Observer

Observer Design Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically

Overview

The Observer pattern is a behavioral design pattern that lets you define a subscription mechanism to notify multiple objects about any events that happen to the object they're observing. It's also known as the Publish-Subscribe pattern.

Implementation Details

This implementation demonstrates a product availability notification system where users can subscribe to get notified when a product becomes available:

Key Components:

• ISubject Interface: Defines methods for managing observers (RegisterObserver, RemoveObserver, NotifyObservers)
• IObserver Interface: Defines the Update method that observers must implement
• Subject (Concrete Subject): Maintains product information and notifies observers of state changes
• Observer (Concrete Observer): Represents users who want to be notified about product availability

Classes:

• ISubject - Subject interface with observer management methods
• IObserver - Observer interface with Update(string availability) method
• Subject - Concrete subject that maintains product state (ProductName, ProductPrice, Availability)
• Observer - Concrete observer representing users with UserName property

Usage Example:

// Create a product subject
Subject RedMI = new("Red MI Mobile", 1000, "Out Of Stock");

// Create observers (users)
Observer user1 = new("Tushar");
Observer user2 = new("Karan");
Observer user3 = new("Kuldeep");

// Subscribe users to product notifications
user1.AddSubscriber(RedMI);
user2.AddSubscriber(RedMI);
user3.AddSubscriber(RedMI);

// Check current availability
Console.WriteLine("Red Mi Mobile current state: " + RedMI.GetAvailability());

// Unsubscribe a user
user3.RemoveSubscriber(RedMI);

// Change product availability - notifies remaining subscribers
RedMI.SetAvailability("Available");

Workflow:

1. Subject Creation: Product is created with initial "Out Of Stock" status
2. Observer Registration: Users subscribe to product notifications
3. State Change: Product availability changes to "Available"
4. Notification: All registered observers receive update notifications
5. Observer Removal: Users can unsubscribe from notifications

Benefits:

• Establishes loose coupling between subject and observers
• Supports broadcast communication
• Allows dynamic relationships between objects
• Follows Open/Closed Principle - can add new observers without modifying subject
• Enables event-driven programming model