What is The Event Aggregator in Aurelia?

Aurelia - Interview Questions

If you include the aurelia-event-aggregator plugin using "basicConfiguration" or "standardConfiguration" then the singleton EventAggregator's API will be also present on the Aurelia object. You can also create additional instances of the EventAggregator, if needed, and "merge" them into any object. To do this, import includeEventsIn and invoke it with the object you wish to turn into an event aggregator. For example includeEventsIn(myObject). Now my object has publish and subscribe methods and can be used in the same way as the global event aggregator, detailed below.

Publishing on a Channel : TypeScript

import {autoinject} from 'aurelia-framework';
  import {EventAggregator} from 'aurelia-event-aggregator';
  
  @autoinject
  export class APublisher {
    constructor(private eventAggregator: EventAggregator) { }
  
    publish(): void {
      var payload = {};
      this.eventAggregator.publish('channel name here', payload);
    }
  }

Subscribing to a Channel : TypeScript

import {autoinject} from 'aurelia-framework';
  import {EventAggregator} from 'aurelia-event-aggregator';
  
  @autoinject
  export class ASubscriber {
    constructor(private eventAggregator: EventAggregator) { }
  
    subscribe(): void {
      this.eventAggregator.subscribe('channel name here', payload => {
          ...
      });
    }
  }

Publishing a Message : TypeScript

  //some-message.ts
  export class SomeMessage{ }
  
  //a-publisher.ts
  import {autoinject} from 'aurelia-framework';
  import {EventAggregator} from 'aurelia-event-aggregator';
  import {SomeMessage} from './some-message';
  
  @autoinject
  export class APublisher {
    constructor(private eventAggregator: EventAggregator) { }
  
    publish(): void {
      this.eventAggregator.publish(new SomeMessage());
    }
  }

Subscribing to a Message Type : TypeScript

  import {autoinject} from 'aurelia-framework';
  import {EventAggregator} from 'aurelia-event-aggregator';
  import {SomeMessage} from './some-message';
  
  @autoinject
  export class ASubscriber {
    constructor(private eventAggregator: EventAggregator) { }
  
    subscribe(): void {
      this.eventAggregator.subscribe(SomeMessage, message => {
          ...
      });
    }
  }