public void OnImmovableCreated(Immovable obj)
{
GameObject obj_go = Instantiate(itemsMap[obj.objectType]);
immovableGameObjectMap.Add(obj, obj_go);
obj_go.name = obj.objectType + "_" + obj.tile.X + "_" + obj.tile.Y;
obj_go.transform.position = new Vector3(obj.tile.X, 0, obj.tile.Y);
obj_go.transform.SetParent(this.transform, true);
// register a tile changing type callback
obj.RegisterOnChangedCallback(OnImmovableChanged);
}