The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.

• 定义了一种一对多的关系，让多个观察者对象同时监听某个主题对象。主题对象在状态发生变更时，会通知所有的观察者，使他们自动更新自己。