/设计：Src和Dis两个类, Src中有一个被封装的属性，类型为int（要求为非负值），每当我们通过特定方法更改Src对象中的这个属性后，Dis对象都能得到通知，并向Src发消息获得此属性值。-/ Design: Src and Dis two categories, Src in a package of properties, type int (request a non-negative), whenever we change the Src through specific methods of this object properties, Dis object can be notified to the Src发消息access this property value.