Pass parameters to loading substate

controllers.application.js

import Ember from 'ember';

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

controllers.foo-loading.js

import Ember from 'ember';

// Created as a target where properties can be stored.
// `queryParams` property isn't defined here because it 
// won't have access to the query params until after the transition has finalized
export default Ember.Controller.extend({
});

controllers.foo.js

import Ember from 'ember';

export default Ember.Controller.extend({
  queryParams: ['bar', 'baz']
});

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('foo');
});

export default Router;

routes.foo.js

import Ember from 'ember';
import { Promise } from 'rsvp';
import { getProperties, setProperties } from '@ember/object';

export default Ember.Route.extend({
  queryParams: {
  	bar: {
    	refreshModel: true
    },
    baz: {
    	refreshModel: true
    }
  },
  
  model() {
    return new Promise((resolve) => {
    	setTimeout(resolve, 500);
    });
  },
  
  actions: {
  	loading(transition, originalRoute) {
      const fooLoadingController = this.controllerFor('foo-loading');
      const queryParams = transition.queryParams;

      // Need to set the properties here because the controller doesn't have access direct access to the query parameters.
      setProperties(fooLoadingController, queryParams);
      
      return true;
    }
  }
});

templates.application.hbs

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

{{#link-to "foo" (query-params bar=true baz=null)}}Foo Bar=true{{/link-to}}
{{#link-to "foo" (query-params bar=null baz=true)}}Foo Baz=true{{/link-to}}

<br>
<br>
{{outlet}}
<br>
<br>

templates.foo-loading.hbs

Foo-loading!<br>

{{#if bar}}
Bar!
{{else if baz}}
Baz!
{{else}}
I dunno...
{{/if}}

templates.foo.hbs

Foo!<br>

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"
  }
}