New Twiddle

components.my-component\.js

import Component from '@glimmer/component';
import { inject as service } from '@ember/service'

export default class extends Component {
	@service foo;  
  
  get internalComponentProp() {
    return this.foo.intermediateProp;
  }
  
  get internalRootProp() {
    return this.foo.rootProp;
  }
}

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
}

services.foo\.js

import Service from '@ember/service';
import { readOnly } from '@ember/object/computed';
import { action, get, set } from '@ember/object';

export default class FooService extends Service {
  rootProp = 1;

  @readOnly('rootProp') intermediateProp;

  @action
  updateRootProp() {
    set(this, 'rootProp', get(this, 'rootProp') + 1);
  }
}

templates.application\.hbs

<MyComponent />

templates.components.my-component\.hbs

<ul>
  <li>root: {{this.foo.rootProp}}</li>
  <li>readOnly: {{this.foo.intermediateProp}}</li>
  <li>getter readOnly: {{this.internalComponentProp}}</li>
  <li>getter rootProp: {{this.internalRootProp}}</li>
</ul>

<button {{on "click" this.foo.updateRootProp}} type="button">update</button>

twiddle\.json

{
  "version": "0.17.1",
  "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/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}