[鐵人賽 Day03] ASP.NET Core 2 系列 - Middleware

-- Pageviews

過去 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。

iT 邦幫忙 2018 鐵人賽 - Modern Web 組參賽文章:
[Day03] ASP.NET Core 2 系列 - Middleware

Middleware 概念

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

[鐵人賽 Day03] ASP.NET Core 2 系列 - Middleware - 概念

App.Use

Middleware 的註冊方式是在 Startup.csConfigureIApplicationBuilder 使用 Use 方法註冊。
大部分擴充的 Middleware 也都是以 Use 開頭的方法註冊,例如:

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

一個簡單的 Middleware 範例。如下:

Startup.cs

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// ...
public class Startup
{
// ...
public void Configure(IApplicationBuilder app)
{
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:

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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。資料經過的順序一定是先進後出

Request 流程如下圖:

[鐵人賽 Day03] ASP.NET Core 2 系列 - Middleware

Middleware 也可以作為攔截使用,如下:

Startup.cs

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// ...
public class Startup
{
// ...
public void Configure(IApplicationBuilder app)
{
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:

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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後面的水管傳送。流程如圖:

[鐵人賽 Day03] ASP.NET Core 2 系列 - Middleware - 概念

App.Run

Run 是 Middleware 的最後一個行為,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 及註冊不同的 Use
新增一個路由如下:

Startup.cs

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// ...
public class Startup
{
// ...
public void Configure(IApplicationBuilder app)
{
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.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.Run(async context =>
{
await context.Response.WriteAsync("Hello World! \r\n");
});
}
}

開啟網站任意連結,會顯示:

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First Middleware in. 
Hello World!
First Middleware out.

開啟網站 http://localhost:5000/second,則會顯示:

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

建立 Middleware 類別

如果 Middleware 全部都寫在 Startup.cs,程式碼應該很難維護,所以應該把自製的 Middleware 邏輯獨立出來。
建立 Middleware 類別不需要額外繼承其它類別或介面,一般的類別即可,範例如下:

FirstMiddleware.cs

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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");
}
}

全域註冊

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

Startup.cs

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

區域註冊

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

Controllers\HomeController.cs

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// ..
[MiddlewareFilter(typeof(FirstMiddleware))]
public class HomeController : Controller
{
// ...

[MiddlewareFilter(typeof(SecondMiddleware))]
public IActionResult Index()
{
// ...
}
}

Extensions

大部分擴充的 Middleware 都會用一個靜態方法包裝,如:UseMvc()UseRewriter()等。
自製的 Middleware 當然也可以透過靜態方法包,範例如下:

Extensions\CustomMiddlewareExtensions.cs

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public static class CustomMiddlewareExtensions
{
public static IApplicationBuilder UseFirstMiddleware(this IApplicationBuilder builder)
{
return builder.UseMiddleware<FirstMiddleware>();
}
}

註冊 Extension Middleware 的方式如下:

Startup.cs

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

參考

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