Move interim tile related methods to ol.Tile

src/ol/tile.js

@@ -73,17 +73,67 @@ ol.Tile.prototype.getKey = function() {
 };
 
 /**
- * Get the interim tile if it exists, otherwise returns the tile itself.
- * @return {!ol.Tile} The interim tile, or the tile itself
- *
+ * Get the interim tile most suitable for rendering using the chain of interim
+ * tiles. This corresponds to the  most recent tile that has been loaded, if no
+ * such tile exists, the original tile is returned.
+ * @return {!ol.Tile} Best tile for rendering.
  */
 ol.Tile.prototype.getInterimTile = function() {
-  if (this.interimTile) {
-    return this.interimTile;
+  if (!this.interimTile) {
+    //empty chain
+    return this;
   }
+  var tile = this.interimTile;
+
+  // find the first loaded tile and return it. Since the chain is sorted in
+  // decreasing order of creation time, there is no need to search the remainder
+  // of the list (all those tiles correspond to older requests and will be
+  // cleaned up by refreshInterimChain)
+  do {
+    if (tile.getState() == ol.Tile.State.LOADED) {
+      return tile;
+    }
+    tile = tile.interimTile;
+  } while (tile);
+
+  // we can not find a better tile
   return this;
 };
 
+/**
+ * Goes through the chain of interim tiles and discards sections of the chain
+ * that are no longer relevant.
+ */
+ol.Tile.prototype.refreshInterimChain = function() {
+  if (!this.interimTile) {
+    return;
+  }
+
+  var tile = this.interimTile;
+  var prev = this;
+
+  do {
+    if (tile.getState() == ol.Tile.State.LOADED) {
+      //we have a loaded tile, we can discard the rest of the list
+      //we would could abort any LOADING tile request
+      //older than this tile (i.e. any LOADING tile following this entry in the chain)
+      tile.interimTile = null;
+      break;
+    } else if (tile.getState() == ol.Tile.State.LOADING) {
+      //keep this LOADING tile any loaded tiles later in the chain are
+      //older than this tile, so we're still interested in the request
+      prev = tile;
+    } else if (tile.getState() == ol.Tile.State.IDLE) {
+      //the head of the list is the most current tile, we don't need
+      //to start any other requests for this chain
+      prev.interimTile = tile.interimTile;
+    } else {
+      prev = tile;
+    }
+    tile = prev.interimTile;
+  } while (tile);
+};
+
 /**
  * Get the tile coordinate for this tile.
  * @return {ol.TileCoord} The tile coordinate.