对其小心使用。

这三种范围的执行顺序在设计程序的时候也需要多作考虑：

1. Global范围的前置处理代码
2. Controller范围的前置处理代码
3. Action范围的前置处理代码
4. Action范围的后置处理代码
5. Controller范围的后置处理代码
6. Global范围的后置处理代码

典型的前置处理代码如常见的OnActionExecuting方法，而常见的后置处理代码，则是像OnActionExecuted方法这般的。

.NET Core开发日志——Filter

原文地址：https://www.cnblogs.com/kenwoo/p/9532317.html