class ModerationStateFieldItemList

class ModerationStateFieldItemList extends FieldItemList {
  use ComputedItemListTrait {
    get as traitGet;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function computeValue() {
    $moderation_state = $this->getModerationStateId();
    // Do not store NULL values, in the case where an entity does not have a
    // moderation workflow associated with it, we do not create list items for
    // the computed field.
    if ($moderation_state) {
      // An entity can only have a single moderation state.
      $this->list[0] = $this->createItem(0, $moderation_state);
    }
  }
  
  /**
   * Gets the moderation state ID linked to a content entity revision.
   *
   * @return string|null
   *   The moderation state ID linked to a content entity revision.
   */
  protected function getModerationStateId() {
    $entity = $this->getEntity();
    /** @var \Drupal\content_moderation\ModerationInformationInterface $moderation_info */
    $moderation_info = \Drupal::service('content_moderation.moderation_information');
    if (!$moderation_info->shouldModerateEntitiesOfBundle($entity->getEntityType(), $entity->bundle())) {
      return NULL;
    }
    // Existing entities will have a corresponding content_moderation_state
    // entity associated with them.
    if (!$entity->isNew() && $content_moderation_state = $this->loadContentModerationStateRevision($entity)) {
      return $content_moderation_state->moderation_state->value;
    }
    // It is possible that the bundle does not exist at this point. For example,
    // the node type form creates a fake Node entity to get default values.
    // @see \Drupal\node\NodeTypeForm::form()
    $workflow = $moderation_info->getWorkFlowForEntity($entity);
    return $workflow ? $workflow->getTypePlugin()
      ->getInitialState($entity)
      ->id() : NULL;
  }
  
  /**
   * Load the content moderation state revision associated with an entity.
   *
   * @param \Drupal\Core\Entity\ContentEntityInterface $entity
   *   The entity the content moderation state entity will be loaded from.
   *
   * @return \Drupal\content_moderation\Entity\ContentModerationStateInterface|null
   *   The content_moderation_state revision or FALSE if none exists.
   */
  protected function loadContentModerationStateRevision(ContentEntityInterface $entity) {
    $moderation_info = \Drupal::service('content_moderation.moderation_information');
    $content_moderation_storage = \Drupal::entityTypeManager()->getStorage('content_moderation_state');
    $revisions = $content_moderation_storage->getQuery()
      ->accessCheck(FALSE)
      ->condition('content_entity_type_id', $entity->getEntityTypeId())
      ->condition('content_entity_id', $entity->id())
      ->condition('content_entity_revision_id', $entity->isNewRevision() ? $entity->getLoadedRevisionId() : $entity->getRevisionId())
      ->condition('workflow', $moderation_info->getWorkflowForEntity($entity)
      ->id())
      ->condition('langcode', $entity->language()
      ->getId())
      ->allRevisions()
      ->sort('revision_id', 'DESC')
      ->execute();
    if (empty($revisions)) {
      return NULL;
    }
    /** @var \Drupal\content_moderation\Entity\ContentModerationStateInterface $content_moderation_state */
    $content_moderation_state = $content_moderation_storage->loadRevision(key($revisions));
    if ($entity->getEntityType()
      ->hasKey('langcode')) {
      $langcode = $entity->language()
        ->getId();
      if (!$content_moderation_state->hasTranslation($langcode)) {
        $content_moderation_state->addTranslation($langcode, $content_moderation_state->toArray());
      }
      if ($content_moderation_state->language()
        ->getId() !== $langcode) {
        $content_moderation_state = $content_moderation_state->getTranslation($langcode);
      }
    }
    return $content_moderation_state;
  }
  
  /**
   * {@inheritdoc}
   */
  public function get($index) {
    if ($index !== 0) {
      throw new \InvalidArgumentException('An entity can not have multiple moderation states at the same time.');
    }
    return $this->traitGet($index);
  }
  
  /**
   * {@inheritdoc}
   */
  public function onChange($delta) {
    $this->updateModeratedEntity($this->list[$delta]->value);
    parent::onChange($delta);
  }
  
  /**
   * {@inheritdoc}
   */
  public function setValue($values, $notify = TRUE) {
    parent::setValue($values, $notify);
    $this->valueComputed = TRUE;
    // If the parent created a field item and if the parent should be notified
    // about the change (e.g. this is not initialized with the current value),
    // update the moderated entity.
    if (isset($this->list[0]) && $notify) {
      $this->updateModeratedEntity($this->list[0]->value);
    }
  }
  
  /**
   * Updates the default revision flag and the publishing status of the entity.
   *
   * @param string $moderation_state_id
   *   The ID of the new moderation state.
   */
  protected function updateModeratedEntity($moderation_state_id) {
    $entity = $this->getEntity();
    /** @var \Drupal\content_moderation\ModerationInformationInterface $content_moderation_info */
    $content_moderation_info = \Drupal::service('content_moderation.moderation_information');
    $workflow = $content_moderation_info->getWorkflowForEntity($entity);
    // Change the entity's default revision flag and the publishing status only
    // if the new workflow state is a valid one.
    if ($workflow && $workflow->getTypePlugin()
      ->hasState($moderation_state_id)) {
      /** @var \Drupal\content_moderation\ContentModerationState $current_state */
      $current_state = $workflow->getTypePlugin()
        ->getState($moderation_state_id);
      // This entity is default if it is new, the default revision state, or the
      // default revision is not published.
      if (!$entity->isSyncing()) {
        $update_default_revision = $entity->isNew() || $current_state->isDefaultRevisionState() || !$content_moderation_info->isDefaultRevisionPublished($entity);
        $entity->isDefaultRevision($update_default_revision);
      }
      // Update publishing status if it can be updated and if it needs updating.
      $published_state = $current_state->isPublishedState();
      if ($entity instanceof EntityPublishedInterface && $entity->isPublished() !== $published_state) {
        $published_state ? $entity->setPublished() : $entity->setUnpublished();
      }
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function generateSampleItems($count = 1) {
    // No sample items generated since the starting moderation state is always
    // computed based on the default state of the associated workflow.
  }

}