【转】观察者模式

Python2代码示例：

class AbstractSubject(object):
    def register(self, listener):
        raise NotImplementedError("Must subclass me")
 
    def deregister(self, listener):
        raise NotImplementedError("Must subclass me")
 
    def notify_listeners(self, event):
        raise NotImplementedError("Must subclass me")
 
class Listener(object):
    def __init__(self, name, subject):
        self.name = name
        subject.register(self)
 
    def notify(self, event):
        print self.name, "received event", event
 
class Subject(AbstractSubject):
    def __init__(self):
        self.listeners = []
        self.data = None

    def getUserAction(self):
        self.data = raw_input('Enter something to do:')
        return self.data

    # Implement abstract Class AbstractSubject

    def register(self, listener):
        self.listeners.append(listener)
 
    def deregister(self, listener):
        self.listeners.remove(listener)
 
    def notify_listeners(self, event):
        for listener in self.listeners:
            listener.notify(event)

 
if __name__=="__main__":
    # make a subject object to spy on
    subject = Subject()
 
    # register two listeners to monitor it.
    listenerA = Listener("<listener A>", subject)
    listenerB = Listener("<listener B>", subject)
 
    # simulated event
    subject.notify_listeners ("<event 1>")
    # outputs:
    #     <listener A> received event <event 1>
    #     <listener B> received event <event 1>
 
    action = subject.getUserAction()
    subject.notify_listeners(action)
    #Enter something to do:hello
    # outputs:
    #     <listener A> received event hello
    #     <listener B> received event hello

来源：维基百科