# What is Observer?

One object tells many listeners when something changes. Each listener updates on its own. Nothing polls them one by one.

## In real work

Observer: when the user toggles dark mode, every screen listens through an observer and colors refresh together.

## Related terms

- [Encapsulate What Varies](https://prompthausapp.com/learn/encapsulate-what-varies.md)
- [Composition Over Inheritance](https://prompthausapp.com/learn/composition-over-inheritance.md)
- [Program to Interface](https://prompthausapp.com/learn/program-to-interface.md)
- [Loosely Coupled](https://prompthausapp.com/learn/loosely-coupled.md)
- [Open-Closed Principle](https://prompthausapp.com/learn/open-closed-principle.md)

## How is this different from Googling it?

Google gives a definition. Prompthaus pairs each term with how it shows up in standups, PRs, prompts, and design reviews—then quizzes you until the word sticks. One term a day on iPhone.

- Category: Foundations
- HTML: https://prompthausapp.com/learn/observer
- App Store: https://apps.apple.com/us/app/prompthaus-ai-vocabulary/id6776314407