from abc import ABCMeta, abstractmethod


class Publisher:
  """被观察者：发布/订阅关系中的发布对象"""
  def __init__(self):
    self.subscribers = []
    self.latest_content = None

  def set_content(self, content):
    """有新消息时，发布新的消息"""
    self.latest_content = content
    self.publish()

  def get_latest_content(self):
    """获取最新的消息"""
    return self.latest_content

  def register(self, subscriber):
    """注册一个新的订阅者"""
    self.subscribers.append(subscriber)

  def publish(self):
    """发布消息并通知订阅的用户"""
    for subscriber in self.subscribers:
      subscriber.notify()


class Subscriber(metaclass=ABCMeta):
  """观察者的抽象类：需要定义一个通知接口，用于发布对象通知订阅的用户"""
  @abstractmethod
  def notify(self):
    pass


class SubscriberA(Subscriber):
  """观察者A：发布/订阅关系中的订阅者，当订阅的发布者有新的变化或动态的时候能及时收到通知"""
  def __init__(self):
    self.my_publisher = None

  def subscribe(self, publisher):
    """订阅并进行注册"""
    self.my_publisher = publisher
    self.my_publisher.register(self)

  def notify(self):
    """获取最新消息"""
    latest_content = self.my_publisher.get_latest_content()
    print(self, latest_content)


class SubscriberB(Subscriber):
  """观察者B：发布/订阅关系中的订阅者，当订阅的发布者有新的变化或动态的时候能及时收到通知"""
  def __init__(self):
    self.my_publisher = None

  def subscribe(self, publisher):
    """订阅并进行注册"""
    self.my_publisher = publisher
    self.my_publisher.register(self)

  def notify(self):
    """获取最新消息"""
    latest_content = self.my_publisher.get_latest_content()
    print(self, latest_content)


if __name__ == '__main__':
  new_publisher = Publisher()
  subscriber_a = SubscriberA()
  subscriber_a.subscribe(new_publisher)
  subscriber_b = SubscriberB()
  subscriber_b.subscribe(new_publisher)
  new_publisher.set_content('This is a new message!')