#Observer

#Overview

Define a one-to-many dependency so that when one object changes state, all dependents are notified.

#When to use

• You need event-driven updates.
• Multiple components react to the same change.

#Java example

interface Observer {
    void update(int value);
}

class Subject {
    private final List<Observer> observers = new ArrayList<>();
    void subscribe(Observer o) { observers.add(o); }
    void setValue(int value) {
        observers.forEach(o -> o.update(value));
    }
}

#TypeScript example

type Observer = (value: number) => void;

class Subject {
  private observers: Observer[] = [];
  subscribe(o: Observer): void { this.observers.push(o); }
  setValue(value: number): void { this.observers.forEach(o => o(value)); }
}

#Pros and cons

Pros:

• Loose coupling between subject and observers.
• Easy to add new listeners.

Cons:

• Hard to debug update order.
• Potential memory leaks from subscriptions.

#Common pitfalls

• Forgetting to unsubscribe.
• Triggering cascading updates unintentionally.

#Related patterns

• Mediator
• Strategy

#title: Observer sidebar_position: 7 tags: [design-patterns, behavioral]

#Observer

#Overview

Define a one-to-many dependency so that when one object changes state, all dependents are notified.

#When to use

• You need event-driven updates.
• Multiple components react to the same change.

#Java example

interface Observer {
    void update(int value);
}

class Subject {
    private final List<Observer> observers = new ArrayList<>();
    void subscribe(Observer o) { observers.add(o); }
    void setValue(int value) {
        observers.forEach(o -> o.update(value));
    }
}

#TypeScript example

type Observer = (value: number) => void;

class Subject {
  private observers: Observer[] = [];
  subscribe(o: Observer): void { this.observers.push(o); }
  setValue(value: number): void { this.observers.forEach(o => o(value)); }
}

#Pros and cons

Pros:

• Loose coupling between subject and observers.
• Easy to add new listeners.

Cons:

• Hard to debug update order.
• Potential memory leaks from subscriptions.

#Common pitfalls

• Forgetting to unsubscribe.
• Triggering cascading updates unintentionally.

#Related patterns

• Mediator
• Strategy