Perspective Vue CLI middleware that uses the generic npm middleware

VueDevelopmentServerMiddleware .cs

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.SpaServices;
using Microsoft.AspNetCore.SpaServices.Npm;

namespace ASP.NETCoreWebApplication
{
    /// <summary>
    /// Extension methods for enabling React development server middleware support.
    /// </summary>
    public static class VueDevelopmentServerMiddlewareExtensions
    {
        /// <summary>
        /// Handles requests by passing them through to an instance of the Vue CLI server.
        /// This means you can always serve up-to-date CLI-built resources without having
        /// to run the Vue CLI server manually.
        ///
        /// This feature should only be used in development. For production deployments, be
        /// sure not to enable the Vue CLI server.
        /// </summary>
        /// <param name="spaBuilder">The <see cref="ISpaBuilder"/>.</param>
        /// <param name="npmScript">The name of the script in your package.json file that launches the Vue CLI server.</param>
        public static void UseVueDevelopmentServer(
            this ISpaBuilder spaBuilder,
            string npmScript)
        {
            if (spaBuilder == null)
            {
                throw new ArgumentNullException(nameof(spaBuilder));
            }

            var spaOptions = spaBuilder.Options;

            if (string.IsNullOrEmpty(spaOptions.SourcePath))
            {
                throw new InvalidOperationException(
                    $"To use {nameof(UseVueDevelopmentServer)}, you must supply a non-empty value for the {nameof(SpaOptions.SourcePath)} property of {nameof(SpaOptions)} when calling {nameof(SpaApplicationBuilderExtensions.UseSpa)}.");
            }

            VueDevelopmentServerMiddleware.Attach(spaBuilder, npmScript);
        }
    }

    internal static class VueDevelopmentServerMiddleware
    {
        private static TimeSpan
            RegexMatchTimeout =
                TimeSpan.FromSeconds(5); // This is a development-time only feature, so a very long timeout is fine

        public static void Attach(
            ISpaBuilder spaBuilder,
            string npmScriptName)
        {
            NpmMiddleware.Attach(spaBuilder, npmScriptName, port => null, EnvVars,
                new Regex("Starting development server", RegexOptions.None, RegexMatchTimeout),
                (match, port) => new UriBuilder("http", "localhost", port).Uri);

            IDictionary<string, string> EnvVars(int portNumber) =>
                new Dictionary<string, string>
                {
                    {"PORT", portNumber.ToString()},
                    {
                        "BROWSER", "none"
                    } // We don't want Vie CLI to open its own extra browser window pointing to the internal dev server port
                };
        }
    }
}