Merge pull request #179 from ahocevar/tile-queue

A tile queue that can be aborted. r=@elemoine,@fredj,@tonio

lib/OpenLayers/Tile.js

@@ -25,6 +25,10 @@ OpenLayers.Tile = OpenLayers.Class({
     
     /** 
      * Supported event types:
+     *  - *beforedraw* Triggered before the tile is drawn. Used to defer
+     *    drawing to an animation queue. To defer drawing, listeners need
+     *    to return false, which will abort drawing. The queue handler needs
+     *    to call <draw>(true) to actually draw the tile.
      *  - *loadstart* Triggered when tile loading starts.
      *  - *loadend* Triggered when tile loading ends.
      *  - *reload* Triggered when an already loading tile is reloaded.
@@ -107,7 +111,7 @@ OpenLayers.Tile = OpenLayers.Class({
      * {Boolean} Is the tile loading?
      */
     isLoading: false,
-        
+    
     /** TBD 3.0 -- remove 'url' from the list of parameters to the constructor.
      *             there is no need for the base tile class to have a url.
      * 
@@ -180,15 +184,26 @@ OpenLayers.Tile = OpenLayers.Class({
      *     it should actually be re-drawn. This is an example implementation
      *     that can be overridden by subclasses. The minimum thing to do here
      *     is to call <clear> and return the result from <shouldDraw>.
+     *
+     * Parameters:
+     * deferred - {Boolean} When drawing was aborted by returning false from a
+     *     *beforedraw* listener, the queue manager needs to pass true, so the
+     *     tile will not be cleared and immediately be drawn. Otherwise, the
+     *     tile will be cleared and a *beforedraw* event will be fired.
      * 
      * Returns:
      * {Boolean} Whether or not the tile should actually be drawn.
      */
-    draw: function() {
-        //clear tile's contents and mark as not drawn
-        this.clear();
-        
-        return this.shouldDraw();
+    draw: function(deferred) {
+        if (!deferred) {
+            //clear tile's contents and mark as not drawn
+            this.clear();
+        }
+        var draw = this.shouldDraw();
+        if (draw && !deferred) {
+            draw = this.events.triggerEvent("beforedraw") !== false;
+        }
+        return draw;
     },
     
     /**
@@ -259,10 +274,10 @@ OpenLayers.Tile = OpenLayers.Class({
     /** 
      * Method: clear
      * Clear the tile of any bounds/position-related data so that it can 
-     *     be reused in a new location. To be implemented by subclasses.
+     *     be reused in a new location.
      */
     clear: function(draw) {
-        // to be implemented by subclasses
+        // to be extended by subclasses
     },
     
     CLASS_NAME: "OpenLayers.Tile"