class ViewsFormBase

abstract class ViewsFormBase extends FormBase implements ViewsFormInterface {
    
    /**
     * The ID of the item this form is manipulating.
     *
     * @var string
     */
    protected $id;
    
    /**
     * The type of item this form is manipulating.
     *
     * @var string
     */
    protected $type;
    
    /**
     * Sets the ID for this form.
     *
     * @param string $id
     *   The ID of the item this form is manipulating.
     */
    protected function setID($id) {
        if ($id) {
            $this->id = $id;
        }
    }
    
    /**
     * Sets the type for this form.
     *
     * @param string $type
     *   The type of the item this form is manipulating.
     */
    protected function setType($type) {
        if ($type) {
            $this->type = $type;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormState(ViewEntityInterface $view, $display_id, $js) {
        // $js may already have been converted to a Boolean.
        $ajax = is_string($js) ? $js === 'ajax' : $js;
        return (new FormState())->set('form_id', $this->getFormId())
            ->set('form_key', $this->getFormKey())
            ->set('ajax', $ajax)
            ->set('display_id', $display_id)
            ->set('view', $view)
            ->set('type', $this->type)
            ->set('id', $this->id)
            ->disableRedirect()
            ->addBuildInfo('callback_object', $this);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getForm(ViewEntityInterface $view, $display_id, $js) {
        
        /** @var \Drupal\Core\Form\FormStateInterface $form_state */
        $form_state = $this->getFormState($view, $display_id, $js);
        $view = $form_state->get('view');
        $form_key = $form_state->get('form_key');
        // @todo Remove the need for this.
        \Drupal::moduleHandler()->loadInclude('views_ui', 'inc', 'admin');
        // Reset the cache of IDs. Drupal rather aggressively prevents ID
        // duplication but this causes it to remember IDs that are no longer even
        // being used.
        Html::resetSeenIds();
        // Check to see if this is the top form of the stack. If it is, pop
        // it off; if it isn't, the user clicked somewhere else and the stack is
        // now irrelevant.
        if (!empty($view->stack)) {
            $identifier = implode('-', array_filter([
                $form_key,
                $view->id(),
                $display_id,
                $form_state->get('type'),
                $form_state->get('id'),
            ]));
            // Retrieve the first form from the stack without changing the integer keys,
            // as they're being used for the "2 of 3" progress indicator.
            reset($view->stack);
            $stack_key = key($view->stack);
            $top = current($view->stack);
            next($view->stack);
            unset($view->stack[$stack_key]);
            if (array_shift($top) != $identifier) {
                $view->stack = [];
            }
        }
        // Automatically remove the form cache if it is set and the key does
        // not match. This way navigating away from the form without hitting
        // update will work.
        if (isset($view->form_cache) && $view->form_cache['key'] !== $form_key) {
            unset($view->form_cache);
        }
        $form_class = get_class($form_state->getFormObject());
        $response = $this->ajaxFormWrapper($form_class, $form_state);
        // If the form has not been submitted, or was not set for rerendering, stop.
        if (!$form_state->isSubmitted() || $form_state->get('rerender')) {
            return $response;
        }
        // Sometimes we need to re-generate the form for multi-step type operations.
        if (!empty($view->stack)) {
            $stack = $view->stack;
            $top = array_shift($stack);
            // Build the new form state for the next form in the stack.
            $reflection = new \ReflectionClass($view::$forms[$top[1]]);
            $form_state = $reflection->newInstanceArgs(array_slice($top, 3, 2))
                ->getFormState($view, $top[2], $form_state->get('ajax'));
            $form_class = get_class($form_state->getFormObject());
            $form_state->setUserInput([]);
            $form_url = views_ui_build_form_url($form_state);
            if (!$form_state->get('ajax')) {
                return new RedirectResponse($form_url->setAbsolute()
                    ->toString());
            }
            $form_state->set('url', $form_url);
            $response = $this->ajaxFormWrapper($form_class, $form_state);
        }
        elseif (!$form_state->get('ajax')) {
            // If nothing on the stack, non-js forms just go back to the main view editor.
            $display_id = $form_state->get('display_id');
            return new RedirectResponse(Url::fromRoute('entity.view.edit_display_form', [
                'view' => $view->id(),
                'display_id' => $display_id,
            ], [
                'absolute' => TRUE,
            ])
                ->toString());
        }
        else {
            $response = new AjaxResponse();
            $response->addCommand(new CloseModalDialogCommand());
            $response->addCommand(new ShowButtonsCommand(!empty($view->changed)));
            $response->addCommand(new TriggerPreviewCommand());
            if ($page_title = $form_state->get('page_title')) {
                $response->addCommand(new ReplaceTitleCommand($page_title));
            }
        }
        // If this form was for view-wide changes, there's no need to regenerate
        // the display section of the form.
        if ($display_id !== '') {
            \Drupal::entityTypeManager()->getFormObject('view', 'edit')
                ->rebuildCurrentTab($view, $response, $display_id);
        }
        return $response;
    }
    
