摘要： ★★★★★ 定义对象间的一种一对多的依赖关系，以便当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并自动更新。 Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. 阅读全文