fix for #784 -- adds some commenting and readability to the tile.draw() function

git-svn-id: http://svn.openlayers.org/trunk/openlayers@3462 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Tile.js

@@ -77,14 +77,38 @@ OpenLayers.Tile.prototype = {
     },
 
     /**
-    */
+     * Clear whatever is currently in the tile, then return whether or not 
-    draw:function() {
+     *  it should actually be re-drawn.
+     * 
+     * @returns Whether or not the tile should actually be drawn. Note that 
+     *           this is not really the best way of doing things, but such is 
+     *           the way the code has been developed. Subclasses call this and
+     *           depend on the return to know if they should draw or not.
+     * @type Boolean
+     */
+    draw: function() {
+        
+        //clear tile's contents and mark as not drawn
         this.clear();
-        return ((this.layer.displayOutsideMaxExtent
+        
-                || (this.layer.maxExtent
+        var maxExtent = this.layer.maxExtent;
-                    && this.bounds.intersectsBounds(this.layer.maxExtent, false)))
+        var withinMaxExtent = (maxExtent &&
-                && !(this.layer.buffer == 0
+                               this.bounds.intersectsBounds(maxExtent, false));
-                     && !this.bounds.intersectsBounds(this.layer.map.getExtent(), false)));
+ 
+        var mapExtent = this.layer.map.getExtent();
+        var withinMapExtent = (mapExtent &&
+                               this.bounds.intersectsBounds(mapExtent, false));
+         
+        // There are two cases where we *wouldn't* want to draw the tile:
+        //  
+        // 1) If the tile is outside its layer's maxExtent
+        // 2) When the layer's buffer is 0, if the tile is outside the map's 
+        //    extent (out of view)
+        //
+        // ...what we return is the opposite of the above conditions :-)
+        //
+        return ( (withinMaxExtent || this.layer.displayOutsideMaxExtent) &&
+                 (withinMapExtent || (this.layer.buffer != 0)) );
     },
     
     /**