Make preload a number of levels instead of a boolean

src/ol/layer/tilelayer.js

@@ -23,17 +23,17 @@ ol.layer.TileLayer = function(options) {
   goog.base(this, options);
 
   this.setPreload(
-      goog.isDef(options.preload) ? options.preload : false);
+      goog.isDef(options.preload) ? options.preload : 0);
 
 };
 goog.inherits(ol.layer.TileLayer, ol.layer.Layer);
 
 
 /**
- * @return {boolean} Preload.
+ * @return {number} Preload.
  */
 ol.layer.TileLayer.prototype.getPreload = function() {
-  return /** @type {boolean} */ (this.get(ol.layer.TileLayerProperty.PRELOAD));
+  return /** @type {number} */ (this.get(ol.layer.TileLayerProperty.PRELOAD));
 };
 goog.exportProperty(
     ol.layer.TileLayer.prototype,
@@ -50,7 +50,7 @@ ol.layer.TileLayer.prototype.getTileSource = function() {
 
 
 /**
- * @param {boolean} preload Preload.
+ * @param {number} preload Preload.
  */
 ol.layer.TileLayer.prototype.setPreload = function(preload) {
   this.set(ol.layer.TileLayerProperty.PRELOAD, preload);

src/ol/renderer/layerrenderer.js

@@ -291,7 +291,7 @@ ol.renderer.Layer.prototype.snapCenterToPixel =
  * @param {ol.Projection} projection Projection.
  * @param {ol.Extent} extent Extent.
  * @param {number} currentZ Current Z.
- * @param {boolean} preload Preload low resolution tiles.
+ * @param {number} preload Load low resolution tiles up to 'preload' levels.
  * @param {function(this: T, ol.Tile)=} opt_tileCallback Tile callback.
  * @param {T=} opt_obj Object.
  * @protected
@@ -313,7 +313,7 @@ ol.renderer.Layer.prototype.manageTilePyramid = function(
     tileResolution = tileGrid.getResolution(z);
     for (x = tileRange.minX; x <= tileRange.maxX; ++x) {
       for (y = tileRange.minY; y <= tileRange.maxY; ++y) {
-        if (preload || z == currentZ) {
+        if (currentZ - z <= preload) {
           tile = tileSource.getTile(z, x, y, tileGrid, projection);
           if (tile.getState() == ol.TileState.IDLE) {
             wantedTiles[tile.tileCoord.toString()] = true;