class EntitySchemaSubscriber

class EntitySchemaSubscriber implements EntityTypeListenerInterface, EventSubscriberInterface {
    use EntityTypeEventSubscriberTrait;
    
    /**
     * The entity definition update manager.
     *
     * @var \Drupal\Core\Entity\EntityDefinitionUpdateManagerInterface
     */
    protected $entityDefinitionUpdateManager;
    
    /**
     * The state service.
     *
     * @var \Drupal\Core\State\StateInterface
     */
    protected $state;
    
    /**
     * Constructs a new EntitySchemaSubscriber.
     *
     * @param \Drupal\Core\Entity\EntityDefinitionUpdateManagerInterface $entityDefinitionUpdateManager
     *   The entity definition update manager.
     * @param \Drupal\Core\State\StateInterface $state
     *   The state service.
     */
    public function __construct(EntityDefinitionUpdateManagerInterface $entityDefinitionUpdateManager, StateInterface $state) {
        $this->entityDefinitionUpdateManager = $entityDefinitionUpdateManager;
        $this->state = $state;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        return static::getEntityTypeEvents();
    }
    
    /**
     * {@inheritdoc}
     */
    public function onEntityTypeCreate(EntityTypeInterface $entity_type) {
        // Only add the new base field when a test needs it.
        if (!$this->state
            ->get('entity_test_update.install_new_base_field_during_create', FALSE)) {
            return;
        }
        // Add a new base field when the entity type is created.
        $definitions = $this->state
            ->get('entity_test_update.additional_base_field_definitions', []);
        $definitions['new_base_field'] = BaseFieldDefinition::create('string')->setName('new_base_field')
            ->setLabel(new TranslatableMarkup('A new base field'));
        $this->state
            ->set('entity_test_update.additional_base_field_definitions', $definitions);
        $this->entityDefinitionUpdateManager
            ->installFieldStorageDefinition('new_base_field', 'entity_test_update', 'entity_test_update', $definitions['new_base_field']);
    }
    
    /**
     * {@inheritdoc}
     */
    public function onEntityTypeUpdate(EntityTypeInterface $entity_type, EntityTypeInterface $original) {
        // Only add the new base field when a test needs it.
        if (!$this->state
            ->get('entity_test_update.install_new_base_field_during_update', FALSE)) {
            return;
        }
        // Add a new base field when the entity type is updated.
        $definitions = $this->state
            ->get('entity_test_update.additional_base_field_definitions', []);
        $definitions['new_base_field'] = BaseFieldDefinition::create('string')->setName('new_base_field')
            ->setLabel(new TranslatableMarkup('A new base field'));
        $this->state
            ->set('entity_test_update.additional_base_field_definitions', $definitions);
        $this->entityDefinitionUpdateManager
            ->installFieldStorageDefinition('new_base_field', 'entity_test_update', 'entity_test_update', $definitions['new_base_field']);
    }

}