New Twiddle

components.child-component.js

import Ember from 'ember';
import EmberObject, {
  observer
} from '@ember/object';

export default Ember.Component.extend({
  alertWhenMultipleOfFive: observer('observable.progress', function() {
    if (this.get('observable.progress') % 5 === 0) {
      alert('Child says progress was a multiple of 5!');
    }
  })
});

components.parent-component.js

import Ember from 'ember';

export default Ember.Component.extend({
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  init() {
  	this.set('observable', Ember.Object.create({
      progress: 0
    }));
	},
  
  actions: {
    increaseProgress() {
      this.incrementProperty('observable.progress');
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<button {{action "increaseProgress"}}>Increase progress</button>
{{parent-component observable=observable}}
<br>
<br>

templates.components.child-component.hbs

Child receives value: {{observable.progress}}

templates.components.parent-component.hbs

Parent receives value: {{observable.progress}}
{{child-component observable=observable}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}