ASP.NET Core Middleware 抽丝剥茧

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一. 宏观概念

ASP.NET Core Middleware是在应用程序处理管道pipeline中用于处理请求和操作响应的组件。

每个组件是pipeline 中的一环。

自行决定是否将请求传递给下一个组件
在处理管道的下个组件执行之前和之后执行业务逻辑

二. 特性和行为

ASP.NET Core处理管道由一系列请求委托组成，一环接一环的被调用，pipeline封装了其中的处理环节，下面是更细化的Middleware 处理管道时序图：

从上图可以看出，请求自进入处理管道，经历了四个中间件，每个中间件都包含后续紧邻中间件执行委托（next）的引用，同时每个中间件在交棒之前和交棒之后可以自行决定参与一些Http请求和响应的逻辑处理。

每个中间件还可以决定不将请求转发给下一个委托，这称为请求管道的短路。

短路是有必要的，某些特殊中间件比如 StaticFileMiddleware 可以在完成功能之后，避免请求被转发到其他动态处理过程。

三. 标准Middleware 使用方式

using System.Threading.Tasks;
using Alyio.AspNetCore.ApiMessages;
using Gridsum.WebDissector.Common;
using Microsoft.AspNetCore.Http;
 
namespace Gridsum.WebDissector
{
    sealed class AuthorizationMiddleware
    {
        private readonly RequestDelegate _next;      // 下一个中间件执行委托的引用
 
        public AuthorizationMiddleware(RequestDelegate next)
        {
            _next = next;
        }
 
        public Task Invoke(HttpContext context)      // 贯穿始终的HttpContext对象
        {
            if (context.Request.Path.Value.StartsWith("/api/"))
            {
                return _next(context);
            }
            if (context.User.Identity.IsAuthenticated && context.User().DisallowBrowseWebsite)
            {
                throw new ForbiddenMessage("You are not allow to browse the website.");
            }
            return _next(context);
        }
    }
}
 
 public static IApplicationBuilder UserAuthorization(this IApplicationBuilder app)
 {
      return app.UseMiddleware<AuthorizationMiddleware>();
 }
 // 启用该中间件，也就是注册该中间件
 app.UserAuthorization();

四. 观察标准中间件的定义方式，带着几个问题探究源码实现

　　① 中间件传参是怎样完成的： app.UseMiddleware<Authorization>(AuthOption); 我们传参的时候，为什么能自动注入中间件构造函数非第1个参数？

　　 ② 编写中间件的时候，为什么必须要定义特定的 Invoke/InvokeAsync 函数？

　　 ③ 设定中间件的顺序很重要，中间件的嵌套顺序是怎么确定的？

思考标准中间件的行为：

输入下一个中间件的执行委托Next；
定义当前中间件的执行委托Invoke/InvokeAsync；

每个中间件完成了 Func<RequestDelegate，RequestDelegate>这样的行为；

通过参数next与下一个中间件的执行委托Invoke/InvokeAsync 建立"链式"关系。

public delegate Task RequestDelegate(HttpContext context);

//-----------------节选自 Microsoft.AspNetCore.Builder.UseMiddlewareExtensions------------------
        /// <summary>
        /// Adds a middleware type to the application's request pipeline.
        /// </summary>
        /// <typeparam name="TMiddleware">The middleware type.</typeparam>
        /// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
        /// <param name="args">The arguments to pass to the middleware type instance's constructor.</param>
        /// <returns>The <see cref="IApplicationBuilder"/> instance.</returns>
        public static IApplicationBuilder UseMiddleware<TMiddleware>(this IApplicationBuilder app, params object[] args)
        {
            return app.UseMiddleware(typeof(TMiddleware), args);
        }
        /// <summary>
        /// Adds a middleware type to the application's request pipeline.
        /// </summary>
        /// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
        /// <param name="middleware">The middleware type.</param>
        /// <param name="args">The arguments to pass to the middleware type instance's constructor.</param>
        /// <returns>The <see cref="IApplicationBuilder"/> instance.</returns>
        public static IApplicationBuilder UseMiddleware(this IApplicationBuilder app, Type middleware, params object[] args)
        {
            if (typeof(IMiddleware).GetTypeInfo().IsAssignableFrom(middleware.GetTypeInfo()))
            {
                // IMiddleware doesn't support passing args directly since it's
                // activated from the container
                if (args.Length > 0)
                {
                    throw new NotSupportedException(Resources.FormatException_UseMiddlewareExplicitArgumentsNotSupported(typeof(IMiddleware)));
                }
 
                return UseMiddlewareInterface(app, middleware);
            }
 
            var applicationServices = app.ApplicationServices;
            return app.Use(next =>
            {
                var methods = middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public);
// 执行委托名称被限制为Invoke/InvokeAsync
                var invokeMethods = methods.Where(m =>
                    string.Equals(m.Name, InvokeMethodName, StringComparison.Ordinal)
                    || string.Equals(m.Name, InvokeAsyncMethodName, StringComparison.Ordinal)
                    ).ToArray();
 
                if (invokeMethods.Length > 1)
                {
                    throw new InvalidOperationException(Resources.FormatException_UseMiddleMutlipleInvokes(InvokeMethodName, InvokeAsyncMethodName));
                }
 
                if (invokeMethods.Length == 0)
                {
                    throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoInvokeMethod(InvokeMethodName, InvokeAsyncMethodName, middleware));
                }
 
                var methodInfo = invokeMethods[0];
                if (!typeof(Task).IsAssignableFrom(methodInfo.ReturnType))
                {
                    throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNonTaskReturnType(InvokeMethodName, InvokeAsyncMethodName, nameof(Task)));
                }
 
                var parameters = methodInfo.GetParameters();
                if (parameters.Length == 0 || parameters[0].ParameterType != typeof(HttpContext))
                {
                    throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoParameters(InvokeMethodName, InvokeAsyncMethodName, nameof(HttpContext)));
                }
 
                var ctorArgs = new object[args.Length + 1];
                ctorArgs[0] = next;
                Array.Copy(args, 0, ctorArgs, 1, args.Length);
                //  通过反射形成中间件实例的时候，构造函数第一个参数被指定为 下一个中间件的执行委托
                var instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middleware, ctorArgs);
                if (parameters.Length == 1)
                {
                    return (RequestDelegate)methodInfo.CreateDelegate(typeof(RequestDelegate), instance);
                }
               
 // 当前执行委托除了可指定HttpContext参数以外， 还可以注入更多的依赖参数   
                var factory = Compile<object>(methodInfo, parameters);
 
                return context =>                                 
                {
                    var serviceProvider = context.RequestServices ?? applicationServices;
                    if (serviceProvider == null)
                    {
                        throw new InvalidOperationException(Resources.FormatException_UseMiddlewareIServiceProviderNotAvailable(nameof(IServiceProvider)));
                    }
 
                    return factory(instance, context, serviceProvider);
                };
            });
        }
 
//-------------------节选自 Microsoft.AspNetCore.Builder.Internal.ApplicationBuilder-------------------
private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();
 
publicIApplicationBuilder Use(Func<RequestDelegate,RequestDelegate> middleware)
{
    this._components.Add(middleware);
    return this;
}
 
public RequestDelegate Build()
{
     RequestDelegate app = context =>
     {
         context.Response.StatusCode = 404;
         return Task.CompletedTask;
     };
 
     foreach (var component in _components.Reverse())
    {
        app = component(app);
    }
 
   return app;
}