Define an object that encapsulates how a set of objects interact.Mdeiator promtoes loose coupling
by keeping objects from referring to eachother explicitly,and it lets you vary their interaction
independently (用一个中介对象封装一系列的对象交互，中介者使各对象不需要显示地相互作用，
从而使其耦合松散，而且可以独立地改变它们之间的交互)