原文:ASP.NET Core 2 學習筆記（三）中間件

之前ASP.NET中使用的HTTP Modules及HTTP Handlers，在ASP.NET Core中已不復存在，取而代之的是Middleware。Middleware除了簡化了HTTP Modules/Handlers的使用方式，還帶入了Pipeline的概念。
本篇將介紹ASP.NET Core的Middleware概念及用法。

Middleware 概念

ASP.NET Core在Middleware的官方說明中，使用了Pipeline這個名詞，意指Middleware像水管一樣可以串聯(lián)在一起，所有的Request及Response都會層層經(jīng)過這些水管。
用圖例可以很容易理解，如下圖：

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App.Use

Middleware的注冊方式是在Startup.cs的Configure對IApplicationBuilder使用Use方法注冊。
大部分擴展的Middleware也都是以Use開頭的方法注冊，例如：

• UseMvc()：MVC的Middleware
• UseRewriter()：URL rewriting的Middleware

一個簡單的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");});}} }

用瀏覽器打開網(wǎng)站任意連結，輸出結果：　

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，注冊順序是很重要的事情。請求經(jīng)過的順序一定是先進后出。

Request 流程如下圖：?

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?Middleware 也可以作為攔截使用，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");});}} }

輸出結果：

First Middleware in. Second Middleware in. Second Middleware out. First Middleware out.

在Second Middleware 中，因為沒有達成條件，所以封包也就不在往后面的水管傳送。流程如圖：

App.Run

Run是Middleware的最后一個行為，以上面圖例來說，就是最末端的Action。
它不像Use能串聯(lián)其他Middleware，但Run還是能完整的使用Request及Response。

App.Map

Map是能用來處理一些簡單路由的Middleware，可依照不同的URL指向不同的Run及注冊不同的Use。
新增一個路由如下：

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");});}} }

開啟網(wǎng)站任意連結，會顯示：

First Middleware in. Third Middleware in. Hello World! Third Middleware out. First Middleware out.

開啟網(wǎng)站http://localhost:5000/second，則會顯示：

First Middleware in. Second Middleware in. Second. Second Middleware out. First Middleware out.

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創(chuàng)建Middleware 類

如果Middleware全部都寫在Startup.cs，代碼將很難維護，所以應該把自定義的Middleware邏輯獨立出來。
建立Middleware類不需要額外繼承其它類或接口，一般的類即可，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");}} }

全局注冊

在Startup.Configure注冊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>();// ...} }

局部注冊

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

Controllers\HomeController.cs

// .. [MiddlewareFilter(typeof(FirstMiddleware))] public class HomeController : Controller {// ...[MiddlewareFilter(typeof(SecondMiddleware))]public IActionResult Index(){// ...} }

Extensions

大部分擴展的Middleware都會用一個靜態(tài)方法包裝，如：UseMvc()、UseRewriter()等。
自定義的Middleware當然也可以透過靜態(tài)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>();}} }

注冊Extension Middleware 的方式如下：

Startup.cs

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

參考

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

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老司機發(fā)車啦：https://github.com/SnailDev/SnailDev.NETCore2Learning

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