EventEmitter bubbling through composition.

merging-event-emitters.js

/**
 * Demonstrates a method for creating bubbling behavior through EventEmitter
 * composition. With this method a parent class (ConsumerClass) is composed
 * of instances of helper classes (HelpfulClass), and wants to bubble all of
 * their events up to itself. The goal is that client code can listen at the
 * parent level (to receive all events) or at the instance level.
 *
 * Limitations without a little more scaffolding:
 * - no context for determining where an event originated from.   
 * - this implementation assumes that HelpfulClass.emitter will not have
 *   listeners attached at the time it's replaced, as they would not be
 *   copied into the new upstream emitter.
 */

const EventEmitter = require('events').EventEmitter;

class HelpfulClass {
  constructor() {
    this.emitter = new EventEmitter;
  }
  
  someFunc() {
    this.emitter.trigger('doingImportantThings');
    
    // ...
    
    this.emitter.trigger('didImportantThings');
  }
}

class ConsumerClass {
  constructor() {
    this.helperA = new HelpfulClass(/* ... */);
    this.helperB = new HelpfulClass(/* ... */);
    this.helperA.emitter = this.helperB.emitter = this.emitter = new EventEmitter;
  }
}


function main() {
  const mainDoer = new ConsumerClass();
  mainDoer.on('didImportantThings', () => /* ... */ );
}