New Twiddle

components.my-checkbox\.js

import Component from '@glimmer/component';
import { get } from "@ember/object";
export default class extends Component {
  // get isRowSelected(){
  // 	return this.args.rowStates[this.args.rowId];
  // };
}

components.parent-component\.js

import Component from '@glimmer/component';
import { action } from '@ember/object';
import { tracked } from '@glimmer/tracking';
import { set } from "@ember/object";
import { A } from '@ember/array';
import EmberObject from '@ember/object';

export default class extends Component {
  constructor(){
    super(...arguments);
    this.rowStates = this.args.shareholders.reduce((result, item) => {
      result[String(item.id)] = false;
      return result;
    }, EmberObject)
    
    this.numberOfRows = Object.keys(this.rowStates).length;
  };
  
  @tracked rowStates = {};
	@tracked numberOfRows;

  get isMasterCheckboxSelected() {
    const numberOfSelectedRows = Object.keys(this.rowStates).filter((key) => this.rowStates[key]).length
    
    return numberOfSelectedRows == this.numberOfRows;
  };

  @action
  toggleAllCheckboxes(event) {
    Object.keys(this.rowStates).forEach((key) => {
      this.rowStates[key] = event.target.checked;
    });
    
    this.rowStates = this.rowStates;
    return false;
  };
 
  @action
  updateSelectedRows(event) {
    const rowId = event.target.getAttribute("data-row-id");
    this.rowStates[rowId] = event.target.checked
    
    this.rowStates =this.rowStates;
    return false;
  };
  
}

controllers.application\.js

import Controller from '@ember/controller';
import { tracked } from '@glimmer/tracking';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
	@tracked shareholders = [
    {
      name: 'SH 1',
      id: '1'
    },
    {
      name: 'SH 2',
      id: '2'
    },
    {
      name: 'SH 3',
      id: '3'
    }
  ]
}

templates.application\.hbs

<ParentComponent @shareholders={{this.shareholders}} />

templates.components.my-checkbox\.hbs

<input class="custom-control-input"
  type="checkbox"
  checked={{@isRowSelected}}
  data-row-id={{@rowId}}
  {{on "input" @updateSelectedRows}} />

{{@label}}

templates.components.parent-component\.hbs

<div>
<input class="custom-control-input" 
  type="checkbox"
  checked={{this.isMasterCheckboxSelected}}
  {{on "input" this.toggleAllCheckboxes}} />
Master
</div>

{{#each @shareholders as |shareholder|}}
  <div style="margin-left:10px">
    <MyCheckbox
      @updateSelectedRows={{this.updateSelectedRows}}
      @isRowSelected={{get this.rowStates shareholder.id}}
      @rowId={{shareholder.id}}
      @label={{shareholder.name}}
    />
  </div>
{{/each}}

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"
  }
}