Observers

application.controller.js

import Ember from 'ember';

var Person = Ember.Object.extend({
  // these will be supplied by `create`
  firstName: null,
  lastName: null,

  fullName: Ember.computed('firstName', 'lastName', function() {
    return `${this.get('firstName')} ${this.get('lastName')}`;
  }),

  fullNameChanged: Ember.observer('fullName', function() {
    // deal with the change
    document.write(`fullName changed to: ${this.get('fullName')}`);
  })
});

var person = Person.create({
  firstName: 'Yehuda',
  lastName: 'Katz'
});

// observer won't fire until `fullName` is consumed first
person.get('fullName'); // "Yehuda Katz"
//person.set('firstName', 'Brohuda'); // fullName changed to: Brohuda Katz

application.template.hbs

twiddle.json

{
  "version": "0.5.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}