阅读源码探索实现NetCore中间件

1.在Core2.2 Startup 中，Configure 方法写入中间件，app.Use

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    #region Middleware
    app.Use(next =>
    {
        Console.WriteLine("this is Middleware1");
        return new RequestDelegate(async context =>
        {
            context.Response.ContentType = "text/html; charset=utf-8";
            await context.Response.WriteAsync("<span><h3>This is Middleware1 start</h3></span>");
            await next.Invoke(context);
            await context.Response.WriteAsync("<span><h3>This is Middleware1 end</h3></span>");
        });
    });
    app.Use(next =>
    {
        return new RequestDelegate(async context =>
        {
            await context.Response.WriteAsync("<span><h3>This is Middleware2 start</h3></span>");
            await next.Invoke(context);
            await context.Response.WriteAsync("<span><h3>This is Middleware2 end</h3></span>");
        });
    });
    app.Use(next =>
    {
        Console.WriteLine("this is Middleware3");
        return new RequestDelegate(async context =>
        {
            await context.Response.WriteAsync("<span><h3>This is Middleware3 start</h3></span>");
            await next.Invoke(context);
            await context.Response.WriteAsync("<span><h3>This is Middleware3 end</h3></span>");
        });
    });
    app.Run(async context => await context.Response.WriteAsync("跑完了.")); //这一句得加上，才能看出
    #endregion
    if (env.IsDevelopment()) app.UseDeveloperExceptionPage();
    else
    {
        app.UseExceptionHandler("/Home/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseMvc(routes =>
    {
        routes.MapRoute(
            name: "default",
            template: "{controller=Home}/{action=Index}/{id?}");
    });
}

2.调试运行输出

3.通过查看源码，得知app.Use方法其实去添加了一个Func委托到 IList<Func<RequestDelegate, RequestDelegate>>,

public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware)
{
    //_components 就是这个 private readonly IList<Func<RequestDelegate, RequestDelegate>> _components 
    //= new List<Func<RequestDelegate, RequestDelegate>>();
   _components.Add(middleware);// _components 
   return this;
}

接着看处理中间件的逻辑看这段代码

public RequestDelegate Build()
{
    RequestDelegate app = context =>
    {
        // If we reach the end of the pipeline, but we have an endpoint, then something unexpected has happened.
        // This could happen if user code sets an endpoint, but they forgot to add the UseEndpoint middleware.
        var endpoint = context.GetEndpoint();
        var endpointRequestDelegate = endpoint?.RequestDelegate;
        if (endpointRequestDelegate != null)
        {
            var message =
                $"The request reached the end of the pipeline without executing the endpoint: '{endpoint.DisplayName}'. " +
                $"Please register the EndpointMiddleware using '{nameof(IApplicationBuilder)}.UseEndpoints(...)' if using " +
                $"routing.";
            throw new InvalidOperationException(message);
        }

        context.Response.StatusCode = 404;
        return Task.CompletedTask;
    };

    foreach (var component in _components.Reverse())
    {
        app = component(app);
    }
    return app;
}

透过这段源码可以发现，处理整个中间件逻辑是，所有的中间件被存入到集合 _components ，核心就是把这个集合，转换成单个委托 RequestDelege 。方便调用者调用，执行委托里面的方法。
借助于这个想法和思路，自己来写个demo
新建一个Core测试程序：

namespace Test
{
    /// <summary>
    /// 给入字符串，返回 Task
    /// </summary>
    /// <param name="str"></param>
    /// <returns></returns>
    public delegate Task Conduit(string str);

    public class MiddlewareDemo
    {
        [Test]
        public void Implement()
        {
            CustomMiddleware middle = new CustomMiddleware();

            #region C

            middle.Add(next =>
            {
                return zfc =>
               {
                   Console.WriteLine("第一阶段开始");
                   System.Diagnostics.Debug.WriteLine("第一阶段开始");
                   return next(zfc);
               };
            });

            middle.Add(next =>
            {
                return zfc =>
               {
                   Console.WriteLine("第二阶段开始");
                   System.Diagnostics.Debug.WriteLine("第二阶段开始");
                   return next(zfc);
               };
            });

            middle.Add(next =>
            {
                return zfc =>
               {
                   Console.WriteLine("第三阶段开始");
                   System.Diagnostics.Debug.WriteLine("第三阶段开始");
                   return next(zfc);
               };
            });
            #endregion

