class CurrentUserContext

class CurrentUserContext implements ContextProviderInterface {
  use StringTranslationTrait;

  /**
   * The current user.
   *
   * @var \Drupal\Core\Session\AccountInterface
   */
  protected $account;

  /**
   * The user storage.
   *
   * @var \Drupal\user\UserStorageInterface
   */
  protected $userStorage;

  /**
   * Constructs a new CurrentUserContext.
   *
   * @param \Drupal\Core\Session\AccountInterface $account
   *   The current user.
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   */
  public function __construct(AccountInterface $account, EntityTypeManagerInterface $entity_type_manager) {
    $this->account = $account;
    $this->userStorage = $entity_type_manager
      ->getStorage('user');
  }

  /**
   * {@inheritdoc}
   */
  public function getRuntimeContexts(array $unqualified_context_ids) {
    $current_user = $this->userStorage
      ->load($this->account
      ->id());
    if ($current_user) {

      // @todo Do not validate protected fields to avoid bug in TypedData,
      //   remove this in https://www.drupal.org/project/drupal/issues/2934192.
      $current_user->_skipProtectedUserFieldConstraint = TRUE;
      $context = EntityContext::fromEntity($current_user, $this
        ->t('Current user'));
    }
    else {

      // If not user is available, provide an empty context object.
      $context = EntityContext::fromEntityTypeId('user', $this
        ->t('Current user'));
    }
    $cacheability = new CacheableMetadata();
    $cacheability
      ->setCacheContexts([
      'user',
    ]);
    $context
      ->addCacheableDependency($cacheability);
    $result = [
      'current_user' => $context,
    ];
    return $result;
  }

  /**
   * {@inheritdoc}
   */
  public function getAvailableContexts() {
    return $this
      ->getRuntimeContexts([]);
  }

}