New Twiddle

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({  
  // Computed property doesn't get the num changes when we have a didReceiveAttrs() defined. If we remove didReceiveAttrs(), it will work.
  didReceiveAttrs() {
    this._super(...arguments);
  },  	

  
  // Defining will make the computed property to work even if didReceiveAttrs() is defined.
  //num: null,
  
  computedNum: Ember.computed('num', function() {
    return "Number is: " + this.attrs.num;
  })
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  num: 0,
  
  actions: {
    incrementNum() {
      this.set('num', this.get('num') + 1);
    }
  }
});

templates.application.hbs

{{my-component num=(readonly num)}}
<button {{action "incrementNum"}}>Increment Number</button>

templates.components.my-component.hbs

<p>{{computedNum}}</p>

twiddle.json

{
  "version": "0.4.8",
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.8/ember.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/1.13.9/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.8/ember-template-compiler.js"
  }
}