Computed set with after promise resolve, proxy

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  _internalAppName: 'Ember Twiddle',
  longRunningTask: new Ember.RSVP.defer(),
  
  appName: Ember.computed('_internalAppName', {
    get() {
      return this.get('_internalAppName');
    },
    
    set(key, value) {
      this.get('longRunningTask').promise.then(() => {
        this.set('_internalAppName', value);
      });
      return this.get('_internalAppName');
    }
  }),
  
  actions: {
    btn() {
    	this.set('appName', 'Delayed computed setter after promise resolves');
      
      Ember.run.later(() => {
        this.get('longRunningTask').resolve();
      }, 1000);
  	}
  } 
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
<button {{action 'btn'}}>Test</button>

<p><strong>_internalAppName value</strong>: {{_internalAppName}}</p>

twiddle.json

{
  "version": "0.10.5",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.8.0",
    "ember-data": "2.8.0",
    "ember-template-compiler": "2.8.0",
    "ember-testing": "2.8.0"
  },
  "addons": {}
}