Add EMPTY tile state instead of using null for empty tiles

src/ol/source/debugtilesource.js

@@ -19,9 +19,7 @@ goog.require('ol.tilegrid.TileGrid');
  */
 ol.DebugTile_ = function(tileCoord, tileGrid) {
 
-  goog.base(this, tileCoord);
-
-  this.state = ol.TileState.LOADED;
+  goog.base(this, tileCoord, ol.TileState.LOADED);
 
   /**
    * @private
@@ -127,7 +125,7 @@ ol.source.DebugTileSource.prototype.expireCache = function(usedTiles) {
 ol.source.DebugTileSource.prototype.getTile = function(tileCoord) {
   var key = tileCoord.toString();
   if (this.tileCache_.containsKey(key)) {
-    return /** @type {ol.DebugTile_} */ (this.tileCache_.get(key));
+    return /** @type {!ol.DebugTile_} */ (this.tileCache_.get(key));
   } else {
     var tile = new ol.DebugTile_(tileCoord, this.tileGrid);
     this.tileCache_.set(key, tile);

src/ol/source/imagetilesource.js

@@ -7,6 +7,7 @@ goog.require('ol.ImageTile');
 goog.require('ol.Projection');
 goog.require('ol.Tile');
 goog.require('ol.TileCache');
+goog.require('ol.TileState');
 goog.require('ol.TileUrlFunction');
 goog.require('ol.TileUrlFunctionType');
 goog.require('ol.source.TileSource');
@@ -89,18 +90,17 @@ ol.source.ImageTileSource.prototype.getTile =
     function(tileCoord, tileGrid, projection) {
   var key = tileCoord.toString();
   if (this.tileCache_.containsKey(key)) {
-    return /** @type {ol.Tile} */ (this.tileCache_.get(key));
+    return /** @type {!ol.Tile} */ (this.tileCache_.get(key));
   } else {
     goog.asserts.assert(tileGrid);
     goog.asserts.assert(projection);
     var tileUrl = this.tileUrlFunction(tileCoord, tileGrid, projection);
-    var tile;
-    if (goog.isDef(tileUrl)) {
-      tile = new ol.ImageTile(tileCoord, tileUrl, this.crossOrigin_);
-      this.tileCache_.set(key, tile);
-    } else {
-      tile = null;
-    }
+    var tile = new ol.ImageTile(
+        tileCoord,
+        goog.isDef(tileUrl) ? ol.TileState.IDLE : ol.TileState.EMPTY,
+        goog.isDef(tileUrl) ? tileUrl : '',
+        this.crossOrigin_);
+    this.tileCache_.set(key, tile);
     return tile;
   }
 };

src/ol/source/tilesource.js

@@ -125,7 +125,7 @@ ol.source.TileSource.prototype.getResolutions = function() {
  * @param {ol.TileCoord} tileCoord Tile coordinate.
  * @param {ol.tilegrid.TileGrid=} opt_tileGrid Tile grid.
  * @param {ol.Projection=} opt_projection Projection.
- * @return {ol.Tile} Tile.
+ * @return {!ol.Tile} Tile.
  */
 ol.source.TileSource.prototype.getTile = goog.abstractMethod;