class PasswordItem

class PasswordItem extends StringItem {
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['value'] = DataDefinition::create('string')->setLabel(new TranslatableMarkup('The hashed password'))
      ->setSetting('case_sensitive', TRUE);
    $properties['existing'] = DataDefinition::create('string')->setLabel(new TranslatableMarkup('Existing password'));
    $properties['pre_hashed'] = DataDefinition::create('boolean')->setLabel(new TranslatableMarkup('Determines if a password needs hashing'));
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public function preSave() {
    parent::preSave();
    $entity = $this->getEntity();
    if ($this->pre_hashed) {
      // Reset the pre_hashed value since it has now been used.
      $this->pre_hashed = FALSE;
    }
    elseif (!$entity->isNew() && empty($this->value)) {
      // If the password is empty, that means it was not changed, so use the
      // original password.
      $this->value = $entity->original->{$this->getFieldDefinition()
        ->getName()}->value;
    }
    elseif ($entity->isNew() || strlen(trim($this->value)) > 0 && $this->value != $entity->original->{$this->getFieldDefinition()
      ->getName()}->value) {
      // Allow alternate password hashing schemes.
      $this->value = \Drupal::service('password')->hash(trim($this->value));
      // Abort if the hashing failed and returned FALSE.
      if (!$this->value) {
        throw new EntityMalformedException('The entity does not have a password.');
      }
    }
    // Ensure that the existing password is unset to minimise risks of it
    // getting serialized and stored somewhere.
    $this->existing = NULL;
  }
  
  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    // We cannot use the parent implementation from StringItem as it does not
    // consider the additional 'existing' property that PasswordItem contains.
    $value = $this->get('value')
      ->getValue();
    $existing = $this->get('existing')
      ->getValue();
    return $value === NULL && $existing === NULL;
  }

}