BuildFeed/BuildFeedApp-Full/Service/ApiCache.cs

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;

namespace BuildFeedApp.Service
{
   public static class ApiCache
   {
      public static int SecondsBeforeStale { get; set; }

      private static Dictionary<string, ApiCacheEntry> _cache = new Dictionary<string, ApiCacheEntry>();

      static ApiCache()
      {
         //ServicePointManager.DefaultConnectionLimit = 20;
         SecondsBeforeStale = 15;
      }

      /// <summary>
      /// Download and parse a JSON document into the specified type
      /// </summary>
      /// <typeparam name="T">Object that JSON Reperesents</typeparam>
      /// <param name="url">The URL that the JSON can be retreived from</param>
      /// <returns></returns>
      public async static Task<T> GetApi<T>(string url)
      {
         if (_cache.ContainsKey(url)
             && (_cache[url].ExpiryTime > DateTime.Now || !await IsAPIStale(url, _cache[url].LastModified)))
         {
            return (T)(_cache[url].Value);
         }
         else
         {
            var cat = await FetchAPI<T>(url);
            _cache[url] = cat;
            return (T)(cat.Value);
         }
      }

      private async static Task<ApiCacheEntry> FetchAPI<T>(string url)
      {
         HttpWebRequest wreq = HttpWebRequest.CreateHttp(url);
         ApiCacheEntry ret = new ApiCacheEntry();

         using (HttpWebResponse wres = await wreq.GetResponseAsync() as HttpWebResponse)
         {
            ret.ExpiryTime = DateTime.Now.AddSeconds(SecondsBeforeStale);
            //ret.LastModified = wres.LastModified;

            using (Stream s = wres.GetResponseStream())
            using (StreamReader sr = new StreamReader(s))
            using (JsonReader jr = new JsonTextReader(sr))
            {

               JsonSerializer jsz = new JsonSerializer();

               ret.Value = jsz.Deserialize<T>(jr);
            }
         }

         return ret;
      }

      private async static Task<bool> IsAPIStale(string url, DateTime lastChecked)
      {
         HttpWebRequest wreq = HttpWebRequest.CreateHttp(url);
         //wreq.IfModifiedSince = lastChecked;
         wreq.Method = "HEAD";

         try
         {
            HttpWebResponse wres = await wreq.GetResponseAsync() as HttpWebResponse;
         }
         catch (WebException wex)
         {
            HttpWebResponse wres = wex.Response as HttpWebResponse;
            if (wres.StatusCode == HttpStatusCode.NotModified || wres.StatusCode == HttpStatusCode.NotFound)
            {
               return false;
            }
         }
         return true;
      }
   }

   internal struct ApiCacheEntry
   {
      public DateTime ExpiryTime { get; set; }
      public DateTime LastModified { get; set; }
      public object Value { get; set; }
   }
}