观察者模式

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The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

“观察者模式”：__被观察者对象__通过一个列表维护__观察者对象__的行为（方法），当__被观察者对象__状态改变时会主动呼叫（调用）__观察者对象__行为（方法）。

PS：观察者模式有时又被称为发布（publish）-订阅（Subscribe）模式、模型-视图（View）模式、源-收听者(Listener)模式或从属者模式。

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'''
Python示例代码，来自网络
'''
class Observable:
    def __init__(self):
        self.__observers = []
    
    def register_observer(self, observer):
        self.__observers.append(observer)
    
    def notify_observers(self, *args, **kwargs):
        for observer in self.__observers:
            observer.notify(self, *args, **kwargs)

class Observer:
    def __init__(self, observable):
        observable.register_observer(self)
    
    def notify(self, observable, *args, **kwargs):
        print('Got', args, kwargs, 'From', observable)


subject = Observable()
observer = Observer(subject)
subject.notify_observers('test')