class PasswordConfirm

class PasswordConfirm extends FormElement {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = get_class($this);
        return [
            '#input' => TRUE,
            '#markup' => '',
            '#process' => [
                [
                    $class,
                    'processPasswordConfirm',
                ],
            ],
            '#theme_wrappers' => [
                'form_element',
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
        if ($input === FALSE) {
            $element += [
                '#default_value' => [],
            ];
            return $element['#default_value'] + [
                'pass1' => '',
                'pass2' => '',
            ];
        }
        $value = [
            'pass1' => '',
            'pass2' => '',
        ];
        // Throw out all invalid array keys; we only allow pass1 and pass2.
        foreach ($value as $allowed_key => $default) {
            // These should be strings, but allow other scalars since they might be
            // valid input in programmatic form submissions. Any nested array values
            // are ignored.
            if (isset($input[$allowed_key]) && is_scalar($input[$allowed_key])) {
                $value[$allowed_key] = (string) $input[$allowed_key];
            }
        }
        return $value;
    }
    
    /**
     * Expand a password_confirm field into two text boxes.
     */
    public static function processPasswordConfirm(&$element, FormStateInterface $form_state, &$complete_form) {
        $element['pass1'] = [
            '#type' => 'password',
            '#title' => t('Password'),
            '#value' => empty($element['#value']) ? NULL : $element['#value']['pass1'],
            '#required' => $element['#required'],
            '#attributes' => [
                'class' => [
                    'password-field',
                    'js-password-field',
                ],
                'autocomplete' => [
                    'new-password',
                ],
            ],
            '#error_no_message' => TRUE,
        ];
        $element['pass2'] = [
            '#type' => 'password',
            '#title' => t('Confirm password'),
            '#value' => empty($element['#value']) ? NULL : $element['#value']['pass2'],
            '#required' => $element['#required'],
            '#attributes' => [
                'class' => [
                    'password-confirm',
                    'js-password-confirm',
                ],
                'autocomplete' => [
                    'new-password',
                ],
            ],
            '#error_no_message' => TRUE,
        ];
        $element['#element_validate'] = [
            [
                get_called_class(),
                'validatePasswordConfirm',
            ],
        ];
        $element['#tree'] = TRUE;
        if (isset($element['#size'])) {
            $element['pass1']['#size'] = $element['pass2']['#size'] = $element['#size'];
        }
        return $element;
    }
    
    /**
     * Validates a password_confirm element.
     */
    public static function validatePasswordConfirm(&$element, FormStateInterface $form_state, &$complete_form) {
        $pass1 = trim($element['pass1']['#value']);
        $pass2 = trim($element['pass2']['#value']);
        if (strlen($pass1) > 0 || strlen($pass2) > 0) {
            if (strcmp($pass1, $pass2)) {
                $form_state->setError($element, t('The specified passwords do not match.'));
            }
        }
        elseif ($element['#required'] && $form_state->getUserInput()) {
            $form_state->setError($element, t('Password field is required.'));
        }
        // Password field must be converted from a two-element array into a single
        // string regardless of validation results.
        $form_state->setValueForElement($element['pass1'], NULL);
        $form_state->setValueForElement($element['pass2'], NULL);
        $form_state->setValueForElement($element, $pass1);
        return $element;
    }

}