cms_content_sync-3.0.x-dev/modules/cms_content_sync_custom_field_example/src/Plugin/Field/FieldFormatter/CustomFieldFormatter.php

<?php

namespace Drupal\cms_content_sync_custom_field_example\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;

/**
 * Plugin implementation of the 'CustomFieldFormatter' formatter.
 *
 * @FieldFormatter(
 *   id = "cs_custom_field_formatter",
 *   label = @Translation("Content Sync - Custom Field Formatter"),
 *   field_types = {
 *     "cs_custom_field",
 *   }
 * )
 */
class CustomFieldFormatter extends FormatterBase {

  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];

    foreach ($items as $delta => $item) {
      if (isset($item->cs_text_field)) {
        $elements[$delta] = [
          '#type' => 'processed_text',
          '#text' => $item->cs_text_field,
          '#langcode' => $item->getLangcode(),
        ];
      }
    }

    return $elements;
  }

}