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;
}