How does the Observer pattern fit into SOLID and DI?

Short answer: Observer supports SRP by separating notification logic from core business logic and DIP by depending on abstractions (observers). It fits DI because observers can be injected, allowing flexible runtime subscriptions and loose coupling.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.