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。

版权声明:本文为kenwoo原创文章,遵循 CC 4.0 BY-SA 版权协议,转载请附上原文出处链接和本声明。
本文链接:https://www.cnblogs.com/kenwoo/p/9532317.html