Mediator

Centralises communication between components so they don't reference each other directly. Components only know the mediator, not each other — reducing coupling from O(n²) to O(n).

When to Use

• Many objects communicate in complex ways, creating tight coupling
• Reusing components is hard because they carry too many references
• A UI form with many interacting controls (enabling/disabling fields based on others)
• Chat rooms, event buses, air traffic control, workflow engines

How to Apply

1. Define a Mediator interface with a notify(sender, event) method
2. Create a ConcreteMediator that knows all components and coordinates their interactions
3. Components hold a reference to the mediator and call mediator.notify(this, event) instead of calling each other
4. Move interaction logic from components into the mediator

Example

interface Mediator { notify(sender: Component, event: string): void; }

abstract class Component { constructor(protected mediator: Mediator) {} }

class AuthForm extends Component {
  login() { this.mediator.notify(this, 'login'); }
}
class Dashboard extends Component {
  show() { console.log('Dashboard visible'); }
}

class AppMediator implements Mediator {
  constructor(private auth: AuthForm, private dash: Dashboard) {}
  notify(_sender: Component, event: string) {
    if (event === 'login') this.dash.show();
  }
}

const auth = new AuthForm(null!);
const dash = new Dashboard(null!);
const med  = new AppMediator(auth, dash);
(auth as any).mediator = med;
(dash as any).mediator = med;
auth.login(); // → Dashboard visible

References

• refactoring.guru/design-patterns/mediator