class UserData

1. 8.x drupal/core/modules/user/lib/Drupal/user/UserData.php \Drupal\user\UserData
2. 8.x drupal/core/modules/user/lib/Drupal/user/Plugin/views/field/UserData.php \Drupal\user\Plugin\views\field\UserData

OverlaySubscriber.php in drupal/core/modules/overlay/lib/Drupal/overlay/EventSubscriber/OverlaySubscriber.php

Contains \Drupal\overlay\EventSubscriber\OverlaySubscriber.

user.services.yml in drupal/core/modules/user/user.services.yml

drupal/core/modules/user/user.services.yml

user.data in drupal/core/modules/user/user.services.yml

Drupal\user\UserData

class UserData implements UserDataInterface {

  /**
   * The database connection to use.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $connection;

  /**
   * Constructs a new user data service.
   *
   * @param \Drupal\Core\Database\Connection $connection
   *   The database connection to use.
   */
  public function __construct(Connection $connection) {
    $this->connection = $connection;
  }

  /**
   * Implements \Drupal\user\UserDataInterface::get().
   */
  public function get($module, $uid = NULL, $name = NULL) {
    $query = $this->connection
      ->select('users_data', 'ud')
      ->fields('ud')
      ->condition('module', $module);
    if (isset($uid)) {
      $query
        ->condition('uid', $uid);
    }
    if (isset($name)) {
      $query
        ->condition('name', $name);
    }
    $result = $query
      ->execute();

    // If $module, $uid, and $name was passed, return the value.
    if (isset($name) && isset($uid)) {
      $result = $result
        ->fetchAllAssoc('uid');
      if (isset($result[$uid])) {
        return $result[$uid]->serialized ? unserialize($result[$uid]->value) : $result[$uid]->value;
      }
      return NULL;
    }
    elseif (isset($uid)) {
      $return = array();
      foreach ($result as $record) {
        $return[$record->name] = $record->serialized ? unserialize($record->value) : $record->value;
      }
      return $return;
    }
    elseif (isset($name)) {
      $return = array();
      foreach ($result as $record) {
        $return[$record->uid] = $record->serialized ? unserialize($record->value) : $record->value;
      }
      return $return;
    }
    else {
      $return = array();
      foreach ($result as $record) {
        $return[$record->uid][$record->name] = $record->serialized ? unserialize($record->value) : $record->value;
      }
      return $return;
    }
  }

  /**
   * Implements \Drupal\user\UserDataInterface::set().
   */
  public function set($module, $uid, $name, $value) {
    $serialized = 0;
    if (!is_scalar($value)) {
      $value = serialize($value);
      $serialized = 1;
    }
    $this->connection
      ->merge('users_data')
      ->key(array(
      'uid' => $uid,
      'module' => $module,
      'name' => $name,
    ))
      ->fields(array(
      'value' => $value,
      'serialized' => $serialized,
    ))
      ->execute();
  }

  /**
   * Implements \Drupal\user\UserDataInterface::delete().
   */
  public function delete($module = NULL, $uid = NULL, $name = NULL) {
    $query = $this->connection
      ->delete('users_data');
    if (isset($module)) {
      $query
        ->condition('module', $module);
    }
    if (isset($uid)) {
      $query
        ->condition('uid', $uid);
    }
    if (isset($name)) {
      $query
        ->condition('name', $name);
    }
    $query
      ->execute();
  }

}