class PathWidget

class PathWidget extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $entity = $items->getEntity();
        $element += [
            '#element_validate' => [
                [
                    get_class($this),
                    'validateFormElement',
                ],
            ],
        ];
        $element['alias'] = [
            '#type' => 'textfield',
            '#title' => $element['#title'],
            '#default_value' => $items[$delta]->alias,
            '#required' => $element['#required'],
            '#maxlength' => 255,
            '#description' => $this->t('Specify an alternative path by which this data can be accessed. For example, type "/about" when writing an about page.'),
        ];
        $element['pid'] = [
            '#type' => 'value',
            '#value' => $items[$delta]->pid,
        ];
        $element['source'] = [
            '#type' => 'value',
            '#value' => !$entity->isNew() ? '/' . $entity->toUrl()
                ->getInternalPath() : NULL,
        ];
        $element['langcode'] = [
            '#type' => 'value',
            '#value' => $items[$delta]->langcode,
        ];
        // If the advanced settings tabs-set is available (normally rendered in the
        // second column on wide-resolutions), place the field as a details element
        // in this tab-set.
        if (isset($form['advanced'])) {
            $element += [
                '#type' => 'details',
                '#title' => t('URL path settings'),
                '#open' => !empty($items[$delta]->alias),
                '#group' => 'advanced',
                '#access' => $entity->get('path')
                    ->access('edit'),
                '#attributes' => [
                    'class' => [
                        'path-form',
                    ],
                ],
                '#attached' => [
                    'library' => [
                        'path/drupal.path',
                    ],
                ],
            ];
            $element['#weight'] = 30;
        }
        return $element;
    }
    
    /**
     * Form element validation handler for URL alias form element.
     *
     * @param array $element
     *   The form element.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The form state.
     */
    public static function validateFormElement(array &$element, FormStateInterface $form_state) {
        // Trim the submitted value of whitespace and slashes.
        $alias = rtrim(trim($element['alias']['#value']), " \\/");
        if ($alias !== '') {
            $form_state->setValueForElement($element['alias'], $alias);
            
            /** @var \Drupal\path_alias\PathAliasInterface $path_alias */
            $path_alias = \Drupal::entityTypeManager()->getStorage('path_alias')
                ->create([
                'path' => $element['source']['#value'],
                'alias' => $alias,
                'langcode' => $element['langcode']['#value'],
            ]);
            $violations = $path_alias->validate();
            foreach ($violations as $violation) {
                // Newly created entities do not have a system path yet, so we need to
                // disregard some violations.
                if (!$path_alias->getPath() && $violation->getPropertyPath() === 'path') {
                    continue;
                }
                $form_state->setError($element['alias'], $violation->getMessage());
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function errorElement(array $element, ConstraintViolationInterface $violation, array $form, FormStateInterface $form_state) {
        return $element['alias'];
    }

}