ASP.NETCore2学习笔记（十四）Filters

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Filter是延续ASP.NET MVC的产物，同样保留了五种的Filter，分别是Authorization Filter、Resource Filter、Action Filter、Exception Filter及Result Filter。
通过不同的Filter可以有效处理封包进出的加工，本篇将介绍ASP.NET Core的五种Filter运作方式。

Filter 介绍

Filter的作用是在Action 执行前或执行后做一些加工处理。
某种程度来看，会跟Middleware很像，但执行的顺序略有不同，用对Filter不仅可以减少代码，还可以提高执行效率。

ASP.NET Core 有以下五种Filter 可以使用：

Authorization Filter
Authorization是五种Filter中优先级最高的，通常用于验证Request合不合法，不合法后面就直接跳过。
Resource Filter
Resource是第二优先，会在Authorization之后，Model Binding之前执行。通常会是需要对Model加工处理才用。
Action Filter
最常使用的Filter，封包进出都会经过它，使用上没什么需要特别注意的。跟Resource Filter很类似，但并不会经过Model Binding。
Exception Filter
异常处理的Filter。
Result Filter
当Action完成后，最终会经过的Filter。

Filter 运作方式

ASP.NET Core的每个Request都会先经过已注册的Middleware接着才会执行Filter，除了会依照上述的顺序外，同类型的Filter预设都会以先进后出的方式处里封包。
Response在某些Filter并不会做处理，会直接被pass。Request及Response的运作流程如下图：

黄色箭头是正常情况流程
灰色箭头是异常处理流程

建立Filter

ASP.NET Core的Filter基本上跟ASP.NET MVC的差不多。
上述的五种Filter范例分别如下：

Authorization Filter

AuthorizationFilter.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Filters;

namespace MyWebsite.Filters
{
    public class AuthorizationFilter : IAuthorizationFilter
    {
        public void OnAuthorization(AuthorizationFilterContext context)
        {
            context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");
        }
    }
}

非同步的方式：

// ...
public class AuthorizationFilter : IAsyncAuthorizationFilter
{
    public async Task OnAuthorizationAsync(AuthorizationFilterContext context)
    {
        await context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");
    }
}

Resource Filter

ResourceFilter.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Filters;

namespace MyWebsite.Filters
{
    public class ResourceFilter : IResourceFilter
    {
        public void OnResourceExecuting(ResourceExecutingContext context)
        {
            context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");
        }

        public void OnResourceExecuted(ResourceExecutedContext context)
        {
            context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n");
        }
    }
}

非同步的方式：

// ...
public class ResourceFilter : IAsyncResourceFilter
{
    public async Task OnResourceExecutionAsync(ResourceExecutingContext context, ResourceExecutionDelegate next)
    {
        await context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");

        await next();

        await context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n");
    }
}

Action Filter

ActionFilter.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Filters;

namespace MyWebsite.Filters
{
    public class ActionFilter : IActionFilter
    {
        public void OnActionExecuting(ActionExecutingContext context)
        {
            context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");
        }

        public void OnActionExecuted(ActionExecutedContext context)
        {
            context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n");
        }
    }
}

非同步的方式：

// ...
public class ActionFilter : IAsyncActionFilter
{
    public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
    {
        await context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");

        await next();

        await context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n");
    }
}

Result Filter

ResultFilter.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Filters;

namespace MyWebsite.Filters
{
    public class ResultFilter : IResultFilter
    {
        public void OnResultExecuting(ResultExecutingContext context)
        {
            context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");
        }

        public void OnResultExecuted(ResultExecutedContext context)
        {
            context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n");
        }
    }
}

非同步的方式：

// ...
public class ResultFilter : IAsyncResultFilter
{
    public async Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next)
    {
        await context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");

        await next();

        await context.HttpContext.Response.WriteAsync($"{GetType().Name} out. \r\n");
    }
}

Exception Filter

ExceptionFilter.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Filters;

namespace MyWebsite.Filters
{
    public class ExceptionFilter : IExceptionFilter
    {
        public void OnException(ExceptionContext context)
        {
            context.ExceptionHandled = true; // 表明异常已处理，客户端可得到正常返回
            context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");
        }
    }
}

