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);
+};

test/spec/ol/tilecache.test.js

@@ -0,0 +1,39 @@
+goog.require('ol.Tile');
+goog.require('ol.TileCache');
+goog.require('ol.tilecoord');
+
+
+describe('ol.TileCache', function() {
+
+  describe('#pruneExceptNewestZ()', function() {
+    it('gets rid of all entries that are not at the newest z', function() {
+      var tiles = [
+        new ol.Tile([0, 0, 0]),
+        new ol.Tile([1, 0, 0]),
+        new ol.Tile([1, 1, 0]),
+        new ol.Tile([2, 0, 0]),
+        new ol.Tile([2, 1, 0]),
+        new ol.Tile([2, 2, 0]),
+        new ol.Tile([2, 3, 0]) // newest tile at z: 2
+      ];
+      var cache = new ol.TileCache();
+
+      sinon.spy(tiles[0], 'dispose');
+
+      tiles.forEach(function(tile) {
+        cache.set(ol.tilecoord.getKey(tile.tileCoord), tile);
+      });
+
+      cache.pruneExceptNewestZ();
+
+      expect(cache.getKeys()).to.eql([
+        '2/3/0',
+        '2/2/0',
+        '2/1/0',
+        '2/0/0'
+      ]);
+
+      expect(tiles[0].dispose.calledOnce).to.be(true);
+    });
+  });
+});