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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.

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.