This pattern shines in scenarios where:

A straightforward approach is to directly call update methods on other objects whenever something changes. For example, a NewsPublisher might call update() on a MobileApp, EmailService, and WebsiteFeed every time a new article is published.

But as the number of subscribers grows, this approach becomes rigid, hard to scale, and violates the Open/Closed Principle. Adding or removing subscribers requires modifying the publisher class. It also tightly couples the publisher to all its subscribers.

The Observer Pattern solves this by decoupling the subject and its observers, allowing them to interact through a common interface. Observers can be added or removed at runtime, and the subject doesn’t need to know who they are.

Let’s walk through a real-world example to see how we can apply the Observer Pattern to build a flexible, extensible, and loosely coupled notification system that’s perfect for event-driven applications.

1. The Problem: Broadcasting Fitness Data

Imagine you are building a Fitness Tracker App that connects to a wearable device. The device continuously streams real-time fitness data: steps taken, active minutes, and calories burned. This data flows into a central FitnessData object.

Now, multiple modules within your app need to react to these updates:

The Naive Approach

In a simple approach, the FitnessData object directly holds and manages references to all its dependent modules.

FitnessDataNaive

FitnessAppNaiveClient

This works initially, but let us think about what happens as the application grows.

Problems with This Approach

1. Tight Coupling

The FitnessData class now has intimate knowledge of every module that cares about its data. It holds direct references to LiveActivityDisplay, ProgressLogger, and NotificationService.

If any of these classes change their interface, or if you want to replace one with a different implementation, you must modify FitnessData.

2. Violates the Open/Closed Principle

What happens when you want to add a WeeklySummaryGenerator? Or a SocialSharingService that posts achievements to social media?

Each new feature requires you to:

The class is open for modification when it should be closed.

3. Inflexible and Static Design

Modules like the NotificationService or ProgressLogger can’t be added or removed at runtime. What if the user disables notifications in their settings?

You’ll need to add even more conditionals to manually enable/disable parts of the code — making things fragile and error-prone.

4. Responsibility Bloat

The FitnessData class should have one job: managing fitness metrics. Instead, it is now responsible for UI updates, database logging, and notification logic. This violates the Single Responsibility Principle and makes the class difficult to test in isolation.

5. Scalability Bottlenecks

As the number of dependents grows, newFitnessDataPushed() becomes a lengthy sequence of method calls, each potentially with different parameters and error handling requirements. The method becomes a bottleneck that every developer must understand and modify.

What We Really Need

We need a better, scalable way to solve this problem, something that allows:

This is exactly what the Observer pattern provides.

2. Understanding the Observer Pattern

The Observer Design Pattern provides a clean and flexible solution to the problem of broadcasting changes from one central object (the Subject) to many dependent objects (the Observers) — all while keeping them loosely coupled.

In our Fitness Tracker App, the Observer Pattern allows the FitnessData object to notify all registered modules (like LiveActivityDisplay, ProgressLogger, and GoalNotifier) automatically whenever new fitness data is received — without needing to know what those modules are or how they respond.

Class Diagram

1. Observer Interface (e.g., FitnessDataObserver)

2. Subject Interface (e.g., FitnessDataSubject)

Declares methods to:

The subject doesn't care who the observers are — it just sends updates.

3. ConcreteSubject (e.g., FitnessData)

4. ConcreteObservers (e.g., LiveActivityDisplay)

3. Implementing Observer Pattern

Let’s refactor our fitness tracker system using the Observer Pattern.

1. Define the FitnessDataObserver Interface

Each observer receives a reference to the subject and can pull whatever data it needs. This keeps the interface stable even as FitnessData evolves.

2. Define the FitnessDataSubject Interface

This interface allows observers to register and unregister themselves, and lets the subject notify all observers of changes.

3. Implement the FitnessData Class (ConcreteSubject)

This class will hold the actual fitness data, manage a list of observers, and notify them upon data changes.

4. Implement Observer Modules

Each observer defines its own update() logic based on the new fitness data.

LiveActivityDisplay

ProgressLogger

GoalNotifier

5. Client Code

Now we wire everything together and simulate some fitness updates:

What We Achieved