loading state demo

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {

}

router\.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route('my-route');
});

export default Router;

routes.application\.js

import Route from '@ember/routing/route';
import RSVP from 'rsvp';

export default class ApplicationRoute extends Route {
   async beforeModel() {
      await new RSVP.Promise(function(resolve) {
        setTimeout(resolve, 3000);
      });
    }
};

routes.my-route\.js

import Route from '@ember/routing/route';

export default Route.extend({
     async beforeModel() {
      await new Promise(function(resolve) {
        setTimeout(resolve, 3000);
      });
    }
});

templates.application-loading\.hbs

AWAIT

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
<LinkTo @route="my-route">My route</LinkTo>

{{outlet}}

templates.loading\.hbs

NO, PLEASE

templates.my-route\.hbs

<h2>My Route</h2>

twiddle\.json

{
  "version": "0.17.0",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.17.0",
    "ember-template-compiler": "3.17.0",
    "ember-testing": "3.17.0"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}