class MachineName

class MachineName extends Textfield {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = get_class($this);
        return [
            '#input' => TRUE,
            '#default_value' => NULL,
            '#required' => TRUE,
            '#maxlength' => 64,
            '#size' => 60,
            '#autocomplete_route_name' => FALSE,
            '#process' => [
                [
                    $class,
                    'processMachineName',
                ],
                [
                    $class,
                    'processAutocomplete',
                ],
                [
                    $class,
                    'processAjaxForm',
                ],
            ],
            '#element_validate' => [
                [
                    $class,
                    'validateMachineName',
                ],
            ],
            '#pre_render' => [
                [
                    $class,
                    'preRenderTextfield',
                ],
            ],
            '#theme' => 'input__textfield',
            '#theme_wrappers' => [
                'form_element',
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
        if ($input !== FALSE && $input !== NULL) {
            // This should be a string, but allow other scalars since they might be
            // valid input in programmatic form submissions.
            return is_scalar($input) ? (string) $input : '';
        }
        return NULL;
    }
    
    /**
     * Processes a machine-readable name form element.
     *
     * @param array $element
     *   The form element to process. See main class documentation for properties.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     * @param array $complete_form
     *   The complete form structure.
     *
     * @return array
     *   The processed element.
     */
    public static function processMachineName(&$element, FormStateInterface $form_state, &$complete_form) {
        // We need to pass the langcode to the client.
        $language = \Drupal::languageManager()->getCurrentLanguage();
        // Apply default form element properties.
        $element += [
            '#title' => t('Machine-readable name'),
            '#description' => t('A unique machine-readable name. Can only contain lowercase letters, numbers, and underscores.'),
            '#machine_name' => [],
            '#field_prefix' => '',
            '#field_suffix' => '',
            '#suffix' => '',
        ];
        // A form element that only wants to set one #machine_name property (usually
        // 'source' only) would leave all other properties undefined, if the defaults
        // were defined by an element plugin. Therefore, we apply the defaults here.
        $element['#machine_name'] += [
            'source' => [
                'label',
            ],
            'target' => '#' . $element['#id'],
            'label' => t('Machine name'),
            'replace_pattern' => '[^a-z0-9_]+',
            'replace' => '_',
            'standalone' => FALSE,
            'field_prefix' => $element['#field_prefix'],
            'field_suffix' => $element['#field_suffix'],
        ];
        // Store the initial value in form state. The machine name needs this to
        // ensure that the exists function is not called for existing values when
        // editing them.
        $initial_values = $form_state->get('machine_name.initial_values') ?: [];
        // Store the initial values in an array so we can differentiate between a
        // NULL default value and a new machine name element.
        if (!array_key_exists($element['#name'], $initial_values)) {
            $initial_values[$element['#name']] = $element['#default_value'];
            $form_state->set('machine_name.initial_values', $initial_values);
        }
        // By default, machine names are restricted to Latin alphanumeric characters.
        // So, default to LTR directionality.
        if (!isset($element['#attributes'])) {
            $element['#attributes'] = [];
        }
        $element['#attributes'] += [
            'dir' => LanguageInterface::DIRECTION_LTR,
        ];
        // The source element defaults to array('name'), but may have been overridden.
        if (empty($element['#machine_name']['source'])) {
            return $element;
        }
        // Retrieve the form element containing the human-readable name from the
        // complete form in $form_state. By reference, because we may need to append
        // a #field_suffix that will hold the live preview.
        $key_exists = NULL;
        $source = NestedArray::getValue($form_state->getCompleteForm(), $element['#machine_name']['source'], $key_exists);
        if (!$key_exists) {
            return $element;
        }
        $suffix_id = $source['#id'] . '-machine-name-suffix';
        $element['#machine_name']['suffix'] = '#' . $suffix_id;
        if ($element['#machine_name']['standalone']) {
            $element['#suffix'] = $element['#suffix'] . ' <small id="' . $suffix_id . '">&nbsp;</small>';
        }
        else {
            // Append a field suffix to the source form element, which will contain
            // the live preview of the machine name.
            $source += [
                '#field_suffix' => '',
            ];
            $source['#field_suffix'] = $source['#field_suffix'] . ' <small id="' . $suffix_id . '">&nbsp;</small>';
            $parents = array_merge($element['#machine_name']['source'], [
                '#field_suffix',
            ]);
            NestedArray::setValue($form_state->getCompleteForm(), $parents, $source['#field_suffix']);
        }
        $element['#attached']['library'][] = 'core/drupal.machine-name';
        $options = [
            'replace_pattern',
            'replace_token',
            'replace',
            'maxlength',
            'target',
            'label',
            'field_prefix',
            'field_suffix',
            'suffix',
        ];
        
        /** @var \Drupal\Core\Access\CsrfTokenGenerator $token_generator */
        $token_generator = \Drupal::service('csrf_token');
        $element['#machine_name']['replace_token'] = $token_generator->get($element['#machine_name']['replace_pattern']);
        $element['#attached']['drupalSettings']['machineName']['#' . $source['#id']] = array_intersect_key($element['#machine_name'], array_flip($options));
        $element['#attached']['drupalSettings']['langcode'] = $language->getId();
        return $element;
    }
    
    /**
     * Form element validation handler for machine_name elements.
     *
     * Note that #maxlength is validated by _form_validate() already.
     *
     * This checks that the submitted value:
     * - Does not contain the replacement character only.
     * - Does not contain disallowed characters.
     * - Is unique; i.e., does not already exist.
     * - Does not exceed the maximum length (via #maxlength).
     * - Cannot be changed after creation (via #disabled).
     */
    public static function validateMachineName(&$element, FormStateInterface $form_state, &$complete_form) {
        // Verify that the machine name not only consists of replacement tokens.
        if (preg_match('@^' . $element['#machine_name']['replace'] . '+$@', $element['#value'])) {
            $form_state->setError($element, t('The machine-readable name must contain unique characters.'));
        }
        // Verify that the machine name contains no disallowed characters.
        if (preg_match('@' . $element['#machine_name']['replace_pattern'] . '@', $element['#value'])) {
            if (!isset($element['#machine_name']['error'])) {
                // Since a hyphen is the most common alternative replacement character,
                // a corresponding validation error message is supported here.
                if ($element['#machine_name']['replace'] == '-') {
                    $form_state->setError($element, t('The machine-readable name must contain only lowercase letters, numbers, and hyphens.'));
                }
                else {
                    $form_state->setError($element, t('The machine-readable name must contain only lowercase letters, numbers, and underscores.'));
                }
            }
            else {
                $form_state->setError($element, $element['#machine_name']['error']);
            }
        }
        // Verify that the machine name is unique. If the value matches the initial
        // default value then it does not need to be validated as the machine name
        // element assumes the form is editing the existing value.
        $initial_values = $form_state->get('machine_name.initial_values') ?: [];
        if (!array_key_exists($element['#name'], $initial_values) || $initial_values[$element['#name']] !== $element['#value']) {
            $function = $element['#machine_name']['exists'];
            if (call_user_func($function, $element['#value'], $element, $form_state)) {
                $form_state->setError($element, t('The machine-readable name is already in use. It must be unique.'));
            }
        }
    }

}