Drupal 8 Search API processor example

ReferenceLastDate.php

<?php

namespace Drupal\hc_core\Plugin\search_api\processor;

use Drupal\node\Entity\Node;
use Drupal\search_api\Datasource\DatasourceInterface;
use Drupal\search_api\IndexInterface;
use Drupal\search_api\Item\ItemInterface;
use Drupal\search_api\Processor\ProcessorPluginBase;
use Drupal\search_api\Processor\ProcessorProperty;

/**
 * Indexes only the last date of a reference.
 *
 * @SearchApiProcessor(
 *   id = "hc_reference_last_date",
 *   label = @Translation("Reference last date"),
 *   description = @Translation("Only gets the last date of the content"),
 *   stages = {
 *     "add_properties" = 0,
 *   },
 *   locked = true,
 *   hidden = false,
 * )
 */
class ReferenceLastDate extends ProcessorPluginBase {

  /**
   * {@inheritdoc}
   */
  public static function supportsIndex(IndexInterface $index) {
    foreach ($index->getDatasources() as $datasource) {
      $allowed = ['reference'];
      if ($datasource->getEntityTypeId() == 'node' && !empty(array_intersect(array_keys($datasource->getBundles()), $allowed))) {
        return TRUE;
      }
    }
    return FALSE;
  }

  /**
   * {@inheritdoc}
   */
  public function getPropertyDefinitions(DatasourceInterface $datasource = NULL) {
    $properties = array();

    if (!$datasource) {
      $definition = array(
        'label' => $this->t('Reference last date'),
        'description' => $this->t('The last date of the reference'),
        'type' => 'date',
        'processor_id' => $this->getPluginId(),
      );
      $properties['search_api_hc_reference_last_date'] = new ProcessorProperty($definition);
    }

    return $properties;
  }

  /**
   * {@inheritdoc}
   */
  public function addFieldValues(ItemInterface $item) {
    /** @var Node $node */
    $node = $item->getOriginalObject()->getValue();

    // Default value in case there is no timeline.
    $value = $node->getCreatedTime();

    // Loop through paragraphs to find a timeline.
    foreach ($node->field_description as $description_item) {
      $paragraph = $description_item->entity;
      if ($paragraph->bundle() === 'timeline') {
        $delta = $paragraph->field_timeline_dates->count() - 1;
        $value = $paragraph->field_timeline_dates->get($delta)->entity->field_date->value;
        break;
      }
    }

    $fields = $this->getFieldsHelper()
      ->filterForPropertyPath($item->getFields(), NULL, 'search_api_hc_reference_last_date');
    foreach ($fields as $field) {
      if (!$field->getDatasourceId()) {
        $field->addValue($value);
      }
    }
  }

}