Create new top-level DefaultNodeInstance concept that will soon hold the "connection draining" logic

src/Microsoft.AspNetCore.NodeServices/Configuration/Configuration.cs

@@ -0,0 +1,58 @@
+using System;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.NodeServices.HostingModels;
+
+namespace Microsoft.AspNetCore.NodeServices
+{
+    public static class Configuration
+    {
+        public static void AddNodeServices(this IServiceCollection serviceCollection)
+            => AddNodeServices(serviceCollection, new NodeServicesOptions());
+
+        public static void AddNodeServices(this IServiceCollection serviceCollection, NodeServicesOptions options)
+        {
+            serviceCollection.AddSingleton(typeof(INodeServices), serviceProvider =>
+            {
+                // Since this instance is being created through DI, we can access the IHostingEnvironment
+                // to populate options.ProjectPath if it wasn't explicitly specified.
+                var hostEnv = serviceProvider.GetRequiredService<IHostingEnvironment>();
+                if (string.IsNullOrEmpty(options.ProjectPath))
+                {
+                    options.ProjectPath = hostEnv.ContentRootPath;
+                }
+
+                return new NodeServicesImpl(options, () => CreateNodeInstance(options));
+            });
+        }
+
+        public static INodeServices CreateNodeServices(NodeServicesOptions options)
+        {
+            return new NodeServicesImpl(options, () => CreateNodeInstance(options));
+        }
+
+        private static INodeInstance CreateNodeInstance(NodeServicesOptions options)
+        {
+            if (options.NodeInstanceFactory != null)
+            {
+                // If you've explicitly supplied an INodeInstance factory, we'll use that. This is useful for
+                // custom INodeInstance implementations.
+                return options.NodeInstanceFactory();
+            }
+            else
+            {
+                // Otherwise we'll construct the type of INodeInstance specified by the HostingModel property,
+                // which itself has a useful default value.
+                switch (options.HostingModel)
+                {
+                    case NodeHostingModel.Http:
+                        return new HttpNodeInstance(options.ProjectPath, /* port */ 0, options.WatchFileExtensions);
+                    case NodeHostingModel.Socket:
+                        return new SocketNodeInstance(options.ProjectPath, options.WatchFileExtensions);
+                    default:
+                        throw new ArgumentException("Unknown hosting model: " + options.HostingModel);
+                }
+            }
+        }
+    }
+}