Implement cache priming prototype

Microsoft.AspNet.AngularServices/PrimeCacheHelper.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Net;
+using System.Net.Http;
+using System.Threading.Tasks;
+using Microsoft.AspNet.Mvc.Rendering;
+using Microsoft.Extensions.Logging;
+using Newtonsoft.Json;
+
+namespace Microsoft.AspNet.AngularServices {
+    public static class PrimeCacheHelper {
+        public static async Task<HtmlString> 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
+            // HTTP requests made during server-side rendering, without risking unnecessary duplicate requests.
+            
+            if (string.IsNullOrEmpty(url)) {
+                throw new ArgumentException("Value cannot be null or empty", "url");
+            }
+
+            try {
+                var request = html.ViewContext.HttpContext.Request;
+                var baseUri = new Uri(string.Concat(request.Scheme, "://", request.Host.ToUriComponent(), request.PathBase.ToUriComponent(), request.Path.ToUriComponent(), request.QueryString.ToUriComponent()));
+                var fullUri = new Uri(baseUri, url);
+                var response = await new HttpClient().GetAsync(fullUri.ToString());
+                var responseBody = await response.Content.ReadAsStringAsync();
+                return new HtmlString(FormatAsScript(url, response.StatusCode, responseBody));
+            } catch (Exception ex) {
+                var logger = (ILogger)html.ViewContext.HttpContext.ApplicationServices.GetService(typeof (ILogger));
+                if (logger != null) {
+                    logger.LogWarning("Error priming cache for URL: " + url, ex);
+                }
+                return new HtmlString(string.Empty);
+            }
+        }
+
+        private static string FormatAsScript(string url, HttpStatusCode responseStatusCode, string responseBody)
+        {
+            return string.Format(@"<script>window.__preCachedResponses = window.__preCachedResponses || {{}}; window.__preCachedResponses[{0}] = {1};</script>",
+                JsonConvert.SerializeObject(url),
+                JsonConvert.SerializeObject(new { statusCode = responseStatusCode, body = responseBody })
+            );
+        }
+    }
+}