            #region D
            //middle.Add(next =>
            //{
            //    return async zfc =>
            //    {
            //        Console.WriteLine("第一阶段开始");
            //        System.Diagnostics.Debug.WriteLine("第一阶段开始");
            //        await next(zfc);
            //        Console.WriteLine("第一阶段结束");
            //        System.Diagnostics.Debug.WriteLine("第一阶段结束");
            //    };
            //});

            //middle.Add(next =>
            //{
            //    return async zfc =>
            //    {
            //        Console.WriteLine("第二阶段开始");
            //        System.Diagnostics.Debug.WriteLine("第二阶段开始");
            //        await next(zfc);
            //        System.Diagnostics.Debug.WriteLine("第二阶段结束");
            //        Console.WriteLine("第二阶段结束");
            //    };
            //});

            //middle.Add(next =>
            //{
            //    return async zfc =>
            //    {
            //        Console.WriteLine("第三阶段开始");
            //        System.Diagnostics.Debug.WriteLine("第三阶段开始");
            //        await next(zfc);
            //        System.Diagnostics.Debug.WriteLine("第三阶段结束");
            //        Console.WriteLine("第三阶段结束");
            //    };
            //});
            #endregion
            var ak = middle.GetDelegate();
            ak.Invoke("正序执行中间件"); 
        System.Diagnostics.Debug.WriteLine("**********分界线**********");
            var ff = middle.Reverse();
            ff.Invoke("开始执行中间件！");

            System.Diagnostics.Debug.WriteLine("跑完了ddd");
            Console.WriteLine("跑完了ddd");

        }
    }

    /// <summary>
    /// 模拟中间件
    /// </summary>
    public class CustomMiddleware
    {
        public IList<Func<Conduit, Conduit>> _middlelist = new List<Func<Conduit, Conduit>>();
        // Add也就是NetCore中StartUp中的 app.Use ，同理     
        public void Add(Func<Conduit, Conduit> func)
        {
            _middlelist.Add(func);
        }

        //为了方便理解做比较，这个没有集合倒序
        //把集合整合成一个 Conduit委托
        public Conduit GetDelegate()
        {
            Conduit conduit = str =>
             {
                 str = "初始化!";
                 return Task.CompletedTask;
             };
            foreach (Func<Conduit, Conduit> item in _middlelist)//正序
              //核心在这里，看似在赋新值，其实是在一层又一层包裹这一个执行方法实例，
              //给人感觉就像在叠加一样，如果这个地方没有倒序，那么执行方法顺序就是反的
               conduit = item(conduit); 
            return conduit;
        }

        /// <summary>
        /// 把集合进行倒叙，然后整合成一个 Conduit委托
        /// </summary>
        /// <returns></returns>
        public Conduit Reverse()
        {
            Conduit app = str =>
             {
                 str = "初始化!";
                 return Task.CompletedTask;
             };
            foreach (var component in _middlelist.Reverse())//倒序
                app = component(app);
            return app;
        }
        /*
           public RequestDelegate Build()
           {

            IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();

               RequestDelegate app = context =>
               {
                   // If we reach the end of the pipeline, but we have an endpoint, then something unexpected has happened.
                   // This could happen if user code sets an endpoint, but they forgot to add the UseEndpoint middleware.
                   var endpoint = context.GetEndpoint();
                   var endpointRequestDelegate = endpoint?.RequestDelegate;
                   if (endpointRequestDelegate != null)
                   {
                       var message =
                           $"The request reached the end of the pipeline without executing the endpoint: '{endpoint.DisplayName}'. " +
                           $"Please register the EndpointMiddleware using '{nameof(IApplicationBuilder)}.UseEndpoints(...)' if using " +
                           $"routing.";
                       throw new InvalidOperationException(message);
                   }

                   context.Response.StatusCode = 404;
                   return Task.CompletedTask;
               };

               foreach (var component in _components.Reverse())
               {
                   app = component(app);
               }
               return app;
           }
           */
    }
}