/**
 * Implements hook_entity_view_mode_alter().
 */
function {{ machine_name }}_entity_view_mode_alter(&$view_mode, Drupal\Core\Entity\EntityInterface $entity, $context) {
  // For nodes, change the view mode when it is teaser.
  if ($entity->getEntityTypeId() == 'node' && $view_mode == 'teaser') {
    $view_mode = 'my_custom_view_mode';
  }
}