ASP.NET Core 的pipeline中間件
using System;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;
using System.Linq;
public class HttpContext
{
public string Request{get;set;}
public string Response {get;set;}
}
public delegate Task RequestDelegate(HttpContext context); // 委托申明
public interface IApplicationBuilder
{
IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware);
RequestDelegate Build();
}
public static class RunExtensions
{
/// <summary>
/// Adds a terminal middleware delegate to the application's request pipeline.
/// </summary>
/// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
/// <param name="handler">A delegate that handles the request.</param>
public static void Run(this IApplicationBuilder app, RequestDelegate handler)
{
if (app == null)
{
throw new ArgumentNullException(nameof(app));
}
if (handler == null)
{
throw new ArgumentNullException(nameof(handler));
}
app.Use(_ => handler); // Here "_" represents the argument variable "next". In c# "_" is a valid variable.
}
}
public static class UseExtensions
{
/// <summary>
/// Adds a middleware delegate defined in-line to the application's request pipeline.
/// </summary>
/// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
/// <param name="middleware">A function that handles the request or calls the given next function.</param>
/// <returns>The <see cref="IApplicationBuilder"/> instance.</returns>
public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware)
{
return app.Use(next =>
{
return context =>
{
Func<Task> simpleNext = () => next(context);
return middleware(context, simpleNext);
};
});
}
}
public class ApplicationBuilder:IApplicationBuilder
{
private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();
public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware)
{
_components.Add(middleware);
return this;
}
public RequestDelegate Build()
{
// Here is the default middleware in the end of pipeline. Ofcourse it can be bypassed by Run() method.
RequestDelegate app = async ctx=>{
System.Console.WriteLine("I am the default(end) middleware.");
await Task.CompletedTask;
};
foreach (var component in _components.Reverse())
{
app = component(app);
}
return app;
}
}
class Program
{
public static void Main()
{
IApplicationBuilder appbuilder = new ApplicationBuilder();
appbuilder.Use(next =>
{
return async ctx =>
{
System.Console.WriteLine("Enter middleware 1");
await next(ctx);
System.Console.WriteLine("Exit middleware 1");
};
});
appbuilder.Use(next =>
{
return async ctx =>
{
System.Console.WriteLine("Enter middleware 2");
await next(ctx);
System.Console.WriteLine("Exit middleware 2");
};
});
appbuilder.Use(next =>
{
return async ctx =>
{
System.Console.WriteLine("Enter middleware 3");
await next(ctx);
System.Console.WriteLine("Exit middleware 3");
};
});
// This Use() is extension method of IApplicationBuilder
appbuilder.Use(async (context, next) =>
{
System.Console.WriteLine("Enter middleware 4");
await next.Invoke();
System.Console.WriteLine("Enter middleware 4");
});
// This Run() is extension method of IApplicationBuilder
// The Run() method can add the end middleware.
// All the middleware added by Use() after Run() will be bypassed by Run().
appbuilder.Run(async ctx=>{
System.Console.WriteLine("Enter middleware 5");
await Task.CompletedTask;
});
// Get the first middleware.
RequestDelegate fstapp = appbuilder.Build();
HttpContext context = new HttpContext(){
Request = "Request is comming.",
Response = "Response to you."
};
// Run the first middleware will run all the middleware in the pipeline.
fstapp(context);
Console.Read();
}
}
/*
Enter middleware 1
Enter middleware 2
Enter middleware 3
Enter middleware 4
Enter middleware 5
Enter middleware 4
Exit middleware 3
Exit middleware 2
Exit middleware 1
*/
上面的代碼假設注釋了122-135行，結果如下:
/*
Enter middleware 1
Enter middleware 2
Enter middleware 3
I am the default(end) middleware.
Exit middleware 3
Exit middleware 2
Exit middleware 1
*/
為了說明ASP.NET Core中pipeline的結構，我們模拟了上面這麼一個pipeline和middleware的代碼。
代碼模拟了HttpContext類（實際ASP.NET Core中這個類是abstract類，這裡為了模拟，做了很多簡化），IApplicationBuilder, ApplicationBuilder，RequestDelegate類。
這段代碼參考了實際的實作，真實模拟了中間件的建構過程，使用List類将delegate委托巧妙的形成了連結清單。
代碼中自定義了兩個委托：RequestDelegate和Func<RequestDelegate,RequestDelegate>。
我們稱RequestDelegate是"具體的中間件"，說它是具體的中間件，是因為其實它就是一個具體的方法。
Func<T1,T2>是.NET自帶的的泛型委托類型，它代表一個類似如下的方法：
RequestDelegate function(RequestDelegate)
也就是這個方法參數是RequestDelegate,傳回值也是一個RequestDelegate。參數的RequestDelegate代表的是要執行的"下一個中間件"，傳回的RequestDelegate則表示了"目前中間件"。
Func<RequestDelegate,RequestDelegate>這委托就是為了讓"目前中間件"調用"下一個中間件"，也就是利用Func<RequestDelegate,RequestDelegate>委托可以實作一個中間件調用"下一個中間件";
我們稱Func<RequestDelegate,RequestDelegate>為一個"pipeline單元";
ApplicationBuilder中包含一個List清單_compoments，這個清單中每個元素是一個Func<RequestDelegate,RequestDelegate>委托。
ApplicationBuilder的Use()方法就是向_compoments連結清單中增加一個形式為Func<RequestDelegate,RequestDelegate>的"中間件連結單元"，也就是插入一個一個Func<RequestDelegate,RequestDelegate>方法。代碼中這些方法都是以匿名形式給出的，但是即使是匿名方法，在記憶體中也會真實的生成這些方法，注意此時具體的RequestDelegate方法并沒有生成，如下圖所示(這裡的圖是假設注釋了122-135行的代碼)：
上圖中藍色部分表示List連結清單，黑色的部分表示一個個Func<RequestDelegate,RequestDelegate>方法，調動完Use後這些匿名方法就在記憶體中存在了，隻不過還沒有調用它們。
當我們調用Build()方法的時候，Build()方法首先會生成一個預設的RequestDelegate中間件（真實的ASP.NET Core同樣會生成一個預設的中間件，不過比我們給的例子複雜一些），這個預設的中間件是會連結到pipeline尾部，作為最後一個中間件，明顯最後的中間件是不需要next中間件的。接下來，Build()會倒序執行連結清單中的Func<RequestDelegate,RequestDelegate>方法，執行這些方法的目的是為了在記憶體中生成一個一個的RequestDelegate方法，因為這些方法才是具體需要的中間件，并且通過Func<RequestDelegate,RequestDelegate>方法中的next參數具體化每個RequestDelegate方法（顯然，這裡的RequestDelegate方法也都是匿名方法）：
調用完Build()方法後，記憶體中就出現了如上圖的一個個連結好的RequestDelegate方法。
至此，pipeline及整個中間件都實作了。
最後，調用第一個middleware委托實作一連串中間件的調用。
應該說微軟使用Delegate成精了。