探究ASP.NET Core Middleware实现方法

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();

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;}