ASP.NET Core中間件計算Http請求時間

ASP.NET Core經過RequestDelegate這個委託類型來定義中間件

public delegate Task RequestDelegate(HttpContext context);

可將一個單獨的請求委託並行指定爲匿名方法（稱爲並行中間件），或在類中對其進行定義。可經過Use，或在Middleware類中配置要傳遞給委託執行的方法(參數類型HttpContext，返回值類型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);

經過定義一箇中間件類 來計算http請求的時間，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);
    }
}