app.Use(async (context, next) =>
{
    var endpoint = context.Features.Get<IEndpointFeature>()?.Endpoint;
    if (endpoint != null)
    {
        ResponseCacheAttribute responseCache = endpoint.Metadata.GetMetadata<ResponseCacheAttribute>();
        if (responseCache != null)
        {
            //做一些事情
        }
    }
    await next();
});

public class RequestCultureMiddleware
{
    private readonly RequestDelegate _next;
    public RequestCultureMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        var cultureQuery = context.Request.Query["culture"];
        if (!string.IsNullOrWhiteSpace(cultureQuery))
        {
            var culture = new CultureInfo(cultureQuery);
            CultureInfo.CurrentCulture = culture;
            CultureInfo.CurrentUICulture = culture;
        }
        await _next(context);
    }
}

app.UseMiddleware<RequestCultureMiddleware>();

public class RequestCultureOtherMiddleware:IMiddleware
{
    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        var cultureQuery = context.Request.Query["culture"];
        if (!string.IsNullOrWhiteSpace(cultureQuery))
        {
            var culture = new CultureInfo(cultureQuery);
            CultureInfo.CurrentCulture = culture;
            CultureInfo.CurrentUICulture = culture;
        }
        await next(context);
    }
}

services.AddSingleton<IMiddleware,RequestCultureOtherMiddleware>();

app.UseMiddleware<RequestCultureOtherMiddleware>();

public interface IMiddleware
{
 /// <summary>
 /// 請(qǐng)求處理方法
 /// </summary>
 /// <param name="context">當(dāng)前請(qǐng)求上下文</param>
 /// <param name="next">請(qǐng)求管道中下一個(gè)中間件的委托</param>
 Task InvokeAsync (HttpContext context, RequestDelegate next);
}

/// <summary> 
/// 將中間件委托添加到應(yīng)用程序的請(qǐng)求管道。
/// </summary> 
/// <param name="middleware">中間件委托</param> 
/// <returns>The <see cref="IApplicationBuilder"/>.</returns> 
IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware); 

public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware)
{
   //將middleware轉(zhuǎn)換為Use(Func<RequestDelegate, RequestDelegate> middleware)的形式
    return app.Use(next =>
    {
        return context =>
        {
            Func<Task> simpleNext = () => next(context);
            return middleware(context, simpleNext);
        };
    });
}

/// <summary> 
/// 將中間件類(lèi)型添加到應(yīng)用程序的請(qǐng)求管道.
/// </summary> 
/// <typeparam name="TMiddleware">中間件類(lèi)型</typeparam> 
/// <param name="args">傳遞給中間件類(lèi)型實(shí)例的構(gòu)造函數(shù)的參數(shù).</param> 
/// <returns>The <see cref="IApplicationBuilder"/> instance.</returns> 
public static IApplicationBuilder UseMiddleware<[DynamicallyAccessedMembers(MiddlewareAccessibility)]TMiddleware>(this IApplicationBuilder app, params object[] args) 
{ 
    return app.UseMiddleware(typeof(TMiddleware), args); 
} 

internal const string InvokeMethodName = "Invoke"; 
internal const string InvokeAsyncMethodName = "InvokeAsync"; 

public static IApplicationBuilder UseMiddleware(this IApplicationBuilder app, [DynamicallyAccessedMembers(MiddlewareAccessibility)] Type middleware, params object[] args)
{
    //判斷自定義的中間件是否是實(shí)現(xiàn)了IMiddleware接口
    if (typeof(IMiddleware).GetTypeInfo().IsAssignableFrom(middleware.GetTypeInfo()))
    {
        //Middleware不支持直接傳遞參數(shù)
        //因?yàn)樗亲?cè)到容器中的，所以不能通過(guò)構(gòu)造函數(shù)傳遞自定義的參數(shù)，否則拋出異常
        if (args.Length > 0)
        {
            throw new NotSupportedException(Resources.FormatException_UseMiddlewareExplicitArgumentsNotSupported(typeof(IMiddleware)));
        }
        //實(shí)現(xiàn)IMiddleware接口的中間件走的是這個(gè)邏輯，咱們待會(huì)看
        return UseMiddlewareInterface(app, middleware);
    }

