Observer Pattern옵저버 패턴

Subject (Observable)
  - observers: List
  - subscribe(observer)
  - unsubscribe(observer)
  - notify()
       ↓ (상태 변화 시 notify 호출)
  Observer 1  Observer 2  Observer 3
  update()    update()    update()

from abc import ABC, abstractmethod
from typing import List

class Observer(ABC):
    @abstractmethod
    def update(self, event: str, data: dict): ...

class Subject:
    def __init__(self):
        self._observers: List[Observer] = []

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

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

    def notify(self, event: str, data: dict):
        for observer in self._observers:
            observer.update(event, data)

# 실제 Subject
class OrderService(Subject):
    def place_order(self, order):
        # ... 주문 처리 ...
        self.notify("order_placed", {"order_id": order.id})

# Observer 구현
class EmailNotifier(Observer):
    def update(self, event, data):
        if event == "order_placed":
            send_email(f"주문 확인: #{data['order_id']}")

class InventoryUpdater(Observer):
    def update(self, event, data):
        if event == "order_placed":
            update_stock(data['order_id'])

# 사용
service = OrderService()
service.subscribe(EmailNotifier())
service.subscribe(InventoryUpdater())
service.place_order(order)
# → EmailNotifier와 InventoryUpdater 자동 실행

const EventEmitter = require('events');

class OrderService extends EventEmitter {
    placeOrder(order) {
        // ... 처리 ...
        this.emit('order_placed', order);
    }
}

const service = new OrderService();
service.on('order_placed', (order) => sendEmail(order));
service.on('order_placed', (order) => updateInventory(order));