🔍 Observer vs Mediator

✅ Purpose of Comparison

Observer and Mediator are both design patterns used to manage communication between multiple objects.
However, they differ fundamentally in their direction of communication and structural centralization.
This comparison highlights the structural distinctions and usage scenarios to help in selecting the appropriate pattern.

✅ Comparison Overview

Aspect	Observer	Mediator
Intent	Notify dependent objects of state changes	Centralize communication between multiple components
Primary Use	Reactive updates, event notification	UI coordination, chat systems, multi-component workflows
Structure	Publisher holds and notifies Subscribers	Mediator coordinates interactions among Participants
Reusability	Notification logic decoupled from recipients	Components reusable without depending on each other
Extensibility	Add new Subscribers for extended behavior	Change Mediator to alter interaction patterns centrally
Downsides	Can become tangled with complex dependency chains	Mediator tends to grow large and complex

✅ Similarities

• Both manage collaboration between multiple objects
• Triggered by state changes or events
• Promote loose coupling by decoupling direct interactions

✅ Key Differences

Aspect	Observer	Mediator
Communication	Unidirectional (Subject → Observer)	Bidirectional or multi-directional (via Mediator)
Centralization	None (decentralized notification)	Present (centralized coordination)
Scalability	Becomes harder to manage as observers grow	Easier to manage via central coordination
Dependency Structure	Can lead to many-to-many dependencies	Reduces direct dependencies via a single point
Typical Use Case	State change notifications (UI updates, monitoring)	Complex coordination (chat apps, UI integration)

✅ When to Choose Which

• ✅ Use Observer when you want to broadcast state changes
• ✅ Use Mediator when you want to coordinate multiple participants
• ✅ Choose Observer for event-driven reactions
• ✅ Choose Mediator for structured control flow between components

✅ UML Class Diagram

Observer Pattern

Mediator Pattern

✅ Practical Design Notes

• ✅ Observer is useful for live updates (e.g., UI refresh, metric notifications), but too many observers can lead to complex control logic.
• ✅ Mediator excels when multiple components interact with each other, offering a centralized structure to coordinate their behavior.
• ▶️ Example: If you want to notify multiple parts about a change, use Observer; if you want to manage interdependent UI logic centrally, use Mediator.

✅ Summary

• Observer is designed for state change notification and fits decentralized reaction-based designs
• Mediator is intended for interaction coordination, offering centralized control among components
• Both patterns reduce tight coupling, but differ in structure: decentralized vs centralized