cleanups to async tile fetch/draw routine

@@ -139,20 +139,32 @@
             }
             else
             {
-                // for non-local tiles, consult cache directly first, else fetch asynchronously
+                // for non-local tiles, consult cache directly first (if possible)
                 //
-                tileImage = [[_mapView tileCache] cachedImage:RMTileMake(x, y, zoom) withCacheKey:[_tileSource uniqueTilecacheKey]];
+                if (_tileSource.isCacheable)
+                    tileImage = [[_mapView tileCache] cachedImage:RMTileMake(x, y, zoom) withCacheKey:[_tileSource uniqueTilecacheKey]];
 
                 if ( ! tileImage)
                 {
+                    // fire off an asynchronous retrieval
+                    //
                     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void)
                     {
-                        if ([_tileSource imageForTile:RMTileMake(x, y, zoom) inCache:[_mapView tileCache]])
+                        // ensure only one request for a URL at a time
+                        //
+                        @synchronized ([(RMAbstractWebMapSource *)_tileSource URLForTile:RMTileMake(x, y, zoom)])
                         {
-                            dispatch_async(dispatch_get_main_queue(), ^(void)
+                            // this will return quicker if cached since above attempt, else block on fetch
+                            //
+                            if (_tileSource.isCacheable && [_tileSource imageForTile:RMTileMake(x, y, zoom) inCache:[_mapView tileCache]])
                             {
-                                [self.layer setNeedsDisplay];
-                            });
+                                dispatch_async(dispatch_get_main_queue(), ^(void)
+                                {
+                                    // do it all again for this tile, next time synchronously from cache
+                                    //
+                                    [self.layer setNeedsDisplayInRect:rect];
+                                });
+                            }
                         }
                     });
                 }