Observer Pattern

Video Lecture

Description

The observer pattern is a software design pattern in which an object, called the subject or observable, manages a list of dependents, called observers, and notifies them automatically of any internal state changes, and calls one of their methods.

Source Code

observer.py

"""
Observer Design Pattern
"""

from abc import ABCMeta, abstractmethod


class IObservable(metaclass=ABCMeta):
    @staticmethod
    @abstractmethod
    def subscribe(observer):
        """The subscribe method"""

    @staticmethod
    @abstractmethod
    def unsubscribe(observer):
        """The unsubscribe method"""

    @staticmethod
    @abstractmethod
    def notify(observer):
        """The notify method"""


class Subject(IObservable):
    def __init__(self):
        self._observers = set()

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

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

    def notify(self, *args, **kwargs):
        for observer in self._observers:
            observer.notify(self, *args, **kwargs)


class IObserver(metaclass=ABCMeta):
    @staticmethod
    @abstractmethod
    def notify(observable, *args, **kwargs):
        """Receive notifications"""


class Observer(IObserver):
    def __init__(self, observable):
        observable.subscribe(self)

    def notify(self, observable, *args, **kwargs):
        print("Observer received", args, kwargs)


SUBJECT = Subject()
OBSERVERA = Observer(SUBJECT)
OBSERVERB = Observer(SUBJECT)

SUBJECT.notify("Hello Observers")

SUBJECT.unsubscribe(OBSERVERB)
SUBJECT.notify("Hello Observers")