ASP.NET Core中间件计算Http请求时间

ASP.NET Core通过RequestDelegate这个委托类型来定义中间件

public delegate Task RequestDelegate(HttpContext context);

可将一个单独的请求委托并行指定为匿名方法（称为并行中间件），或在类中对其进行定义。可通过Use，或在Middleware类中配置要传递给委托执行的方法(参数类型HttpContext，返回值类型Task)。

public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware);

public static IApplicationBuilder UseMiddleware<TMiddleware>(this IApplicationBuilder app, params object[] args);

通过定义一个中间件类 来计算http请求的时间，例：

public class ResponseTimeMiddleware
{
    // Name of the Response Header, Custom Headers starts with "X-"  
    private const string RESPONSE_HEADER_RESPONSE_TIME = "X-Response-Time-ms";
    // Handle to the next Middleware in the pipeline  
    private readonly RequestDelegate _next;
    public ResponseTimeMiddleware(RequestDelegate next)
    {
        _next = next;
    }
    public Task InvokeAsync(HttpContext context)
    {
        // Start the Timer using Stopwatch  
        var watch = new Stopwatch();
        watch.Start();
        context.Response.OnStarting(() => {
            // Stop the timer information and calculate the time   
            watch.Stop();
            var responseTimeForCompleteRequest = watch.ElapsedMilliseconds;
            // Add the Response time information in the Response headers.   
            context.Response.Headers[RESPONSE_HEADER_RESPONSE_TIME] = responseTimeForCompleteRequest.ToString();
            return Task.CompletedTask;
        });
        // Call the next delegate/middleware in the pipeline   
        return this._next(context);
    }
}