rsvplist.module

<?php

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_alter()
 * Alter the form Add/Edit form to include admin setting for displaying
 * RSVPBlock with content
 */

function rsvplist_form_node_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $formState, $form_id) {
  $node = $formState->getFormObject()->getEntity();
  $current_node_type = $node->getType();
  $config = \Drupal::config('rsvplist.settings');
  $types = $config->get('allowed_types', []);
  // RSVP options for administrators
  if (in_array($current_node_type, $types)) {
    $form['rsvplist'] = [
      '#type' => 'details',
      '#title' => t('RSVP Collections'),
      '#access' => \Drupal::currentUser()->hasPermission('administer rsvplist'),
      '#group' => 'advanced',
      '#weight' => 100,
    ];
    /** @var \Drupal\rsvplist\EnablerService $enabler */
    $enabler = Drupal::service('rsvplist.enabler');
    $form['rsvplist']['rsvplist_enabled'] = [
      '#type' => 'checkbox',
      '#title' => t('Collect RSVP e-mail addresses for this node.'),
      '#default_value' => $enabler->isEnabled($node),
    ];
    foreach (array_keys($form['actions']) as $action) {
      if ($action != 'preview' && isset($form['actions'][$action]['#type']) && $form['actions'][$action]['#type'] === 'submit') {
        $form['actions'][$action]['#submit'][] = 'rsvplist_form_node_form_submit';
      }
    }
  }
}

/**
 * Form submission handler for RSVP item field on the node form.
 *
 * @see \rsvplist_form_node_form_alter()
 */
function rsvplist_form_node_form_submit(&$form, \Drupal\Core\Form\FormStateInterface $formState) {
  /** @var \Drupal\rsvplist\EnablerService $enabler */
  $enabler = Drupal::service('rsvplist.enabler');
  $node = $formState->getFormObject()->getEntity();
  if ($enabled = $formState->getValue('rsvplist_enabled')) {
    $enabler->setEnabled($node);
  }
  else {
    $enabler->delEnabled($node);
  }

}