ChangedItem.php

<?php

namespace Drupal\Core\Field\Plugin\Field\FieldType;

use Drupal\Core\Entity\SynchronizableInterface;
use Drupal\Core\Field\Attribute\FieldType;
use Drupal\Core\Field\ChangedFieldItemList;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Defines the 'changed' entity field type.
 *
 * Based on a field of this type, entity types can easily implement the
 * EntityChangedInterface.
 *
 * @see \Drupal\Core\Entity\EntityChangedInterface
 */
class ChangedItem extends CreatedItem {
    
    /**
     * {@inheritdoc}
     */
    public function preSave() {
        parent::preSave();
        // Set the timestamp to request time if it is not set.
        if (!$this->value) {
            $this->value = \Drupal::time()->getRequestTime();
        }
        else {
            // On an existing entity translation, the changed timestamp will only be
            // set to the request time automatically if at least one other field value
            // of the entity has changed. This detection does not run on new entities
            // and will be turned off if the changed timestamp is set manually before
            // save, for example during migrations or by using
            // \Drupal\content_translation\ContentTranslationMetadataWrapperInterface::setChangedTime().
            
            /** @var \Drupal\Core\Entity\ContentEntityInterface $entity */
            $entity = $this->getEntity();
            if (!$entity instanceof SynchronizableInterface || !$entity->isSyncing()) {
                $original = $entity->getOriginal();
                $langcode = $entity->language()
                    ->getId();
                if (!$entity->isNew() && $original && $original->hasTranslation($langcode)) {
                    $original_value = $original->getTranslation($langcode)
                        ->get($this->getFieldDefinition()
                        ->getName())->value;
                    if ($this->value == $original_value && $entity->hasTranslationChanges()) {
                        $this->value = \Drupal::time()->getRequestTime();
                    }
                }
            }
        }
    }

}