visitor(访问者)模式
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我个人比较喜欢吕震宇的博客(www.cnblogs.com/zhenyulu)。
这是我学习访问模式时自己做的一个小例子,访问者模式是为了使你在不改变各元素的类的前提
下定义作用于这些元素的新操作。
在我的这个例子里,基类animal只有一个默认的方法Run(),子类继承时获得了这个方法,子类也可
以覆写这个方法,但是这种方式会导致一个问题,就是无法动态辨析子类,调用合适的方法,如果
要加入多态特性的话,就要用到虚方法或抽象方法,这里这个Run()只是个示例,也许这样用并不合
适。
现在回到正题,如果要给这个类层次体系统一填加一个方法,我们可以修改源代码,在基类中加
入一个抽象方法,子类来实现这个方法,但是这样违背了“开--闭原则”,所以这里引入了访问者
角色,不同的具体的访问者角色表示不同的行为,通过两次多态的动态辨析,为特定的子类填加了
特定的方法。当有新的行为要填加时,只需继承抽象访问者角色,建立一个新的具体的访问者类即
可。
任何设计模式都不是万能的,设计模式是为了应对变化,封装变化。每一个论计模式都有自己适
用的情况,但没有万能的设计模式,我们总是假设什么是变的,什么是不变的,在项目设计初期,
我们要预计将来,什么变什么不变,从而进行良好的设计应对变化。
在访问者模式里,我们把类层次结构看做不变的,而类层次结构的形为是多变的,如果类层次结
构变化的话,我们就要修改任何一个访问者类,这是我们所不想看到的