trait AddEventFormTrait

trait AddEventFormTrait {
  
  /**
   * The Rules event manager.
   *
   * @var \Drupal\rules\Core\RulesEventManager
   */
  protected $eventManager;
  
  /**
   * The entity type bundle information manager.
   *
   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
   */
  protected $entityBundleInfo;
  
  /**
   * {@inheritdoc}
   */
  public function buildEventForm(array $form, FormStateInterface $form_state) {
    $event_definitions = $this->eventManager
      ->getGroupedDefinitions();
    $options = [];
    foreach ($event_definitions as $group => $definitions) {
      foreach ($definitions as $id => $definition) {
        $options[$group][$id] = $definition['label'];
      }
    }
    $form['#entity_builders'][] = '::entityBundleBuilder';
    $form['selection'] = [
      '#type' => 'details',
      '#title' => $this->t('Event selection'),
      '#open' => TRUE,
    ];
    $form['selection']['events'] = [
      '#tree' => TRUE,
    ];
    // Selection of an event will trigger an Ajax request to see if this is an
    // entity event; if so, present a select element to choose a bundle type.
    $form['selection']['events'][]['event_name'] = [
      '#type' => 'select',
      '#title' => $this->t('React on event'),
      '#options' => $options,
      '#description' => $this->t('Rule evaluation is triggered whenever the selected event occurs.'),
      '#required' => TRUE,
      '#ajax' => [
        'event' => 'change',
        'wrapper' => 'entity-bundle-restriction',
        'callback' => '::bundleSelectCallback',
      ],
    ];
    // Empty container to hold the bundle selection element, if available
    // for the event chosen above.
    $form['selection']['container'] = [
      '#type' => 'container',
      '#attributes' => [
        'id' => 'entity-bundle-restriction',
      ],
    ];
    $event_name = $form_state->getValue([
      'events',
      0,
      'event_name',
    ]);
    // On form reload via Ajax, the $event_name will be set.
    if (!empty($event_name)) {
      // Add a non-required select element "Restrict by type" to choose from
      // all the bundles defined for the entity type.
      $event_definition = $this->eventManager
        ->getDefinition($event_name);
      $handler_class = $event_definition['class'];
      if (is_subclass_of($handler_class, RulesConfigurableEventHandlerInterface::class)) {
        // We have bundles ...
        $bundles = $this->entityBundleInfo
          ->getBundleInfo($event_definition['entity_type_id']);
        // Transform the $bundles array into a form suitable for select options.
        array_walk($bundles, function (&$value, $key) {
          $value = $value['label'];
        });
        // Bundle selections for this entity type.
        $form['selection']['container']['bundle'] = [
          '#type' => 'select',
          '#title' => $this->t('Restrict by type'),
          '#empty_option' => '- None -',
          '#empty_value' => 'notselected',
          '#options' => $bundles,
          '#description' => $this->t('If you need to filter for multiple values, either add multiple events or use the "Entity is of bundle" condition. These options are available after saving this form.'),
        ];
      }
    }
    return $form;
  }
  
  /**
   * Ajax callback for the entity bundle restriction select element.
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   *
   * @return array
   *   The form structure.
   */
  public function bundleSelectCallback(array $form, FormStateInterface $form_state) {
    // Replace the entire container placeholder element.
    return $form['selection']['container'];
  }
  
  /**
   * Callback method for the #entity_builder form property.
   *
   * Used to qualify the selected event name with a bundle suffix.
   *
   * @param string $entity_type
   *   The type of the entity.
   * @param \Drupal\Core\Config\Entity\ConfigEntityInterface $entity
   *   The entity whose form is being built.
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   */
  public function entityBundleBuilder($entity_type, ConfigEntityInterface $entity, array $form, FormStateInterface $form_state) {
    $bundle = $form_state->getValue('bundle');
    if (!empty($bundle) && $bundle != 'notselected') {
      $event_name = $form_state->getValue([
        'events',
        0,
        'event_name',
      ]);
      // Fully-qualify the event name if a bundle was selected.
      $form_state->setValue([
        'events',
        0,
        'event_name',
      ], $event_name . '--' . $bundle);
    }
  }

}