Observer を Python で

Observer は、振る舞いに関するデザインパターンの一つで、オブジェクトが別のオブジェクトに状態の変化を通知できるようにします。

Observer パターンは、サブスクライバー・インターフェースを実装するいかなるオブジェクトのイベント通知の申し込みと停止をする方法を提供します。

Observer の詳細

複雑度：

人気度：

使用例： Observer パターンは、 Python コードではよく見かけます。特に、 GUI コンポーネントで。他のオブジェクトで起きるイベントに、そのクラスに結合することなく、反応する方法を提供します。

見つけ方： このパターンは、オブジェクトをリストに保存するサブスクリプション・メソッドと、そのリスト中のオブジェクトの更新メソッドの呼び出しにより、識別できます。

概念的な例

この例は、 Observer デザインパターンの構造を説明するためのものです。以下の質問に答えることを目的としています：

どういうクラスからできているか？
それぞれのクラスの役割は？
パターンの要素同士はどう関係しているのか？

main.py: 概念的な例

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: 実行結果

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