ASP.NET Core 2 学习笔记（三）中间件

原文:ASP.NET Core 2 学习笔记（三）中间件

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：

Startup.cs

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=398940public 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：

Startup.cs

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=398940public 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：

Startup.cs

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=398940public 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($"{nameof(FirstMiddleware)} in. \r\n");await _next(context);await context.Response.WriteAsync($"{nameof(FirstMiddleware)} 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：

Startup.cs

// ...
public class Startup
{// ...public void Configure(IApplicationBuilder app){app.UseMiddleware<FirstMiddleware>();// ...}
}

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<FirstMiddleware>();}}
}

注册Extension Middleware 的方式如下：

Startup.cs

// ...
public class Startup
{// ...public void Configure(IApplicationBuilder app){app.UseFirstMiddleware();// ...}
}

参考

ASP.NET Core Middleware Fundamentals
Creating Custom Middleware In ASP.Net Core

老司机发车啦：https://github.com/SnailDev/SnailDev.NETCore2Learning