class Program
    {
        static void Main(string[] args)
        {
            IApplicationBuilder appBuidler=new ApplicationBuilder(null);
            appBuidler.UseMiddleware<LogMiddleWare>();
            appBuidler.Use(async (context,next)=>{
               await Console.Out.WriteLineAsync("Call before next.");
               await next(context);
               await Console.Out.WriteLineAsync("Call after next.");
            });

            appBuidler.Run(async (context)=>{
               await Console.Out.WriteLineAsync("hello world!");
            });

            var app=appBuidler.Build();

            var response=app.Invoke(new RequestContext(){ Url="http://localhost"});
            response.Wait();
            Console.ReadLine();
        }
    }

    //LogMiddleWare类
    public class LogMiddleWare
    {
        private readonly RequestDelegate _next;
        public LogMiddleWare(RequestDelegate next)
        {
            if (next == null)
            {
                throw new ArgumentNullException(nameof(next));
            }

            _next = next;
        }
        
        public async Task Invoke(RequestContext context){
            await Console.Out.WriteLineAsync("before log!");
            await _next(context);
            await Console.Out.WriteLineAsync("after log!");
        }
    }

//RequestContext 类模拟了请求上下文
public class RequestContext
{
   public string Url{ get; set;}
}
//RequestDelegate模拟了这一类请求的委托
public delegate Task RequestDelegate(RequestContext context);
//IApplicationBuilder用来创建注册委托和并创建应用
public interface IApplicationBuilder
{
    IServiceProvider ApplicationServices{get;set;}
    void Add(Func<RequestDelegate,RequestDelegate> middleware);
    RequestDelegate Build();
}
//ApplicationBuilderExtension对IApplication进行了一些方法的扩展，UseMiddleWare,Use,Run等方法
void UseMiddleware<T>(this IApplicationBuilder builder, params object[] args)
void Use(this IApplicationBuilder builder,Func<RequestContext,RequestDelegate,Task> func)
void Run(this IApplicationBuilder builder,Func<RequestContext,Task> last)

static void Main(string[] args)
{
    var host = new WebHostBuilder()
       .UseTestServer()
       .Configure(app =>
       {
           app.UseMiddleware<LogMiddleWare>();
           app.Use(async (context, next) => {
               await Console.Out.WriteLineAsync(context.Url + "Call before next.");
               await next(context);
               await Console.Out.WriteLineAsync(context.Url + "Call after next.");
           });
           app.Run(async (context) => {
               await Console.Out.WriteLineAsync("hello world!");
           });
       })
       .Build()
    host.Run();
    Console.ReadLine();
}