computed update

components.component-1.js

import Ember from 'ember';
import { computed } from '@ember/object';

export default Ember.Component.extend({
  bar: 1,
  
  shouldShow: true,
  
  bannerText: computed('bar', 'shouldShow', function() {
    if (!this.get('shouldShow')) {
      console.log('you will never see this in the console!!!!');
      return 'bannerText';
    }
    console.log('get showed banner text');
    return this.get('bar') + '_bannerText';
  }),
  
  foo: computed('bar', function(propertyName) {
    return this.get('bar') + '_propertyName: ' + propertyName;
  }),
  
  actions: {
    increaseBar() {
      this.set('bar', this.get('bar') + 1);
    },
    
    toggleBanner() {
      this.toggleProperty('shouldShow');
    }
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}

{{component-1}}
<br>
<br>

templates.components.component-1.hbs

foo: {{foo}}

<button {{action "increaseBar"}}>increase bar</button>

<div>----------------------------------------</div>

{{#if shouldShow}}
  <div>{{bannerText}}</div>
{{/if}}
<button {{action "toggleBanner"}}>toggle banner</button>

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"
  }
}