transform cartesian y to flash native ; every object group has its own itemRenderer ;
mxml friendly : use variable instead of function, so we can declare class instance in mxml ; dispatch complete event when nothing will be loaded, so the context doesn't need to handle special case ;
In order to solve asynchronous arguments passing, I  put TiledMapData in TiledMap , use TiledMap to accept arguments and handle asynchronism, and then pass them to TiledMapData.