New Twiddle

components.smart-wow.js

import Ember from 'ember';

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

components.x-loaded.js

import Ember from 'ember';

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

components.x-loading.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',
  test: 'smart-wow'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{#smart-wow isLoading=true as |ui|}}
  {{#ui.loading}}
  {{/ui.loading}}
  {{#ui.loaded}}
  {{/ui.loaded}}
{{/smart-wow}}
<br>
<br>

templates.components.smart-wow.hbs

<div>
<div>Header</div>
 {{#if isLoading}}
   {{yield (hash loading='x-loading')}}
 {{else}}
   {{yield (hash loaded='x-loaded')}}
 {{/if}}
<div>Footer</div>
</div>

templates.components.x-loaded.hbs

loaded

templates.components.x-loading.hbs

..loading

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"
  }
}