How to disable validation on previous (and other) buttons on a Drupal 8 webform.

You'll need to add the following to any buttons in hook_form_alter(), which prevents validation of all fields:

'#limit_validation_errors' => array(),

Recently, I have a request about skipping required validation in the multi-step form. The required field is only checked in the last page of the form (or when users submit the form).

My solution is to create a new webform handler. In this handler, I override the function validateForm, so I only validate if the next page is the confirm page or review page.

Here is my piece of code

/**
* Required Validation Webform handler.
*
* @WebformHandler(
* id = "required_validation",
* label = @Translation("Required Validation"),
* category = @Translation("Required Validation"),
* description = @Translation("Required Validation."),
* cardinality = \Drupal\webform\Plugin\WebformHandlerInterface::CARDINALITY_SINGLE,
* results = \Drupal\webform\Plugin\WebformHandlerInterface::RESULTS_IGNORED,
* submission = \Drupal\webform\Plugin\WebformHandlerInterface::SUBMISSION_REQUIRED,
* )
*/
class RequiredValidationWebformHandler extends WebformHandlerBase
{
use WebformAjaxElementTrait;

public function __construct(array $configuration, $plugin_id, $plugin_definition, LoggerChannelFactoryInterface $logger_factory, ConfigFactoryInterface $config_factory, EntityTypeManagerInterface $entity_type_manager, WebformSubmissionConditionsValidatorInterface $conditions_validator, WebformElementManagerInterface $element_manager) {
parent::__construct($configuration, $plugin_id, $plugin_definition, $logger_factory, $config_factory, $entity_type_manager, $conditions_validator);

$this->elementManager = $element_manager;
}

/**
* {@inheritdoc}
*/
public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
return new static(
$configuration,
$plugin_id,
$plugin_definition,
$container->get('logger.factory'),
$container->get('config.factory'),
$container->get('entity_type.manager'),
$container->get('webform_submission.conditions_validator'),
$container->get('plugin.manager.webform.element')
);
}

public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
$webform = $this->getWebform();
$elements = $webform->getElementsInitializedAndFlattened();

$options = [];
foreach ($elements as $element) {
$webform_element = $this->elementManager->getElementInstance($element);
if (!$webform_element->isContainer($element)) {
$webform_key = $element['#webform_key'];
$options[$webform_key] = $webform_element->getAdminLabel($element);
}
}

$form['element'] = [
'#type' => 'fieldset',
'#title' => $this->t('Element settings'),
];

$form['element']['element_key'] = array(
'#type' => 'select',
'#title' => $this->t('Element'),
'#options' => $options,
'#default_value' => $this->configuration['element_key'],
'#required' => TRUE,
'#empty_option' => (empty($this->configuration['element_key'])) ? $this->t('- Select -') : NULL,
);

$form['element']['required_error'] = [
'#type' => 'textfield',
'#title' => $this->t('Required message'),
'#description' => $this->t('If set, this message will be used when a required webform element is empty, instead of the default "Field x is required." message.'),
'#default_value' => $this->configuration['required_error'],
];

return $form;
}

public function defaultConfiguration() {
return [
'element_key' => '',
'required_error' => ''
];
}

public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
parent::submitConfigurationForm($form, $form_state);

$elementValues = $form_state->getValue('element');
$this->configuration['element_key'] = $elementValues['element_key'];
$this->configuration['required_error'] = $elementValues['required_error'];
}

public function validateForm(array &$form, FormStateInterface $form_state, WebformSubmissionInterface $webform_submission) {
$pages = $form_state->get('pages');
$page = $form_state->get('current_page');
$next = WebformArrayHelper::getNextKey($pages, $page);
if ($next && ($next === WebformInterface::PAGE_PREVIEW || $next === WebformInterface::PAGE_CONFIRMATION)) {
$conf = $this->getConfiguration();
if (!empty($conf['settings'])) {
$confSettings = $conf['settings'];
$element = $confSettings['element_key'];
$value = $form_state->getValue($element);
if (empty($value)) {
$form_state->setErrorByName($element, $confSettings['required_error']);
}
}
}
}
}