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.
    }

}