Reduce string allocations in PrimeCache

Return Task<IHtmlContent> instead of Task<HtmlString> and write
directly to the TextWriter instead of concatenating/formatting
strings.

Author: justinvp

src/Microsoft.AspNetCore.AngularServices/PrimeCacheHelper.cs

@@ -1,17 +1,19 @@
 using System;
+using System.IO;
 using System.Net;
 using System.Net.Http;
+using System.Text.Encodings.Web;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Mvc.Rendering;
 using Microsoft.AspNetCore.Html;
+using Microsoft.AspNetCore.Mvc.Rendering;
 using Microsoft.Extensions.Logging;
 using Newtonsoft.Json;
 
 namespace Microsoft.AspNetCore.AngularServices
 {
     public static class PrimeCacheHelper
     {
-        public static async Task<HtmlString> PrimeCache(this IHtmlHelper html, string url)
+        public static async Task<IHtmlContent> PrimeCache(this IHtmlHelper html, string url)
         {
             // TODO: Consider deduplicating the PrimeCache calls (that is, if there are multiple requests to precache
             // the same URL, only return nonempty for one of them). This will make it easier to auto-prime-cache any
@@ -33,26 +35,55 @@ public static async Task<HtmlString> PrimeCache(this IHtmlHelper html, string ur
                     request.Path.ToUriComponent(),
                     request.QueryString.ToUriComponent());
                 var fullUri = new Uri(new Uri(baseUriString), url);
-                var response = await new HttpClient().GetAsync(fullUri.ToString());
-                var responseBody = await response.Content.ReadAsStringAsync();
-                return new HtmlString(FormatAsScript(url, response.StatusCode, responseBody));
+                var response = await new HttpClient().GetAsync(fullUri.ToString()).ConfigureAwait(false);
+                var responseBody = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
+                return new PrimeCacheScript(url, response.StatusCode, responseBody);
             }
             catch (Exception ex)
             {
                 var logger = (ILogger)html.ViewContext.HttpContext.RequestServices.GetService(typeof(ILogger));
                 logger?.LogWarning("Error priming cache for URL: " + url, ex);
-                return new HtmlString(string.Empty);
+                return HtmlString.Empty;
             }
         }
 
-        private static string FormatAsScript(string url, HttpStatusCode responseStatusCode, string responseBody)
+        private sealed class PrimeCacheScript : IHtmlContent
         {
-            var preCachedUrl = JsonConvert.SerializeObject(url);
-            var preCachedJson = JsonConvert.SerializeObject(new { statusCode = responseStatusCode, body = responseBody });
-            return "<script>"
-                + "window.__preCachedResponses = window.__preCachedResponses || {};"
-                + $"window.__preCachedResponses[{preCachedUrl}] = {preCachedJson};"
-                + "</script>";
+            private readonly string _url;
+            private readonly HttpStatusCode _responseStatusCode;
+            private readonly string _responseBody;
+
+            public PrimeCacheScript(string url, HttpStatusCode responseStatusCode, string responseBody)
+            {
+                _url = url;
+                _responseStatusCode = responseStatusCode;
+                _responseBody = responseBody;
+            }
+
+            // These properties exist to be serialized as JSON without having to allocate an anonymous object.
+            public HttpStatusCode statusCode => _responseStatusCode;
+            public string body => _responseBody;
+
+            public void WriteTo(TextWriter writer, HtmlEncoder encoder)
+            {
+                if (writer == null)
+                {
+                    throw new ArgumentNullException(nameof(writer));
+                }
+
+                if (encoder == null)
+                {
+                    throw new ArgumentNullException(nameof(encoder));
+                }
+
+                var serializer = new JsonSerializer();
+                var jsonWriter = new JsonTextWriter(writer);
+                jsonWriter.WriteRaw("<script>window.__preCachedResponses=window.__preCachedResponses||{},window.__preCachedResponses[");
+                serializer.Serialize(jsonWriter, _url);
+                jsonWriter.WriteRaw("]=");
+                serializer.Serialize(jsonWriter, this);
+                jsonWriter.WriteRaw(";</script>");
+            }
         }
     }
 }