非同步的方式：

// ...
public class ExceptionFilter : IAsyncExceptionFilter
{
        public Task OnExceptionAsync(ExceptionContext context)
        {
            context.ExceptionHandled = true;// 表明异常已处理，客户端可得到正常返回
            context.HttpContext.Response.WriteAsync($"{GetType().Name} in. \r\n");
            return Task.CompletedTask;
        }
}

注册Filter

Filter有两种注册方式，一种是全局注册，另一种是用[Attribute]局部注册的方式，只套用在特定的Controller或Action。

全局注册

在Startup.ConfigureServices的MVC服务中注册Filter，这样就可以套用到所有的Request。如下：

// ...
public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc(config =>
        {
            config.Filters.Add(new ResultFilter());
            config.Filters.Add(new ExceptionFilter());
            config.Filters.Add(new ResourceFilter());
        });
    }
}

局部注册

ASP.NET Core在局部注册Filter的方式跟ASP.NET MVC有一点不一样，要通过[TypeFilter(type)]。
在Controller或Action上面加上[TypeFilter(type)]就可以局部注册Filter。如下：

// ...
namespace MyWebsite.Controllers
{
    [TypeFilter(typeof(AuthorizationFilter))]
    public class HomeController : Controller
    {
        [TypeFilter(typeof(ActionFilter))]
        public void Index()
        {
            Response.WriteAsync("Hello World! \r\n");
        }
        
        [TypeFilter(typeof(ActionFilter))]
        public void Error()
        {
            throw new System.Exception("Error");
        }
    }
}

[TypeFilter(type)]用起来有点冗长，想要像过去ASP.NET MVC用[Attribute]注册Filter的话，只要将Filter继承Attribute即可。如下：

public class AuthorizationFilter : Attribute, IAuthorizationFilter
{
    // ...
}
public class ActionFilter : Attribute, IActionFilter
{
    // ...
}

[Attribute] 注册就可以改成如下方式：

// ...
namespace MyWebsite.Controllers
{
    [AuthorizationFilter]
    public class HomeController : Controller
    {
        [ActionFilter]
        public void Index()
        {
            Response.WriteAsync("Hello World! \r\n");
        }
        
        [ActionFilter]
        public void Error()
        {
            throw new System.Exception("Error");
        }
    }
}

执行结果

http://localhost:5000/Home/Index 输出结果如下：

AuthorizationFilter in.
ResourceFilter in.
ActionFilter in.
Hello World!
ActionFilter out.
ResultFilter in.
ResultFilter out.
ResourceFilter out.

http://localhost:5000/Home/Error 输出结果如下：

AuthorizationFilter in.
ResourceFilter in.
ActionFilter in.
ActionFilter out.
ExceptionFilter in.
ResourceFilter out.

执行顺序

预设注册同类型的Filter 是以先进后出的方式处里封包，注册层级也会影响执行顺序。

但也可以通过实现 IOrderedFilter 更改执行顺序。例如：

public class ActionFilter : Attribute, IActionFilter, IOrderedFilter
{
    public string Name { get; set; }

    public int Order { get; set; } = 0;

    public void OnActionExecuting(ActionExecutingContext context)
    {
        context.HttpContext.Response.WriteAsync($"{GetType().Name}({Name}) in. \r\n");
    }
    public void OnActionExecuted(ActionExecutedContext context)
    {
        context.HttpContext.Response.WriteAsync($"{GetType().Name}({Name}) out. \r\n");
    }
}

在注册Filter 时带上Order，数值越小优先权越高。

// ...
public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc(config =>
        {
            config.Filters.Add(new ActionFilter() { Name = "Global", Order = 3 });
        });
    }
}

// ...
namespace MyWebsite.Controllers
{
    [ActionFilter(Name = "Controller", Order = 2)]
    public class HomeController : Controller
    {
        [ActionFilter(Name = "Action", Order = 1)]
        public void Index()
        {
            Response.WriteAsync("Hello World! \r\n");
        }
    }
}

变更执行顺序后的输出内容：

ActionFilter(Action) in. 
ActionFilter(Controller) in. 
ActionFilter(Global) in. 
Hello World! 
ActionFilter(Global) out. 
ActionFilter(Controller) out. 
ActionFilter(Action) out.