    /**
     * Wrapper for handling AJAX forms.
     *
     * Wrapper around \Drupal\Core\Form\FormBuilderInterface::buildForm() to
     * handle some AJAX stuff automatically.
     * This makes some assumptions about the client.
     *
     * @param \Drupal\Core\Form\FormInterface|string $form_class
     *   The value must be one of the following:
     *   - The name of a class that implements \Drupal\Core\Form\FormInterface.
     *   - An instance of a class that implements \Drupal\Core\Form\FormInterface.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return \Drupal\Core\Ajax\AjaxResponse|string|array
     *   Returns one of three possible values:
     *   - A \Drupal\Core\Ajax\AjaxResponse object.
     *   - The rendered form, as a string.
     *   - A render array with the title in #title and the rendered form in the
     *   #markup array.
     */
    protected function ajaxFormWrapper($form_class, FormStateInterface &$form_state) {
        
        /** @var \Drupal\Core\Render\RendererInterface $renderer */
        $renderer = \Drupal::service('renderer');
        // This won't override settings already in.
        if (!$form_state->has('rerender')) {
            $form_state->set('rerender', FALSE);
        }
        $ajax = $form_state->get('ajax');
        // Do not overwrite if the redirect has been disabled.
        if (!$form_state->isRedirectDisabled()) {
            $form_state->disableRedirect($ajax);
        }
        $form_state->disableCache();
        // Builds the form in a render context in order to ensure that cacheable
        // metadata is bubbled up.
        $render_context = new RenderContext();
        $callable = function () use ($form_class, &$form_state) {
            return \Drupal::formBuilder()->buildForm($form_class, $form_state);
        };
        $form = $renderer->executeInRenderContext($render_context, $callable);
        if (!$render_context->isEmpty()) {
            BubbleableMetadata::createFromRenderArray($form)->merge($render_context->pop())
                ->applyTo($form);
        }
        $output = $renderer->renderRoot($form);
        // These forms have the title built in, so set the title here:
        $title = $form_state->get('title') ?: '';
        if ($ajax && (!$form_state->isExecuted() || $form_state->get('rerender'))) {
            // If the form didn't execute and we're using ajax, build up an
            // Ajax command list to execute.
            $response = new AjaxResponse();
            // Attach the library necessary for using the OpenModalDialogCommand and
            // set the attachments for this Ajax response.
            $form['#attached']['library'][] = 'core/drupal.dialog.ajax';
            $response->setAttachments($form['#attached']);
            $display = '';
            $status_messages = [
                '#type' => 'status_messages',
            ];
            if ($messages = $renderer->renderRoot($status_messages)) {
                $display = '<div class="views-messages">' . $messages . '</div>';
            }
            $display .= $output;
            $options = [
                'classes' => [
                    'ui-dialog' => 'views-ui-dialog js-views-ui-dialog',
                ],
                'width' => '75%',
            ];
            $response->addCommand(new OpenModalDialogCommand($title, $display, $options));
            // Views provides its own custom handling of AJAX form submissions.
            // Usually this happens at the same path, but custom paths may be
            // specified in $form_state.
            $form_url = $form_state->has('url') ? $form_state->get('url')
                ->toString() : Url::fromRoute('<current>')->toString();
            $response->addCommand(new SetFormCommand($form_url));
            if ($section = $form_state->get('#section')) {
                $response->addCommand(new HighlightCommand('.' . Html::cleanCssIdentifier($section)));
            }
            return $response;
        }
        return $title ? [
            '#title' => $title,
            '#markup' => $output,
        ] : $output;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }

}