The Observer Design Pattern is a behavioral design pattern that defines a one-to-many relationship between objects. When one object (the subject) changes its state, all its dependents (the observers) are notified and updated automatically.

Intent

• Decouple the subject and its observers, allowing them to vary independently.
• Ensure that changes in the subject are reflected in the observers without direct coupling.

Key Components

1. Subject: Maintains a list of observers and notifies them of any state changes.
2. Observer Interface: Defines the method(s) used by the subject to notify observers.
3. Concrete Observer: Implements the observer interface and updates itself in response to changes in the subject.

When to Use

• When changes to one object need to automatically trigger updates in dependent objects.
• To implement event-driven programming or publish/subscribe mechanisms.