Mark all low resolution tiles as used to prevent cache expiration

src/ol/renderer/canvas/canvastilelayerrenderer.js

@@ -199,6 +199,7 @@ ol.renderer.canvas.TileLayer.prototype.renderFrame =
   }
 
   this.updateUsedTiles(frameState.usedTiles, tileSource, z, tileRange);
+  this.useLowResolutionTiles(tileSource, z, frameState.extent);
   this.scheduleExpireCache(frameState, tileSource);
 
   var transform = this.transform_;

src/ol/renderer/dom/domtilelayerrenderer.js

@@ -217,6 +217,7 @@ ol.renderer.dom.TileLayer.prototype.renderFrame =
   }
 
   this.updateUsedTiles(frameState.usedTiles, tileSource, z, tileRange);
+  this.useLowResolutionTiles(tileSource, z, frameState.extent);
   this.scheduleExpireCache(frameState, tileSource);
 
 };

src/ol/renderer/layerrenderer.js

@@ -3,6 +3,7 @@ goog.provide('ol.renderer.Layer');
 goog.require('goog.events');
 goog.require('goog.events.EventType');
 goog.require('ol.Attribution');
+goog.require('ol.Extent');
 goog.require('ol.FrameState');
 goog.require('ol.Image');
 goog.require('ol.ImageState');
@@ -258,3 +259,25 @@ ol.renderer.Layer.prototype.updateWantedTiles =
   }
   wantedTiles[tileSourceKey][coordKey] = true;
 };
+
+
+/**
+ * @protected
+ * @param {ol.source.TileSource} tileSource Tile source.
+ * @param {number} z Z.
+ * @param {ol.Extent} extent Extent.
+ */
+ol.renderer.Layer.prototype.useLowResolutionTiles =
+    function(tileSource, z, extent) {
+  var tileGrid = tileSource.getTileGrid();
+  var tileRange, x, y, zKey;
+  // FIXME this should loop up to tileGrid's minZ when implemented
+  for (; z >= 0; --z) {
+    tileRange = tileGrid.getTileRangeForExtentAndZ(extent, z);
+    for (x = tileRange.minX; x <= tileRange.maxX; ++x) {
+      for (y = tileRange.minY; y <= tileRange.maxY; ++y) {
+        tileSource.useTile(new ol.TileCoord(z, x, y));
+      }
+    }
+  }
+};

src/ol/renderer/webgl/webgltilelayerrenderer.js

@@ -459,6 +459,7 @@ ol.renderer.webgl.TileLayer.prototype.renderFrame =
   }
 
   this.updateUsedTiles(frameState.usedTiles, tileSource, z, tileRange);
+  this.useLowResolutionTiles(tileSource, z, frameState.extent);
   this.scheduleExpireCache(frameState, tileSource);
 
   goog.vec.Mat4.makeIdentity(this.texCoordMatrix_);