class ViewsExposedForm

class ViewsExposedForm extends FormBase implements WorkspaceSafeFormInterface {
  
  /**
   * The exposed form cache.
   *
   * @var \Drupal\views\ExposedFormCache
   */
  protected $exposedFormCache;
  
  /**
   * The current path stack.
   *
   * @var \Drupal\Core\Path\CurrentPathStack
   */
  protected $currentPathStack;
  
  /**
   * Constructs a new ViewsExposedForm.
   *
   * @param \Drupal\views\ExposedFormCache $exposed_form_cache
   *   The exposed form cache.
   * @param \Drupal\Core\Path\CurrentPathStack $current_path_stack
   *   The current path stack.
   */
  public function __construct(ExposedFormCache $exposed_form_cache, CurrentPathStack $current_path_stack) {
    $this->exposedFormCache = $exposed_form_cache;
    $this->currentPathStack = $current_path_stack;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container->get('views.exposed_form_cache'), $container->get('path.current'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'views_exposed_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    // Make sure that we validate because this form might be submitted
    // multiple times per page.
    $form_state->setValidationEnforced();
    /** @var \Drupal\views\ViewExecutable $view */
    $view = $form_state->get('view');
    $display =& $form_state->get('display');
    $form_state->setUserInput($view->getExposedInput());
    // Let form plugins know this is for exposed widgets.
    $form_state->set('exposed', TRUE);
    // Check if the form was already created
    if ($cache = $this->exposedFormCache
      ->getForm($view->storage
      ->id(), $view->current_display)) {
      return $cache;
    }
    $form['#info'] = [];
    // Go through each handler and let it generate its exposed widget.
    foreach ($view->display_handler->handlers as $type => $value) {
      /** @var \Drupal\views\Plugin\views\ViewsHandlerInterface $handler */
      foreach ($view->{$type} as $id => $handler) {
        if ($handler->canExpose() && $handler->isExposed()) {
          // Grouped exposed filters have their own forms.
          // Instead of render the standard exposed form, a new Select or
          // Radio form field is rendered with the available groups.
          // When a user chooses an option the selected value is split
          // into the operator and value that the item represents.
          if ($handler->isAGroup()) {
            $handler->groupForm($form, $form_state);
            $id = $handler->options['group_info']['identifier'];
          }
          else {
            $handler->buildExposedForm($form, $form_state);
          }
          if ($info = $handler->exposedInfo()) {
            $form['#info']["{$type}-{$id}"] = $info;
          }
        }
      }
    }
    $form['actions'] = [
      '#type' => 'actions',
    ];
    $form['actions']['submit'] = [
      // Prevent from showing up in \Drupal::request()->query.
'#name' => '',
      '#type' => 'submit',
      '#value' => $this->t('Apply'),
      '#id' => Html::getUniqueId('edit-submit-' . $view->storage
        ->id()),
    ];
    if (!$view->hasUrl()) {
      // On any non views.ajax route, use the current route for the form action.
      if ($this->getRouteMatch()
        ->getRouteName() !== 'views.ajax') {
        $form_action = Url::fromRoute('<current>')->toString();
      }
      else {
        // On the views.ajax route, set the action to the page we were on.
        $form_action = Url::fromUserInput($this->currentPathStack
          ->getPath())
          ->toString();
      }
    }
    else {
      $form_action = $view->getUrl()
        ->toString();
    }
    $form['#action'] = $form_action;
    $form['#theme'] = $view->buildThemeFunctions('views_exposed_form');
    $form['#id'] = Html::cleanCssIdentifier('views_exposed_form-' . $view->storage
      ->id() . '-' . $display['id']);
    // Labels are built too late for inline form errors to work, resulting
    // in duplicated messages.
    $form['#disable_inline_form_errors'] = TRUE;
    /** @var \Drupal\views\Plugin\views\exposed_form\ExposedFormPluginInterface $exposed_form_plugin */
    $exposed_form_plugin = $view->display_handler
      ->getPlugin('exposed_form');
    $exposed_form_plugin->exposedFormAlter($form, $form_state);
    // Save the form.
    $this->exposedFormCache
      ->setForm($view->storage
      ->id(), $view->current_display, $form);
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $view = $form_state->get('view');
    foreach ([
      'field',
      'filter',
    ] as $type) {
      /** @var \Drupal\views\Plugin\views\ViewsHandlerInterface[] $handlers */
      $handlers =& $view->{$type};
      foreach ($handlers as $key => $handler) {
        $handlers[$key]->validateExposed($form, $form_state);
      }
    }
    /** @var \Drupal\views\Plugin\views\exposed_form\ExposedFormPluginInterface $exposed_form_plugin */
    $exposed_form_plugin = $view->display_handler
      ->getPlugin('exposed_form');
    $exposed_form_plugin->exposedFormValidate($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Form input keys that will not be included in $view->exposed_raw_data.
    $exclude = [
      'submit',
      'form_build_id',
      'form_id',
      'form_token',
      'exposed_form_plugin',
      'reset',
    ];
    $values = $form_state->getValues();
    foreach ([
      'field',
      'filter',
    ] as $type) {
      /** @var \Drupal\views\Plugin\views\ViewsHandlerInterface[] $handlers */
      $handlers =& $form_state->get('view')->{$type};
      foreach ($handlers as $key => $info) {
        if ($handlers[$key]->acceptExposedInput($values)) {
          $handlers[$key]->submitExposed($form, $form_state);
        }
        else {
          // The input from the form did not validate, exclude it from the
          // stored raw data.
          $exclude[] = $key;
        }
      }
    }
    $view = $form_state->get('view');
    $view->exposed_data = $values;
    $view->exposed_raw_input = [];
    $exclude = [
      'submit',
      'form_build_id',
      'form_id',
      'form_token',
      'exposed_form_plugin',
      'reset',
    ];
    /** @var \Drupal\views\Plugin\views\exposed_form\ExposedFormPluginBase $exposed_form_plugin */
    $exposed_form_plugin = $view->display_handler
      ->getPlugin('exposed_form');
    $exposed_form_plugin->exposedFormSubmit($form, $form_state, $exclude);
    foreach ($values as $key => $value) {
      if (!empty($key) && !in_array($key, $exclude)) {
        if (is_array($value)) {
          // Handle checkboxes, we only want to include the checked options.
          // @todo revisit the need for this when
          //   https://www.drupal.org/node/342316 is resolved.
          $checked = Checkboxes::getCheckedCheckboxes($value);
          foreach ($checked as $option_id) {
            $view->exposed_raw_input[$key][$option_id] = $value[$option_id];
          }
        }
        else {
          $view->exposed_raw_input[$key] = $value;
        }
      }
    }
  }

}