Code Notes made by Vaibhav Rathod

Observer Pattern

1 min read

Written by Vaibhav Rathod

Explanation

  • The Observer Pattern is a behavioral design pattern where an object (the subject) maintains a list of observers that need to be notified when the subject’s state changes.
  • “Subject-Observer”: The subject is responsible for notifying its observers about changes.
  • Use Case: Ideal for event-driven systems, where you need to update multiple components when the state of an object changes.

Steps

  • Define a Subject class that keeps track of its observers.
  • Define an Observer interface that receives updates from the subject.
  • Implement concrete Observers that react to changes.
class Observer:
    def update(self, state):
        pass
 
class ConcreteObserver(Observer):
    def update(self, state):
        print(f"State updated to: {state}")
 
class Subject:
    def __init__(self):
        self._observers = []
        self._state = None
 
    def add_observer(self, observer):
        self._observers.append(observer)
 
    def set_state(self, state):
        self._state = state
        self.notify()
 
    def notify(self):
        for observer in self._observers:
            observer.update(self._state)
 
# Observer pattern usage
subject = Subject()
observer1 = ConcreteObserver()
subject.add_observer(observer1)
 
subject.set_state(10)  # Output: State updated to: 10

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