    var applicationServices = app.ApplicationServices;
    return app.Use(next =>
    {
        //獲取自定義中間件類(lèi)的非靜態(tài)public方法
        var methods = middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public);
        //查找方法名為Invoke或InvokeAsync的方法
        var invokeMethods = methods.Where(m =>
            string.Equals(m.Name, InvokeMethodName, StringComparison.Ordinal)
            || string.Equals(m.Name, InvokeAsyncMethodName, StringComparison.Ordinal)
            ).ToArray();
        //方法名為Invoke或InvokeAsync的方法只能有有一個(gè)，存在多個(gè)話會(huì)拋出異常
        if (invokeMethods.Length > 1)
        {
            throw new InvalidOperationException(Resources.FormatException_UseMiddleMutlipleInvokes(InvokeMethodName, InvokeAsyncMethodName));
        }
        //自定義的中間件類(lèi)中必須包含名為Invoke或InvokeAsync的方法，否則也會(huì)拋出異常
        if (invokeMethods.Length == 0)
        {
            throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoInvokeMethod(InvokeMethodName, InvokeAsyncMethodName, middleware));
        }
        //名為Invoke或InvokeAsync的方法的返回值類(lèi)型必須是Task類(lèi)型，否則會(huì)拋出異常
        var methodInfo = invokeMethods[0];
        if (!typeof(Task).IsAssignableFrom(methodInfo.ReturnType))
        {
            throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNonTaskReturnType(InvokeMethodName, InvokeAsyncMethodName, nameof(Task)));
        }
        //獲取Invoke或InvokeAsync方法的參數(shù)
        var parameters = methodInfo.GetParameters();
        //如果該方法不存在參數(shù)或方法的第一個(gè)參數(shù)不是HttpContext類(lèi)型的實(shí)例，會(huì)拋出異常
        if (parameters.Length == 0 || parameters[0].ParameterType != typeof(HttpContext))
        {
            throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoParameters(InvokeMethodName, InvokeAsyncMethodName, nameof(HttpContext)));
        }
        //定義新的數(shù)組比傳遞的參數(shù)長(zhǎng)度多一個(gè)，為啥呢？往下看。
        var ctorArgs = new object[args.Length + 1];
        //因?yàn)榉椒〝?shù)組的首元素是RequestDelegate類(lèi)型的next
        //也就是基于約定定義的中間件構(gòu)造函數(shù)的第一個(gè)參數(shù)是RequestDelegate類(lèi)型的實(shí)例
        ctorArgs[0] = next;
        Array.Copy(args, 0, ctorArgs, 1, args.Length);
        //創(chuàng)建基于約定的中間件實(shí)例
        //又看到ActivatorUtilities這個(gè)類(lèi)了，關(guān)于這個(gè)類(lèi)有興趣的可以研究一下，可以根據(jù)容器創(chuàng)建類(lèi)型實(shí)例，非常好用
        var instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middleware, ctorArgs);
        //如果Invoke或InvokeAsync方法只有一個(gè)參數(shù)，則直接創(chuàng)建RequestDelegate委托返回
        if (parameters.Length == 1)
        {
            //RequestDelegate其實(shí)就是public delegate Task RequestDelegate(HttpContext context);
            return (RequestDelegate)methodInfo.CreateDelegate(typeof(RequestDelegate), instance);
        }
        //編譯Invoke或InvokeAsync方法，關(guān)于Compile的實(shí)現(xiàn)等會(huì)咱們?cè)倏?
        var factory = Compile<object>(methodInfo, parameters);
        //返回這個(gè)委托
        //看著這個(gè)委托的格式有點(diǎn)眼熟，其實(shí)就是RequestDelegate即public delegate Task RequestDelegate(HttpContext context);
        return context =>
        {
            var serviceProvider = context.RequestServices ?? applicationServices;
            //serviceProvider不能為空,否則沒(méi)法玩了
            if (serviceProvider == null)
            {
                throw new InvalidOperationException(Resources.FormatException_UseMiddlewareIServiceProviderNotAvailable(nameof(IServiceProvider)));
            }
            //返回委托執(zhí)行結(jié)果
            return factory(instance, context, serviceProvider);
        };
    });
}

private static IApplicationBuilder UseMiddlewareInterface(IApplicationBuilder app, [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicConstructors)] Type middlewareType)
{
    return app.Use(next =>
    {
        return async context =>
        {
            var middlewareFactory = (IMiddlewareFactory?)context.RequestServices.GetService(typeof(IMiddlewareFactory));
            if (middlewareFactory == null)
            {
                // 沒(méi)有middlewarefactory直接拋出異常
                throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoMiddlewareFactory(typeof(IMiddlewareFactory)));
            }
            //創(chuàng)建middleware實(shí)例
            var middleware = middlewareFactory.Create(middlewareType);
            if (middleware == null)
            {
                throw new InvalidOperationException(Resources.FormatException_UseMiddlewareUnableToCreateMiddleware(middlewareFactory.GetType(), middlewareType));
            }

            try
            {
                //執(zhí)行middleware的InvokeAsync方法
                await middleware.InvokeAsync(context, next);
            }
            finally
            {
                //釋放middleware
                middlewareFactory.Release(middleware);
            }
        };
    });
}

public class MiddlewareFactory : IMiddlewareFactory
{
    private readonly IServiceProvider _serviceProvider;

