Observer

components.item\.js

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


export default class extends Component {
  updateActiveItem = observer('activeItems', function() {
    console.log('updateActiveItem');
    this.isActive = this.activeItems.includes(this.item);
  })

  @action
  setActive(event) {
    console.log('setActive');
    event.preventDefault();
    if (this.activeItems.includes(this.item)) {
      this.activeItems.removeObject(this.item);
    } else {
      this.activeItems.pushObject(this.item);
    }
    console.log(this.activeItems, this.activeItems.length);
  }
}

components.items-container\.js

import Component from '@ember/component';
import { observer } from '@ember/object';

export default class extends Component {
  updateActiveItem = observer('activeItems.[]', function() {
    console.log('parent updateActiveItem');
  })
}

controllers.application\.js

import Controller from '@ember/controller';
import { A } from '@ember/array';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
	items = A([1, 2, 3, 4]);
  activeItems = A([1, 2]);
}

templates.application\.hbs

<ItemsContainer
	@items={{this.items}}
	@activeItems={{this.activeItems}}
/>

templates.components.item\.hbs

<input
	type="checkbox"
  checked={{this.isActive}}
  {{on 'click' setActive}}
/> Item #{{@item}}

templates.components.items-container\.hbs

{{#if @items}}
  <ul>
		{{#each @items as |item|}}
    	<li>
      	<Item
        	@item={{item}}
          @activeItems={{@activeItems}}
         />
      </li>
    {{/each}}
  </ul>
{{else}}
  <p>
    No items :'(
  </p>
{{/if}}

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"
  }
}