Observateur en Python
L’Observateur est un patron de conception comportemental qui permet à certains objets d’envoyer des notifications concernant leur état à d’autres objets.
Ce patron fournit la possibilité aux objets qui implémentent une interface de souscription, de s’inscrire et de se désinscrire de ces événements.
Complexité :
Popularité :
Exemples d’utilisation : L’observateur est assez répandu en Python, surtout dans les composants GUI. Il fournit une manière de réagir aux événements qui se produisent chez d’autres objets sans se coupler à leurs classes.
Identification : Ce patron peut être reconnu dans les méthodes de souscription qui stockent des objets dans une liste et par les appels des objets de cette liste à la méthode update.
Exemple conceptuel
Dans cet exemple, nous allons voir la structure de l’Observateur. Nous allons répondre aux questions suivantes :
- Que contiennent les classes ?
- Quels rôles jouent-elles ?
- Comment les éléments du patron sont-ils reliés ?
main.py: Exemple conceptuel
from __future__ import annotations
from abc import ABC, abstractmethod
from random import randrange
from typing import List
class Subject(ABC):
"""
The Subject interface declares a set of methods for managing subscribers.
"""
@abstractmethod
def attach(self, observer: Observer) -> None:
"""
Attach an observer to the subject.
"""
pass
@abstractmethod
def detach(self, observer: Observer) -> None:
"""
Detach an observer from the subject.
"""
pass
@abstractmethod
def notify(self) -> None:
"""
Notify all observers about an event.
"""
pass
class ConcreteSubject(Subject):
"""
The Subject owns some important state and notifies observers when the state
changes.
"""
_state: int = None
"""
For the sake of simplicity, the Subject's state, essential to all
subscribers, is stored in this variable.
"""
_observers: List[Observer] = []
"""
List of subscribers. In real life, the list of subscribers can be stored
more comprehensively (categorized by event type, etc.).
"""
def attach(self, observer: Observer) -> None:
print("Subject: Attached an observer.")
self._observers.append(observer)
def detach(self, observer: Observer) -> None:
self._observers.remove(observer)
"""
The subscription management methods.
"""
def notify(self) -> None:
"""
Trigger an update in each subscriber.
"""
print("Subject: Notifying observers...")
for observer in self._observers:
observer.update(self)
def some_business_logic(self) -> None:
"""
Usually, the subscription logic is only a fraction of what a Subject can
really do. Subjects commonly hold some important business logic, that
triggers a notification method whenever something important is about to
happen (or after it).
"""
print("\nSubject: I'm doing something important.")
self._state = randrange(0, 10)
print(f"Subject: My state has just changed to: {self._state}")
self.notify()
class Observer(ABC):
"""
The Observer interface declares the update method, used by subjects.
"""
@abstractmethod
def update(self, subject: Subject) -> None:
"""
Receive update from subject.
"""
pass
"""
Concrete Observers react to the updates issued by the Subject they had been
attached to.
"""
class ConcreteObserverA(Observer):
def update(self, subject: Subject) -> None:
if subject._state < 3:
print("ConcreteObserverA: Reacted to the event")
class ConcreteObserverB(Observer):
def update(self, subject: Subject) -> None:
if subject._state == 0 or subject._state >= 2:
print("ConcreteObserverB: Reacted to the event")
if __name__ == "__main__":
# The client code.
subject = ConcreteSubject()
observer_a = ConcreteObserverA()
subject.attach(observer_a)
observer_b = ConcreteObserverB()
subject.attach(observer_b)
subject.some_business_logic()
subject.some_business_logic()
subject.detach(observer_a)
subject.some_business_logic()
Output.txt: Résultat de l’exécution
Subject: Attached an observer.
Subject: Attached an observer.
Subject: I'm doing something important.
Subject: My state has just changed to: 0
Subject: Notifying observers...
ConcreteObserverA: Reacted to the event
ConcreteObserverB: Reacted to the event
Subject: I'm doing something important.
Subject: My state has just changed to: 5
Subject: Notifying observers...
ConcreteObserverB: Reacted to the event
Subject: I'm doing something important.
Subject: My state has just changed to: 0
Subject: Notifying observers...
ConcreteObserverB: Reacted to the event
