Ignore initial server requests if cached and stale

Staleness is determined by seeing if the Last-Modified value is older than the current request by the length of the refresh rate plus a 10 second grace period.

This check will only run for the initial request to /live_cache_check/1/, as it's assumed that the other urls, which are time based, won't need such aggressive cache busting.

If the cache is stale, this will set the timeStamp variable to the current time and immediately rerun check().

live-cache.js

@@ -45,6 +45,7 @@
 		};
 
 		this.check = function () {
+			var self = this;
 			if (undefined === runCache || null === runCache) {
 				$.each(self.auto_updates, function () {
 					var selector = "" + this;
@@ -58,13 +59,24 @@
 					url     : ajaxurl + '/live_cache_check/' + timeStamp + '/',
 					success : function (data, s, resp) {
 						var date = resp.getResponseHeader('Date'),
-								serverTime = Math.round((new Date(date)).getTime() / 1000);
+								serverTime = Math.round((new Date(date)).getTime() / 1000),
+								lastModified = resp.getResponseHeader('Last-Modified'),
+								lastModifiedTime;
 
 						data.refresh_rate = getRefreshRate(data.refresh_rate || 0);
 
 						// look only at the tens place of the second counter to refresh cache every ten seconds.
 						timeStamp = formatTimestampEndpoint(int_to_timestamp(serverTime + data.refresh_rate));
 
+						if (lastModified && /\/live_cache_check\/1\/?$/.test(this.url)) {
+							lastModifiedTime = Math.round((new Date(lastModified)).getTime() / 1000);
+							if (lastModifiedTime + data.refresh_rate + 10 < serverTime) {
+								timeStamp = formatTimestampEndpoint(int_to_timestamp(serverTime));
+								self.check();
+								return;
+							}
+						}
+
 						$.each(data, function (key, value) {
 							if (undefined !== callbacks[key] && (undefined === values[key] || values[key] !== value)) {
 								callbacks[key].fire(key, value); //pass key and value so that we can use the same callback for multiple keys