Mediator

Mediator is a behavioural design pattern which helps when there are many objects communicating with each other.

The Mediator itself is an object that encapsulates how one or more objects interact with each other. It controls how these object communication and resticts dependencies required to manage.

Components

Example

In this example, we will have a ConcreteMediator that will ensure certain Participants receive a message based on the Participant sending the Broadcast.

We will use the mediator to ensure messages sent from the FirstParticipant is broadcasted to both FirstParticipant and SecondParticipant while messages from the SecondParticipant are only reached by the FirstPartiicpant and not itself.

The abstract Participant class in this example is the Colleague and the subclasses are the ConcreteColleague classes.

interface Mediator {
  broadcast(msg: String, sender: Participant): void;
}

class ConcreteMediator implements Mediator {
  public firstParticipant: Participant;
  public secondParticipant: Participant;

  broadcast(msg: String, sender: Participant): void {
    if (sender === this.firstParticipant) {
      this.firstParticipant.receiveMsg(msg);
      this.secondParticipant.receiveMsg(msg);
    }

    if (sender === this.secondParticipant) {
      this.firstParticipant.receiveMsg(msg);
    }
  }
}

abstract class Participant {
  public mediator: Mediator;

  constructor(mediator: Mediator) {
    this.mediator = mediator;
  }

  broadcast(msg: String): void {
    this.mediator.broadcast(msg, this);
  }

  receiveMsg(msg: String): void {
    console.log('recieveMsg:', msg);
  }
}

class FirstParticipant extends Participant {
  constructor(mediator: Mediator) {
    super(mediator);
  }

  broadcast(msg: String) {
    console.log('First participant customized logic');
    super.broadcast(msg);
  }
}

class SecondParticipant extends Participant {
  constructor(mediator: Mediator) {
    super(mediator);
  }

  broadcast(msg: String) {
    console.log('Second participant customized logic');
    super.broadcast(msg);
  }
}

(function main() {
  const mediator = new ConcreteMediator();
  const firstParticipant = new FirstParticipant(mediator);
  const secondParticipant = new SecondParticipant(mediator);

  mediator.firstParticipant = firstParticipant;
  mediator.secondParticipant = secondParticipant;

  firstParticipant.broadcast(
    'FirstParticipant sends message to FirstParticipant, SecondParticipant'
  );
  secondParticipant.broadcast(
    'SecondParticipant sends message to FirstParticipant'
  );
})();
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