hook form validate drupal 8 code example

Example: drupal 8 webform hook validation

namespace Drupal\custom_module\Validate;

use Drupal\Core\Field\FieldException;
use Drupal\Core\Form\FormStateInterface;

/**
 * Form API callback. Validate element value.
 */
class MyValidateConstraint {
    /**
     * Validates given element.
     *
     * @param array              $element      The form element to process.
     * @param FormStateInterface $formState    The form state.
     * @param array              $form The complete form structure.
     */
    public static function validate(array &$element, FormStateInterface $formState, array &$form) {
        $webformKey = $element['#webform_key'];
        $value = $formState->getValue($webformKey);

        // Skip empty unique fields or arrays (aka #multiple).
        if ($value === '' || is_array($value)) {
            return;
        }

        // do some validation here...
        // and set some error variable, e.g. $error

        if ($error) {
            if (isset($element['#title'])) {
                $tArgs = [
                    '%name' => empty($element['#title']) ? $element['#parents'][0] : $element['#title'],
                    '%value' => $value,
                ];
                $formState->setError(
                    $element,
                    t('The value %value is not allowed for element %name. Please use a different value.', $tArgs)
                );
            } else {
                $formState->setError($element);
            }
        }
    }
}