||Role's logical attribute set, which is provided to RoleView object
||Background is in charge of the background of game,
and it keeps and updates the viewport of the game.
||DOMTree supports some frequently-used operations on an
||GameContainer is in charge of the game (frame) updating
and the communications among all the roles inside it.
||GameEvent works as the medium of the communication among different roles.
||A game scheduler is a container holding several
||This class represents a logical role who lives and acts
inside a game container.
||StageLoader loads a new stage and reinitialize the game.