Prune all except for the most recent z on URL change

src/ol/source/urltile.js

@@ -155,6 +155,7 @@ ol.source.UrlTile.prototype.setTileLoadFunction = function(tileLoadFunction) {
  */
 ol.source.UrlTile.prototype.setTileUrlFunction = function(tileUrlFunction, opt_key) {
   this.tileUrlFunction = tileUrlFunction;
+  this.tileCache.pruneExceptNewestZ();
   if (typeof opt_key !== 'undefined') {
     this.setKey(opt_key);
   } else {

src/ol/tilecache.js

@@ -2,6 +2,7 @@ goog.provide('ol.TileCache');
 
 goog.require('ol');
 goog.require('ol.structs.LRUCache');
+goog.require('ol.tilecoord');
 
 
 /**
@@ -33,3 +34,22 @@ ol.TileCache.prototype.expireCache = function(usedTiles) {
     }
   }
 };
+
+
+/**
+ * Prune all tiles from the cache that don't have the same z as the newest tile.
+ */
+ol.TileCache.prototype.pruneExceptNewestZ = function() {
+  if (this.getCount() === 0) {
+    return;
+  }
+  var key = this.peekFirstKey();
+  var tileCoord = ol.tilecoord.fromKey(key);
+  var z = tileCoord[0];
+  this.forEach(function(tile) {
+    if (tile.tileCoord[0] !== z) {
+      this.remove(ol.tilecoord.getKey(tile.tileCoord));
+      tile.dispose();
+    }
+  }, this);
+};