FormTestCheckboxesZeroForm.php

<?php

namespace Drupal\form_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\HttpFoundation\JsonResponse;

/**
 * Tests checkboxes zero.
 *
 * @internal
 */
class FormTestCheckboxesZeroForm extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'form_test_checkboxes_zero';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, $json = TRUE) {
    $form_state
      ->set('json', $json);
    $form['checkbox_off'] = [
      '#title' => t('Checkbox off'),
      '#type' => 'checkboxes',
      '#options' => [
        'foo',
        'bar',
        'baz',
      ],
    ];
    $form['checkbox_zero_default'] = [
      '#title' => t('Zero default'),
      '#type' => 'checkboxes',
      '#options' => [
        'foo',
        'bar',
        'baz',
      ],
      '#default_value' => [
        0,
      ],
    ];
    $form['checkbox_string_zero_default'] = [
      '#title' => t('Zero default (string)'),
      '#type' => 'checkboxes',
      '#options' => [
        'foo',
        'bar',
        'baz',
      ],
      '#default_value' => [
        '0',
      ],
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => 'Save',
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    if ($form_state
      ->get('json')) {
      $form_state
        ->setResponse(new JsonResponse($form_state
        ->getValues()));
    }
    else {
      $form_state
        ->disableRedirect();
    }
  }

}