LLD: Observer Pattern (Behavioral)

Definition

The Observer Pattern is a behavioral design pattern where an object (aka the “subject” or “observable” or “publisher”) maintains a list of dependents (called “observers”) and automatically notifies them whenever there is a change in its state. The pattern also allows addition and removal of observers at runtime.

Real-life Examples

Real-life examples of the Observer pattern include:

• Weather Applications: Where multiple devices receive updates from a weather station.
• Social Media Feeds: When we follow someone on Instagram, Facebook, or Twitter, we become observer of their profile. When they post new content, we are automatically notified.
• Subscription Services: YouTube subscriptions, where viewers are notified of new videos, or Content magazine/newspaper/newsletter subscriptions, where publishers send new issues to subscribers.
• Stock Market Trackers: When the price of a stock (or state) changes, the stock’s market system (the observable) sends out notifications to all interested investors (the observers).

Class Diagram