以前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。git
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像水管同樣能夠串聯在一塊兒，全部的Request及Response都會層層通過這些水管。
用圖例能夠很容易理解，以下圖：github
App.Use
Middleware的註冊方式是在Startup.cs的Configure對IApplicationBuilder使用Use方法註冊。
大部分擴展的Middleware也都是以Use開頭的方法註冊，例如：瀏覽器
- UseMvc()：MVC的Middleware
- UseRewriter()：URL rewriting的Middleware
一個簡單的Middleware 範例。以下：bash
Startup.csapp
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"); }); } } }
用瀏覽器打開網站任意連結，輸出結果： async
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，註冊順序是很重要的事情。請求通過的順序必定是先進後出。網站
Request 流程以下圖： ui
Middleware 也能夠做爲攔截使用，以下：this
Startup.csurl
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，封包不日後送 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，由於沒有達成條件，因此封包也就不在日後面的水管傳送。流程如圖：
App.Run
Run是Middleware的最後一個行爲，以上面圖例來講，就是最末端的Action。
它不像Use能串聯其餘Middleware，但Run仍是能完整的使用Request及Response。
App.Map
Map是能用來處理一些簡單路由的Middleware，可依照不一樣的URL指向不一樣的Run及註冊不一樣的Use。
新增一個路由以下：
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=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，封包不日後送 // 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"); }); } } }
開啓網站任意連結，會顯示：
First Middleware in. Third Middleware in. Hello World! Third Middleware out. First Middleware out.
開啓網站http://localhost:5000/second，則會顯示：
First Middleware in. Second Middleware in. Second. Second Middleware out. First Middleware out.
建立Middleware 類
若是Middleware所有都寫在Startup.cs，代碼將很難維護，因此應該把自定義的Middleware邏輯獨立出來。
創建Middleware類不須要額外繼承其它類或接口，通常的類便可，例子以下：
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"); } } }
全局註冊
在Startup.Configure註冊Middleware就能夠套用到全部的Request。以下：
Startup.cs
// ... public class Startup { // ... public void Configure(IApplicationBuilder app) { app.UseMiddleware<FirstMiddleware>(); // ... } }
局部註冊
Middleware 也能夠只套用在特定的Controller 或Action。註冊方式以下：
Controllers\HomeController.cs
// .. [MiddlewareFilter(typeof(FirstMiddleware))] public class HomeController : Controller { // ... [MiddlewareFilter(typeof(SecondMiddleware))] public IActionResult Index() { // ... } }
Extensions
大部分擴展的Middleware都會用一個靜態方法包裝，如：UseMvc()、UseRewriter()等。
自定義的Middleware固然也能夠透過靜態方法包，範例以下：
Extensions\CustomMiddlewareExtensions.cs
using Microsoft.AspNetCore.Builder; namespace MyWebsite { public static class CustomMiddlewareExtensions { public static IApplicationBuilder UseFirstMiddleware(this IApplicationBuilder builder) { return builder.UseMiddleware<FirstMiddleware>(); } } }
註冊Extension Middleware 的方式以下：
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
老司機發車啦：https://github.com/SnailDev/SnailDev.NETCore2Learning