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);
        }
    }

}