ASP.NET Core Middleware抽丝剥茧

//  该接口只有一个固定的函数
public Task InvokeAsync(HttpContext context,RequestDelegate next);

public class FactoryActivatedMiddleware : IMiddleware
    {
        private readonly ILogger _logger;
        public FactoryActivatedMiddleware(ILoggerFactory logger)   // 
        {
            _logger = logger.CreateLogger<FactoryActivatedMiddleware>();
        }
        public async Task InvokeAsync(HttpContext context, RequestDelegate next)
        {
            // TODO  logic handler
            _logger.LogInformation("测试");
            await next(context);
            // TODO  logic handler
        }
    }

public class ConventionalMiddleware
{
    private readonly RequestDelegate _next;
    public ConventionalMiddleware(RequestDelegate next)
    {
        _next = next;
    }
    public async Task InvokeAsync(HttpContext context, AppDbContext db)
    {
        var keyValue = context.Request.Query["key"];
        if (!string.IsNullOrWhiteSpace(keyValue))
        {
            db.Add(new Request()
                {
                    DT = DateTime.UtcNow, 
                    MiddlewareActivation = "ConventionalMiddleware", 
                    Value = keyValue
                });
            await db.SaveChangesAsync();
        }
        await _next(context);
    }
}

//--------节选自 Microsoft.AspNetCore.Builder.Internal.ApplicationBuilder--------
private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();
public IApplicationBuilder Use(Func<RequestDelegate,RequestDelegate> middleware)
{
    this._components.Add(middleware);
    return this;
}
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 = StatusCodes.Status404NotFound;
                return Task.CompletedTask;
            };
            foreach (var component in _components.Reverse())
            {
                app = component(app);
            }
            return app;
}