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;
                }
            }
        }
    }

}