New Twiddle

components.child\.js

import Component from '@glimmer/component';
import { get, set, computed } from '@ember/object';

export default class extends Component {
  // Why does this work? Isn't it like A.?
  get firstArrayObject() {
  	return this.args.myArray[0];
  }
  
  // A. This does not work
  // I believe this is because we need to let the getter know about the dependency. Using `set` is not enough
  get firstArrayObjectCount() {
    return this.args.myArray[0].count;
  }
  
  // B. This works. 
  //I believe it is because using `get`, we let the getter know that there is a dependency in there
  /**
  get firstArrayObjectCount() {
    return get(this.args.myArray[0], 'count');
  }
  **/
}

components.parent\.js

import Component from '@glimmer/component';
import { action, set, get } from '@ember/object';
import { tracked } from '@glimmer/tracking';

export default class extends Component {
	myArray = [{count: 0}, {count: 0}];
	
	@action
	doSomething() {
  	set(this.myArray[0], 'count', this.myArray[0].count + 1);
  }
}

controllers.application\.js

import Controller from '@ember/controller';

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

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
<Parent />
<br>
<br>

templates.components.child\.hbs

<div>
First array object with count property: {{this.firstArrayObject.count}}
</div>
<div>
First array object count computed property: {{this.firstArrayObjectCount}}
</div>

templates.components.parent\.hbs

<button {{on 'click' this.doSomething}}>Click me</button>
<br/>
<br/>
<div>
	<Child @myArray={{this.myArray}} />
</div>

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"
  }
}