ConstraintFactory example

  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
    $this->factory = new ConstraintFactory($this);
    parent::__construct('Plugin/Validation/Constraint', $namespaces, $module_handler, NULL, 'Drupal\Core\Validation\Annotation\Constraint');
    $this->alterInfo('validation_constraint');
    $this->setCacheBackend($cache_backend, 'validation_constraint_plugins');
  }

  /** * {@inheritdoc} */
  protected function getDiscovery() {
    if (!isset($this->discovery)) {
      $this->discovery = parent::getDiscovery();