ASP.NET Core 2.0 中间件

来源：snailteam

cnblogs.com/snaildev/p/9075424.html

之前ASP.NET中使用的HTTP Modules及HTTP Handlers，在ASP.NET Core中已不复存在，取而代之的是Middleware。

Middleware除了简化了HTTP Modules/Handlers的使用方式，还带入了Pipeline的概念。

本篇将介绍ASP.NET Core的Middleware概念及用法。

Middleware 概念

ASP.NET Core在Middleware的官方说明中，使用了Pipeline这个名词，意指Middleware像水管一样可以串联在一起，所有的Request及Response都会层层经过这些水管。

用图例可以很容易理解，如下图：

App.Use

Middleware的注册方式是在Startup.cs的Configure对IApplicationBuilder使用Use方法注册。

大部分扩展的Middleware也都是以Use开头的方法注册，例如：

UseMvc()：MVC的Middleware

UseRewriter()：URL rewriting的Middleware

一个简单的Middleware 范例。如下：

using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

using Microsoft.AspNetCore.Builder;

using Microsoft.AspNetCore.Hosting;

using Microsoft.AspNetCore.Http;

using Microsoft.Extensions.DependencyInjection;

namespace MyWebsite

{

public class Startup

{

// This method gets called by the runtime. Use this method to add services to the container.

// For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940

public void ConfigureServices(IServiceCollection services)

{

}

// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.

public void Configure(IApplicationBuilder app, IHostingEnvironment env)

{

if (env.IsDevelopment())

{

app.UseDeveloperExceptionPage();

}

app.Use(async (context, next) =>

{

await context.Response.WriteAsync("First Middleware in. \r\n");

await next.Invoke();

await context.Response.WriteAsync("First Middleware out. \r\n");

});

app.Use(async (context, next) =>

{

await context.Response.WriteAsync("Second Middleware in. \r\n");

await next.Invoke();

await context.Response.WriteAsync("Second Middleware out. \r\n");

});

app.Use(async (context, next) =>

{

await context.Response.WriteAsync("Third Middleware in. \r\n");

await next.Invoke();

await context.Response.WriteAsync("Third Middleware out. \r\n");

});

app.Run(async (context) =>

{

await context.Response.WriteAsync("Hello World! \r\n");

});

}

}

}

用浏览器打开网站任意连结，输出结果：

First Middleware in.

Second Middleware in.

Third Middleware in.

Hello World!

Third Middleware out.

Second Middleware out.

First Middleware out.

在Pipeline的概念中，注册顺序是很重要的事情。请求经过的顺序一定是先进后出。

Request 流程如下图：

Middleware 也可以作为拦截使用，如下：

using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

using Microsoft.AspNetCore.Builder;

using Microsoft.AspNetCore.Hosting;

using Microsoft.AspNetCore.Http;

using Microsoft.Extensions.DependencyInjection;

namespace MyWebsite

{

public class Startup

{

// This method gets called by the runtime. Use this method to add services to the container.

// For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940

public void ConfigureServices(IServiceCollection services)

{

}

// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.

public void Configure(IApplicationBuilder app, IHostingEnvironment env)

{

if (env.IsDevelopment())

{

app.UseDeveloperExceptionPage();

}

app.Use(async (context, next) =>

{

await context.Response.WriteAsync("First Middleware in. \r\n");

await next.Invoke();

await context.Response.WriteAsync("First Middleware out. \r\n");

});

app.Use(async (context, next) =>

{

await context.Response.WriteAsync("Second Middleware in. \r\n");

// 水管阻塞，封包不往后送

var condition = false;

if (condition)

{

await next.Invoke();

}

await context.Response.WriteAsync("Second Middleware out. \r\n");

});

app.Use(async (context, next) =>

{

await context.Response.WriteAsync("Third Middleware in. \r\n");

await next.Invoke();

await context.Response.WriteAsync("Third Middleware out. \r\n");

});

app.Run(async (context) =>

{

await context.Response.WriteAsync("Hello World! \r\n");

});

}

}

}

输出结果：

First Middleware in.

Second Middleware in.

Second Middleware out.

First Middleware out.

