详解Python设计模式编程中观察者模式与策略模式的运用
观察者模式
观察者模式:又叫发布订阅模式,定义了一种一对多的依赖关系,让多个观察者对象同时监听某一个主题对象,这个主题对象的状态发生变化时,会通知所有观察者对象,是他们能自动更新自己。
代码结构
class Topic(object): """主题类。保存所有观察者实例的引用,每个主题都可以有很多观察者 可以增加和删除观察者""" def __init__(self): self.obs = [] def Attach(self, ob): self.obs.append(ob) def Detach(self, ob): self.obs.remove(ob) def Notify(self): for ob in self.obs: ob.Update() class Observer(object): """抽象观察者类,收到主题的变更通知时,更新自己""" def Update(self): raise NotImplementedError() class ConcreteTopic(object): """一个具体主题""" def __init__(self): self.state = None def ChangeState(self, newState): self.state = newState self.Notify() class ConcreteObserver(object): """一个具体监听类""" def __init__(self, topic): self.topic = topic def Update(self): print self.topic.state def client(): topic = ConcreteTopic() topic.Attach(ConcreteObserver(topic)) topic.ChangeState('New State')
众多MQ中间件都是采用这种模式的思想来实现的。
观察者模式可以让主题和观察者之间解耦,互相之间尽可能少的依赖。不过抽象主题和抽象观察者之间还是有耦合的。
策略模式
策略模式: 定义了算法家族,分别封装起来,让他们之间可以互相替换。此模式让算法的变化不影响使用算法的客户。
代码框架
class Strategy(object): """抽象算法类""" def AlgorithmInterface(self): raise NotImplementedError() class ConcreteStrategyA(Strategy): def AlgorithmInterface(self): print '算法A' class ConcreteStrategyB(Strategy): def AlgorithmInterface(self): print '算法B' class Context(object): """上下文,作用就是封装策略的实现细节,用户只需要知道有哪些策略可用""" def __init__(self, strategy): # 初始化时传入具体的策略实例 self.strategy = strategy def ContextInterface(self): # 负责调用具体的策略实例的接口 self.strategy.AlgorithmInterface() def client(cond): # 策略模式的使用演示 # 用户只需要根据不同的条件,将具体的算法实现类传递给Context, # 然后调用Context暴露给用户的接口就行了。 if cond == 'A': context = Context(ConcreteStrategyA()) elif cond == 'B': context = Context(ConcreteStrategyB()) result = context.ContextInterface()
策略模式解决那类问题
在回答这个问题之前,先说下对策略模式的使用方式的感觉。上面的client函数,怎么看起来就像是简单工厂模式中的工厂函数呢?确实如此,实际上策略模式可以和简工厂模式结合起来,将更多细节封装在策略模式内部,让使用者更容易的使用。
那么策略模式和简单工厂模式有什么不同呢?策略模式中的算法是用来解决同一个问题的,根据时间、条件不同,算法的具体细节有差异,但最终解决的是同一个问题。在需求分析过程中,当听到需要在不同时间应用不同的业务规则,就可以考虑使用策略模式来处理这种变化的可能性。
缺点
使用者需要知道每一种策略的具体含义,并负责选择策略
改进
结合简单工厂模式,将策略选择封装在Context内部,解放client:
class Context(object): def __init__(self, cond): if cond == 'A': self.strategy = Context(ConcreteStrategyA()) elif cond == 'B': self.strategy = Context(ConcreteStrategyB()) def ContextInterface(self): self.strategy.AlgorithmInterface() def client(cond): context = Context(cond) result = context.ContextInterface()
改进后的遗留问题
每次需要增加新的策略时,就需要修改Context的构造函数,增加一个新的判断分支。

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