    public MiddlewareFactory(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public IMiddleware? Create(Type middlewareType)
    {
        //根據(jù)類(lèi)型從容器中獲取IMiddleware實(shí)例
        return _serviceProvider.GetRequiredService(middlewareType) as IMiddleware;
    }

    public void Release(IMiddleware middleware)
    {
        //因?yàn)槿萜骺刂屏藢?duì)象的生命周期，所以這里啥也沒(méi)有
    }
}

private static readonly MethodInfo GetServiceInfo = typeof(UseMiddlewareExtensions).GetMethod(nameof(GetService), BindingFlags.NonPublic | BindingFlags.Static)!;

private static object GetService(IServiceProvider sp, Type type, Type middleware)
{
    var service = sp.GetService(type);
    if (service == null)
    {
        throw new InvalidOperationException(Resources.FormatException_InvokeMiddlewareNoService(type, middleware));
    }
    return service;
}

private static Func<T, HttpContext, IServiceProvider, Task> Compile<T>(MethodInfo methodInfo, ParameterInfo[] parameters)
{
    var middleware = typeof(T);
    //構(gòu)建三個(gè)Parameter名為httpContext、serviceProvider、middleware
    var httpContextArg = Expression.Parameter(typeof(HttpContext), "httpContext");
    var providerArg = Expression.Parameter(typeof(IServiceProvider), "serviceProvider");
    var instanceArg = Expression.Parameter(middleware, "middleware");

    //穿件Expression數(shù)組，且數(shù)組第一個(gè)參數(shù)為httpContextArg
    var methodArguments = new Expression[parameters.Length];
    methodArguments[0] = httpContextArg;
    //因?yàn)镮nvoke或InvokeAsync方法第一個(gè)參數(shù)為HttpContext，且methodArguments第一個(gè)參數(shù)占位，所以跳過(guò)第一個(gè)參數(shù)
    for (int i = 1; i < parameters.Length; i++)
    {
        //獲取方法參數(shù)
        var parameterType = parameters[i].ParameterType;
        //不支持ref類(lèi)型操作
        if (parameterType.IsByRef)
        {
            throw new NotSupportedException(Resources.FormatException_InvokeDoesNotSupportRefOrOutParams(InvokeMethodName));
        }
       
       //構(gòu)建參數(shù)類(lèi)型表達(dá)式，即用戶(hù)構(gòu)建方法參數(shù)的操作
        var parameterTypeExpression = new Expression[]
        {
            providerArg,
            Expression.Constant(parameterType, typeof(Type)),
            Expression.Constant(methodInfo.DeclaringType, typeof(Type))
        };
        //聲明調(diào)用GetServiceInfo的表達(dá)式
        var getServiceCall = Expression.Call(GetServiceInfo, parameterTypeExpression);
        //將getServiceCall操作轉(zhuǎn)換為parameterType
        methodArguments[i] = Expression.Convert(getServiceCall, parameterType);
    }
    //獲取中間件類(lèi)型表達(dá)式
    Expression middlewareInstanceArg = instanceArg;
    if (methodInfo.DeclaringType != null && methodInfo.DeclaringType != typeof(T))
    {
        //轉(zhuǎn)換中間件類(lèi)型表達(dá)式類(lèi)型與聲明類(lèi)型一致
        middlewareInstanceArg = Expression.Convert(middlewareInstanceArg, methodInfo.DeclaringType);
    }
    //調(diào)用middlewareInstanceArg(即當(dāng)前中間件)的methodInfo(即獲取Invoke或InvokeAsync)方法參數(shù)(methodArguments)
    var body = Expression.Call(middlewareInstanceArg, methodInfo, methodArguments);
    //轉(zhuǎn)換為lambda
    var lambda = Expression.Lambda<Func<T, HttpContext, IServiceProvider, Task>>(body, instanceArg, httpContextArg, providerArg);
    return lambda.Compile();
}

public class Middleware
{
    public Task Invoke(HttpContext context, ILoggerFactory loggerFactory)
    {
    }
}

Task Invoke(Middleware instance, HttpContext httpContext, IServiceProvider provider)
{
    return instance.Invoke(httpContext, (ILoggerFactory)UseMiddlewareExtensions.GetService(provider, typeof(ILoggerFactory));
}

public RequestDelegate Build()
{
    //最后的管道處理，即請(qǐng)求未能匹配到任何終結(jié)點(diǎn)的情況
    RequestDelegate app = context =>
    {
        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);
        }
        //執(zhí)行管道的重點(diǎn)是404,只有未命中任何終結(jié)點(diǎn)的情況下才會(huì)走到這里
        context.Response.StatusCode = StatusCodes.Status404NotFound;
        return Task.CompletedTask;
    };
    //_components即我們通過(guò)Use添加的中間件
    foreach (var component in _components.Reverse())
    {
       //得到執(zhí)行結(jié)果即RequestDelegate
        app = component(app);
    }
    //返回第一個(gè)管道中間件
    return app;
}