Demystifying Design Patterns: Observer Design Pattern

  1. Demystifying Design Patterns: Singleton Design Pattern
  2. Demystifying Design Patterns: Factory Method Design Pattern
  3. Demystifying Design Patterns: Abstract Factory Design Pattern
  4. Demystifying Design Patterns: Builder Design Pattern
  5. Demystifying Design Patterns: Prototype Design Pattern
  6. Demystifying Design Patterns: Adapter Design Pattern
  7. Demystifying Design Patterns: Bridge Design Pattern
  8. Demystifying Design Patterns: Composite Design Pattern
  9. Demystifying Design Patterns: Decorator Design Pattern
  10. Demystifying Design Patterns: Proxy Design Pattern
  11. Demystifying Design Patterns: Observer Design Pattern
  12. Demystifying Design Patterns: Strategy Design Pattern
  13. Demystifying Design Patterns: Command Design Pattern
  14. Demystifying Design Patterns: State Design Pattern
  15. Demystifying Design Patterns: Chain of Responsibility Design Pattern
  16. Demystifying Design Patterns: Visitor Design Pattern
  17. Demystifying Design Patterns: Template Method Design Pattern

Welcome back to our ongoing series on Demystifying Design Patterns! In this 11th installment, we delve deep into the Observer Design Pattern, a crucial component in the toolkit of every software developer. This behavioral design pattern enables objects to subscribe and receive notifications about changes or events in another object, fostering loosely coupled and highly maintainable systems. Let’s explore the Observer Pattern comprehensively, covering its core concepts, real-life examples, advanced use cases, and code examples in Java, C#, and Python.¬†

Observing Changes with the Observer Pattern

At its essence, the Observer Pattern manages dependencies between objects by establishing a one-to-many relationship between a subject (publisher) and its observers. It allows multiple observers to be notified of changes in the subject’s state without the subject needing knowledge of who its observers are.

Publisher-Subscriber Model

The Observer Pattern closely follows the publisher-subscriber model:

– Publisher (Subject): The subject holds the data or state of interest and maintains a list of its subscribers. When its state changes, it notifies all subscribers.

– Subscriber (Observer): Observers are entities interested in changes in the subject. They register themselves with the subject and receive notifications when the state changes.

Event Handling and UI Frameworks

The Observer Pattern plays a pivotal role in event-driven programming, particularly in graphical user interfaces (GUIs). UI frameworks often employ this pattern to manage user interactions. Instead of tightly coupling GUI elements like buttons, text fields, and checkboxes to their respective actions, these elements act as publishers, while the actions are subscribers. When a button is clicked, for instance, it notifies all subscribers, ensuring a decoupled and responsive UI.

Implementing Custom Event Systems

Beyond UIs, custom event systems can benefit greatly from the Observer Pattern. These systems are used when you need to handle events or changes that don’t neatly fit into pre-built event frameworks. Custom event systems are common in game development, server-side applications, and various other domains where specific event handling is required.

Real-Life Examples

Example 1: Stock Market Updates

Imagine developing a stock market application where various components need to react to changes in stock prices. Rather than each component constantly polling for updates, the Observer Pattern shines. The stock market data feed becomes the subject, and components displaying stock prices become observers. When a stock’s price changes, all interested components are automatically notified and update accordingly.

Example 2: Weather Station

In a weather monitoring system, sensors collect data like temperature, humidity, and wind speed. Weather displays, mobile apps, and data recorders act as observers. When a sensor records new data, it notifies all observers. This enables multiple components to react to real-time weather changes without being tightly coupled to the sensor logic.

Example 3: Social Media Notifications

Social media platforms rely on the Observer Pattern to notify users of new messages, comments, or likes. Users are subscribers, while the platform’s notifications system acts as the subject. When a new event occurs (e.g., a new message), the system notifies all subscribers, ensuring timely updates without overloading the server.

Code Examples

Now, let’s explore the Observer Pattern with code examples in Java, C#, and Python to demonstrate how to implement it effectively.


Java Example:
import java.util.ArrayList;
import java.util.List;

class WeatherData {
    private List observers = new ArrayList<>();
    private float temperature;

    public void addObserver(Observer observer) {

    public void removeObserver(Observer observer) {

    public void notifyObservers() {
        for (Observer observer : observers) {

    public void setTemperature(float temperature) {
        this.temperature = temperature;

interface Observer {
    void update(float temperature);

class WeatherDisplay implements Observer {
    public void update(float temperature) {
        System.out.println("Weather Display: Temperature is now " + temperature);

public class ObserverPatternDemo {
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
        WeatherDisplay display = new WeatherDisplay();


        weatherData.setTemperature(25.5f); // This triggers an update to the display.
C# Example:
using System;
using System.Collections.Generic;

interface IObserver
    void Update(float temperature);

class WeatherData
    private List observers = new List();
    private float temperature;

    public void AddObserver(IObserver observer)

    public void RemoveObserver(IObserver observer)

    public void NotifyObservers()
        foreach (var observer in observers)

    public void SetTemperature(float temperature)
        this.temperature = temperature;

class WeatherDisplay : IObserver
    public void Update(float temperature)
        Console.WriteLine($"Weather Display: Temperature is now {temperature}");

class ObserverPatternDemo
    static void Main(string[] args)
        WeatherData weatherData = new WeatherData();
        WeatherDisplay display = new WeatherDisplay();


        weatherData.SetTemperature(25.5f); // This triggers an update to the display.
Python Example:
class WeatherData:
    def __init__(self):
        self.observers = []
        self.temperature = 0.0

    def add_observer(self, observer):

    def remove_observer(self, observer):

    def notify_observers(self):
        for observer in self.observers:

    def set_temperature(self, temperature):
        self.temperature = temperature

class Observer:
    def update(self, temperature):

class WeatherDisplay(Observer):
    def update(self, temperature):
        print(f"Weather Display: Temperature is now {temperature}")

if __name__ == "__main__":
    weather_data = WeatherData()
    display = WeatherDisplay()


    weather_data.set_temperature(25.5)  # This triggers an update to the display.

Advanced Observer Patterns

While we’ve covered the basic Observer Pattern, there are more advanced variations, such as the Push vs. Pull Model and the Java’s Built-in Observer Pattern¬†using the `java.util.Observer` and `java.util.Observable` classes. Exploring these advanced concepts can enhance your understanding of how the pattern can be tailored to suit different scenarios.


The Observer Design Pattern is a powerful tool for managing dependencies, promoting flexibility, and building maintainable software systems. By enabling objects to subscribe and receive notifications about changes in other objects, it fosters loose coupling and scalability.

In this article, we’ve explored the core concepts of the Observer Pattern, discussed real-life examples, and provided code examples in Java, C#, and Python to help you implement it in your projects. Additionally, we touched upon advanced Observer Pattern variations to deepen your knowledge.

With this newfound understanding, you can harness the Observer Pattern’s potential to design robust, adaptable, and responsive software systems. Stay tuned for our next installment in the Demystifying Design Patterns series!