在Second Middleware 中，因为没有达成条件，所以封包也就不在往后面的水管传送。流程如图：

App.Run

Run是Middleware的最后一个行为，以上面图例来说，就是最末端的Action。

它不像Use能串联其他Middleware，但Run还是能完整的使用Request及Response。

App.Map

Map是能用来处理一些简单路由的Middleware，可依照不同的URL指向不同的Run及注册不同的Use。

新增一个路由如下：

using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

using Microsoft.AspNetCore.Builder;

using Microsoft.AspNetCore.Hosting;

using Microsoft.AspNetCore.Http;

using Microsoft.Extensions.DependencyInjection;

namespace MyWebsite

{

public class Startup

{

// This method gets called by the runtime. Use this method to add services to the container.

// For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940

public void ConfigureServices(IServiceCollection services)

{

}

// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.

public void Configure(IApplicationBuilder app, IHostingEnvironment env)

{

if (env.IsDevelopment())

{

app.UseDeveloperExceptionPage();

}

app.Use(async (context, next) =>

{

await context.Response.WriteAsync("First Middleware in. \r\n");

await next.Invoke();

await context.Response.WriteAsync("First Middleware out. \r\n");

});

// app.Use(async (context, next) =>

// {

// await context.Response.WriteAsync("Second Middleware in. \r\n");

// // 水管阻塞，封包不往后送

// var condition = false;

// if (condition)

// {

// await next.Invoke();

// }

// await context.Response.WriteAsync("Second Middleware out. \r\n");

// });

app.Map("/second", mapApp =>

{

mapApp.Use(async (context, next) =>

{

await context.Response.WriteAsync("Second Middleware in. \r\n");

await next.Invoke();

await context.Response.WriteAsync("Second Middleware out. \r\n");

});

mapApp.Run(async context =>

{

await context.Response.WriteAsync("Second. \r\n");

});

});

app.Use(async (context, next) =>

{

await context.Response.WriteAsync("Third Middleware in. \r\n");

await next.Invoke();

await context.Response.WriteAsync("Third Middleware out. \r\n");

});

app.Run(async (context) =>

{

await context.Response.WriteAsync("Hello World! \r\n");

});

}

}

}

开启网站任意连结，会显示：

First Middleware in.

Third Middleware in.

Hello World!

Third Middleware out.

First Middleware out.

开启网站http://localhost:5000/second，则会显示：

First Middleware in.

Second Middleware in.

Second.

Second Middleware out.

First Middleware out.

创建Middleware 类

如果Middleware全部都写在Startup.cs，代码将很难维护，所以应该把自定义的Middleware逻辑独立出来。

建立Middleware类不需要额外继承其它类或接口，一般的类即可，例子如下：

FirstMiddleware.cs

using System.Threading.Tasks;

using Microsoft.AspNetCore.Http;

namespace MyWebsite

{

public class FirstMiddleware

{

private readonly RequestDelegate _next;

public FirstMiddleware(RequestDelegate next)

{

_next = next;

}

public async Task Invoke(HttpContext context)

{

await context.Response.WriteAsync($" in. \r\n");

await _next(context);

await context.Response.WriteAsync($" out. \r\n");

}

}

}

全局注册

在Startup.Configure注册Middleware就可以套用到所有的Request。如下：

// ...

public class Startup

{

// ...

public void Configure(IApplicationBuilder app)

{

app.UseMiddleware();

// ...

}

}

局部注册

Middleware 也可以只套用在特定的Controller 或Action。注册方式如下：

Controllers\HomeController.cs

// ..

[MiddlewareFilter(typeof(FirstMiddleware))]

public class HomeController : Controller

{

// ...

[MiddlewareFilter(typeof(SecondMiddleware))]

public IActionResult Index()

{

// ...

}

}

Extensions

大部分扩展的Middleware都会用一个静态方法包装，如：UseMvc()、UseRewriter()等。

自定义的Middleware当然也可以透过静态方法包，范例如下：

Extensions\CustomMiddlewareExtensions.cs

using Microsoft.AspNetCore.Builder;

namespace MyWebsite

{

public static class CustomMiddlewareExtensions

{

public static IApplicationBuilder UseFirstMiddleware(this IApplicationBuilder builder)

{

return builder.UseMiddleware();

}

}

}

注册Extension Middleware 的方式如下：

// ...

public class Startup

{

// ...

public void Configure(IApplicationBuilder app)

{

app.UseFirstMiddleware();

// ...

}

}

源码：https://github.com/SnailDev/SnailDev.NETCore2Learning

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