Observer Pattern

The Observer Pattern is a design pattern in software development used to implement event-driven communication systems. It belongs to the category of behavioral patterns and enables loose coupling between objects that wish to be notified of changes in another object.

The main goal of the Observer Pattern is to create a one-to-many dependency structure where multiple observers can watch a subject. When the state of the subject changes, all its registered observers are notified and automatically updated.

The key components of the Observer Pattern are:

1. Subject: This is the object being observed. It maintains a list of registered observers and provides methods to add, remove, and notify observers when its state changes.

2. Observer: This is the interface or class that defines how observers respond when they receive an update from the subject.

3. ConcreteSubject: This is the concrete implementation of the subject that changes its state and notifies the observers.

4. ConcreteObserver: This is the concrete implementation of the observer that receives notifications from the subject and responds to them.

Benefits of the Observer Pattern:

1. Loose coupling: The pattern enables loose coupling between the subject and its observers, as they are not directly dependent on each other.

2. Extensibility: It's easy to add new observers or remove existing ones without changing the subject's code.

3. Reusability: The Observer Pattern promotes reusability, as different observers can be combined with different subjects.

A common example of the Observer Pattern is the subscription of users for notifications. The notification system (subject) maintains a list of users (observers) waiting for changes. When a new notification is sent, all subscribed users are automatically notified and receive the update.