Python设计模式——责任链模式

责任链模式(Chain of Responsibility Pattern)可将请求的发送方与处理请求的接收方解耦。这样的话，某函数就不用直接调用别的函数了，而是可以把请求发送给一个由诸多接收者所组成的链条。链条中的首个接收者可以处理请求并停止责任链(也就是不再继续往下传递)，也可以把请求发给下一个接收者。而第二个接收者也有这两种选择，此过程可一直延续至最后一个接收者，该接收者可将请求丢弃，也可抛出异常。

使用常规方法实现

#!/usr/bin/env python
# encoding:utf-8class NullHandler(object):def __init__(self, successor=None):self.__successor = successordef handle(self, event):if self.__successor is not None:self.__successor.handle(event)class ConcreteHandler1(NullHandler):def handle(self, event):if event > 0 and event <= 10:print "in handler1", eventelse:super(ConcreteHandler1, self).handle(event)class ConcreteHandler2(NullHandler):def handle(self, event):if event > 10 and event <= 20:print "in handler2", eventelse:super(ConcreteHandler2, self).handle(event)class ConcreteHandler3(NullHandler):def handle(self, event):if event > 20 and event <= 30:print "in handler3", eventelse:super(ConcreteHandler3, self).handle(event)def main():handler = ConcreteHandler3(ConcreteHandler2(ConcreteHandler1(NullHandler())))events = [2, 5, 14, 22, 18, 3, 35, 27, 20]for event in events:handler.handle(event)

所示代码在NullHandler设置一个successor属性，使用它来执行handle，而其他则继承NullHandler，这样不符合自身条件将它抛给NullHandler，让它来执行successor的handle。我们可以从上面赋值中看出successor的顺序，一开始最里面successor为None，后面是ConcreteHandler1类型…依次类推，实现责任链。

使用协程实现责任链

关于协程

协程(coroutine)与生成器一样，也使用yield表达式，但行为不同。协程执行的是无限循环，而且一开始就会停在首个（或仅有的那个）yield表达式那里，等值有值传给它。协程会把收到的值当成yield表达式的值，然后继续执行它所需的操作，等处理完之后，又会再度循环，并在下一个yield表达式那里等着接收下个值。这样的话，我们就能反复调用协程中的send()或throw()方法向其push值。
在Python中，凡是带有yield语句的函数或方法都能充当生成器。而利用@coroutine装饰器及无限循环则可将生成器变为协程。

import functoolsdef coroutine(function):@functools.wraps(function)def wrapper(*args, **kwargs):generator = function(*args, **kwargs)next(generator)#调用一次生成器，令生成器对象前进到首个yield表达式。return generatorreturn wrapper

#!/usr/bin/env python
# encoding:utf-8@coroutine
def mouse_handler(successor=None):while True:event = (yield)if 0 < event <= 10:print("mouse-num:   {}".format(event))elif successor is not None:successor.send(event)@coroutine
def key_handler(successor=None):while True:event = (yield)if 10 < event <= 20:print("key-num:   {}".format(event))elif successor is not None:successor.send(event)@coroutine
def timer_handler(successor=None):while True:event = (yield)if 20 < event <= 30:print("timer-num: {}".format(event))elif successor is not None:successor.send(event)def main():pipeline = key_handler(mouse_handler(timer_handler()))events = [2, 5, 14, 22, 18, 3, 35, 27, 20]for event in events:pipeline.send(event)if __name__ == "__main__":main()