Allow tile cache key calculation to be overridden

src/ol/source/debugtilesource.js

@@ -125,7 +125,7 @@ ol.source.TileDebug.prototype.expireCache = function(usedTiles) {
  * @inheritDoc
  */
 ol.source.TileDebug.prototype.getTile = function(z, x, y) {
-  var tileCoordKey = ol.TileCoord.getKeyZXY(z, x, y);
+  var tileCoordKey = this.getKeyZXY(z, x, y);
   if (this.tileCache_.containsKey(tileCoordKey)) {
     return /** @type {!ol.DebugTile_} */ (this.tileCache_.get(tileCoordKey));
   } else {

src/ol/source/imagetilesource.js

@@ -90,7 +90,7 @@ ol.source.TileImage.prototype.expireCache = function(usedTiles) {
  * @inheritDoc
  */
 ol.source.TileImage.prototype.getTile = function(z, x, y, projection) {
-  var tileCoordKey = ol.TileCoord.getKeyZXY(z, x, y);
+  var tileCoordKey = this.getKeyZXY(z, x, y);
   if (this.tileCache_.containsKey(tileCoordKey)) {
     return /** @type {!ol.Tile} */ (this.tileCache_.get(tileCoordKey));
   } else {
@@ -112,7 +112,7 @@ ol.source.TileImage.prototype.getTile = function(z, x, y, projection) {
  * @inheritDoc
  */
 ol.source.TileImage.prototype.useTile = function(z, x, y) {
-  var tileCoordKey = ol.TileCoord.getKeyZXY(z, x, y);
+  var tileCoordKey = this.getKeyZXY(z, x, y);
   if (this.tileCache_.containsKey(tileCoordKey)) {
     this.tileCache_.get(tileCoordKey);
   }

src/ol/source/tilesource.js

@@ -85,7 +85,7 @@ ol.source.Tile.prototype.findLoadedTiles = function(loadedTilesByZ,
   var tile, tileCoordKey, x, y;
   for (x = tileRange.minX; x <= tileRange.maxX; ++x) {
     for (y = tileRange.minY; y <= tileRange.maxY; ++y) {
-      tileCoordKey = ol.TileCoord.getKeyZXY(z, x, y);
+      tileCoordKey = this.getKeyZXY(z, x, y);
       if (loadedTilesByZ[z] && loadedTilesByZ[z][tileCoordKey]) {
         continue;
       }
@@ -104,6 +104,18 @@ ol.source.Tile.prototype.findLoadedTiles = function(loadedTilesByZ,
 };
 
 
+/**
+ * @param {number} z Z.
+ * @param {number} x X.
+ * @param {number} y Y.
+ * @return {string} Key.
+ * @protected
+ */
+ol.source.Tile.prototype.getKeyZXY = function(z, x, y) {
+  return ol.TileCoord.getKeyZXY(z, x, y);
+};
+
+
 /**
  * @return {boolean} Opaque.
  */