14_observer.py

"""
	Observer
	- behavioral design pattern that lets you define a subscription mechanism to notify
	multiple objects about any events that happen to the object they’re observing
"""
import abc
from random import randrange


class Publisher(abc.ABC):

	@abc.abstractmethod
	def subscribe(self, observer):
		pass

	@abc.abstractmethod
	def unsubscribe(self, observer):
		pass

	@abc.abstractmethod
	def notify(self):
		pass


class ConcretePublisher(Publisher):
	_observers = []
	_state = None

	def subscribe(self, observer):
		self._observers.append(observer)

	def unsubscribe(self, observer):
		self._observers.remove(observer)

	def notify(self):
		print('Notifying observers...')

		for observer in self._observers:
			observer.update(self)

	def operation(self):
		self._state = randrange(0, 10)
		print(f'state changed to {self._state}')
		self.notify()


class Observer(abc.ABC):

	@abc.abstractmethod
	def update(self, publisher):
		pass


class ObserverA(Observer):
	def update(self, publisher):
		if publisher._state <= 5:
			print('ObserverA reacted to the event')


class ObserverB(Observer):
	def update(self, publisher):
		if publisher._state > 5:
			print('ObserverB reacted to the event')


publisher = ConcretePublisher()

observer_a = ObserverA()
observer_b = ObserverB()

publisher.subscribe(observer_a)
publisher.subscribe(observer_b)

publisher.operation()