New Twiddle

components.inner-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: 'inner-component',

	// passed in from outer component
  oneWayProp: null
});

components.outer-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: 'outer-component',

  // passed in from application controller
  mutableProp: null,
  
  oneWayProp: Ember.computed.oneWay('mutableProp')
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Consumerism',
  
  mutableProp: 1,
  
  actions: {
    addOne() {
      this.incrementProperty('mutableProp');
    }
  }
});

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.inner-component,
.outer-component {
  border: 1px solid salmon;
  padding: 5px;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<button {{action "addOne"}}>+1</button>
<br>
<br>
{{outer-component mutableProp=mutableProp}}
<br>
<br>

templates.components.inner-component.hbs

<h2>Inner</h2>

<pre><code>

oneWayProp:  {{oneWayProp}}

</code></pre>

templates.components.outer-component.hbs

<h2>Outer</h2>
<pre><code>

mutableProp: {{mutableProp}}
oneWayProp:  {{oneWayProp}}

</code></pre>

{{inner-component oneWayProp=oneWayProp}}

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"
  }
}