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() || 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 (!$this->value) {
                throw new EntityMalformedException('The entity does not have a password.');
            }
        }
        if (!$entity->isNew()) {
            // If the password is empty, that means it was not changed, so use the
            // original password.
            if (empty($this->value)) {
                $this->value = $entity->original->{$this->getFieldDefinition()
                    ->getName()}->value;
            }
        }
        // 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;
    }

}