preemptive is now true by default + improved documentation

examples/tileutfgrid.js

@@ -12,8 +12,7 @@ var mapLayer = new ol.layer.Tile({
 });
 
 var gridSource = new ol.source.TileUTFGrid({
-  url: 'http://api.tiles.mapbox.com/v3/mapbox.geography-class.json',
-  preemptive: true
+  url: 'http://api.tiles.mapbox.com/v3/mapbox.geography-class.json'
 });
 
 var gridLayer = new ol.layer.Tile({source: gridSource});

externs/olx.js

@@ -3550,9 +3550,12 @@ olx.source.TileUTFGridOptions;
 
 
 /**
- * If true, the TileUTFGrid source loads the tiles based on their "visibility".
- * This can be used to improve the speed of response, but increases traffic.
- * Default is `false`.
+ * If `true` the TileUTFGrid source loads the tiles based on their "visibility".
+ * This improves the speed of response, but increases traffic.
+ * Note that if set to `false`, you need to pass `true` as `opt_request`
+ * to the `forDataAtCoordinateAndResolution` method otherwise no data
+ * will ever be loaded.
+ * Default is `true`.
  * @type {boolean|undefined}
  * @api
  */

src/ol/source/tileutfgridsource.js

@@ -38,7 +38,7 @@ ol.source.TileUTFGrid = function(options) {
    * @type {boolean}
    */
   this.preemptive_ = goog.isDef(options.preemptive) ?
-      options.preemptive : false;
+      options.preemptive : true;
 
   /**
    * @private