Move React server-side rendering into more general SpaServices package

src/Microsoft.AspNet.SpaServices/Prerendering/PrerenderTagHelper.cs

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+using System;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.AspNet.Http;
+using Microsoft.AspNet.Http.Extensions;
+using Microsoft.AspNet.NodeServices;
+using Microsoft.AspNet.Razor.TagHelpers;
+using Microsoft.Extensions.PlatformAbstractions;
+using Newtonsoft.Json;
+
+namespace Microsoft.AspNet.SpaServices.Prerendering
+{
+    [HtmlTargetElement(Attributes = PrerenderModuleAttributeName)]
+    public class PrerenderTagHelper : TagHelper
+    {
+        static INodeServices fallbackNodeServices; // Used only if no INodeServices was registered with DI
+        
+        const string PrerenderModuleAttributeName = "asp-prerender-module";
+        const string PrerenderExportAttributeName = "asp-prerender-export";
+        
+        [HtmlAttributeName(PrerenderModuleAttributeName)]
+        public string ModuleName { get; set; }
+        
+        [HtmlAttributeName(PrerenderExportAttributeName)]
+        public string ExportName { get; set; }
+
+        private IHttpContextAccessor contextAccessor;
+        private INodeServices nodeServices;
+
+        public PrerenderTagHelper(IServiceProvider serviceProvider, IHttpContextAccessor contextAccessor)
+        {
+            this.contextAccessor = contextAccessor;
+            this.nodeServices = (INodeServices)serviceProvider.GetService(typeof (INodeServices)) ?? fallbackNodeServices;
+            
+            // Consider removing the following. Having it means you can get away with not putting app.AddNodeServices()
+            // in your startup file, but then again it might be confusing that you don't need to.
+            if (this.nodeServices == null) {
+                var appEnv = (IApplicationEnvironment)serviceProvider.GetService(typeof(IApplicationEnvironment));
+                this.nodeServices = fallbackNodeServices = Configuration.CreateNodeServices(NodeHostingModel.Http, appEnv.ApplicationBasePath);
+            }
+        }
+
+        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
+        {
+            var request = this.contextAccessor.HttpContext.Request;
+            var result = await Prerenderer.RenderToString(
+                nodeServices: this.nodeServices,
+                componentModuleName: this.ModuleName,
+                componentExportName: this.ExportName,
+                requestAbsoluteUrl: UriHelper.GetEncodedUrl(this.contextAccessor.HttpContext.Request),
+                requestPathAndQuery: request.Path + request.QueryString.Value);
+            output.Content.SetHtmlContent(result.Html);
+            
+            // Also attach any specific globals to the 'window' object. This is useful for transferring
+            // general state between server and client.
+            if (result.Globals != null) {
+                var stringBuilder = new StringBuilder();
+                foreach (var property in result.Globals.Properties()) {
+                    stringBuilder.AppendFormat("window.{0} = {1};",
+                        property.Name,
+                        property.Value.ToString(Formatting.None));
+                }
+                if (stringBuilder.Length > 0) {
+                    output.PostElement.SetHtmlContent($"<script>{ stringBuilder.ToString() }</script>");
+                }
+            }
+        }
+    }
+}

src/Microsoft.AspNet.SpaServices/Prerendering/Prerenderer.cs

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+using System;
+using System.Threading.Tasks;
+using Microsoft.AspNet.NodeServices;
+using Newtonsoft.Json.Linq;
+
+namespace Microsoft.AspNet.SpaServices.Prerendering
+{
+    public static class Prerenderer
+    {
+        private static Lazy<StringAsTempFile> nodeScript;
+        
+        static Prerenderer() {
+            nodeScript = new Lazy<StringAsTempFile>(() => {
+                var script = EmbeddedResourceReader.Read(typeof(Prerenderer), "/Content/Node/prerenderer.js");
+                return new StringAsTempFile(script); // Will be cleaned up on process exit
+            });
+        }
+        
+        public static async Task<RenderToStringResult> RenderToString(INodeServices nodeServices, string componentModuleName, string componentExportName, string requestAbsoluteUrl, string requestPathAndQuery) {
+            return await nodeServices.InvokeExport<RenderToStringResult>(nodeScript.Value.FileName, "renderToString", 
+                /* bootModulePath */ componentModuleName,
+                /* bootModuleExport */ componentExportName,
+                /* absoluteRequestUrl */ requestAbsoluteUrl,
+                /* requestPathAndQuery */ requestPathAndQuery);
+        }
+    }
+    
+    public class RenderToStringResult {
+        public string Html;
+        public JObject Globals;
+    }
+}