class ViewFormBase

abstract class ViewFormBase extends EntityForm {
    
    /**
     * The name of the display used by the form.
     *
     * @var string
     */
    protected $displayID;
    
    /**
     * {@inheritdoc}
     */
    public function init(FormStateInterface $form_state) {
        parent::init($form_state);
        // @todo Remove the need for this.
        $form_state->loadInclude('views_ui', 'inc', 'admin');
        $form_state->set('view', $this->entity);
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, $display_id = NULL) {
        if (isset($display_id) && $form_state->has('display_id') && $display_id !== $form_state->get('display_id')) {
            throw new \InvalidArgumentException('Mismatch between $form_state->get(\'display_id\') and $display_id.');
        }
        $this->displayID = $form_state->has('display_id') ? $form_state->get('display_id') : $display_id;
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    protected function prepareEntity() {
        // Determine the displays available for editing.
        if ($tabs = $this->getDisplayTabs($this->entity)) {
            if (empty($this->displayID)) {
                // If a display isn't specified, use the first one after sorting by
                // #weight.
                uasort($tabs, 'Drupal\\Component\\Utility\\SortArray::sortByWeightProperty');
                foreach ($tabs as $id => $tab) {
                    if (!isset($tab['#access']) || $tab['#access']) {
                        $this->displayID = $id;
                        break;
                    }
                }
            }
            // If a display is specified, but we don't have access to it, return
            // an access denied page.
            if ($this->displayID && !isset($tabs[$this->displayID])) {
                throw new NotFoundHttpException();
            }
            elseif ($this->displayID && (isset($tabs[$this->displayID]['#access']) && !$tabs[$this->displayID]['#access'])) {
                throw new AccessDeniedHttpException();
            }
        }
        elseif ($this->displayID) {
            throw new NotFoundHttpException();
        }
    }
    
    /**
     * Adds tabs for navigating across Displays when editing a View.
     *
     * This function can be called from hook_menu_local_tasks_alter() to implement
     * these tabs as secondary local tasks, or it can be called from elsewhere if
     * having them as secondary local tasks isn't desired. The caller is responsible
     * for setting the active tab's #active property to TRUE.
     *
     * @param \Drupal\views_ui\ViewUI $view
     *   The ViewUI entity.
     *
     * @return array
     *   An array of tab definitions.
     */
    public function getDisplayTabs(ViewUI $view) {
        $executable = $view->getExecutable();
        $executable->initDisplay();
        $display_id = $this->displayID;
        $tabs = [];
        // Create a tab for each display.
        foreach ($view->get('display') as $id => $display) {
            // Get an instance of the display plugin, to make sure it will work in the
            // UI.
            $display_plugin = $executable->displayHandlers
                ->get($id);
            if (empty($display_plugin)) {
                continue;
            }
            $tabs[$id] = [
                '#theme' => 'menu_local_task',
                '#weight' => $display['position'],
                '#link' => [
                    'title' => $this->getDisplayLabel($view, $id),
                    'localized_options' => [],
                    'url' => $view->toUrl('edit-display-form')
                        ->setRouteParameter('display_id', $id),
                ],
            ];
            if (!empty($display['deleted'])) {
                $tabs[$id]['#link']['localized_options']['attributes']['class'][] = 'views-display-deleted-link';
            }
            if (isset($display['display_options']['enabled']) && !$display['display_options']['enabled']) {
                $tabs[$id]['#link']['localized_options']['attributes']['class'][] = 'views-display-disabled-link';
            }
        }
        // If the default display isn't supposed to be shown, don't display its tab, unless it's the only display.
        if (!$this->isDefaultDisplayShown($view) && $display_id != 'default' && count($tabs) > 1) {
            $tabs['default']['#access'] = FALSE;
        }
        // Mark the display tab as red to show validation errors.
        $errors = $executable->validate();
        foreach ($view->get('display') as $id => $display) {
            if (!empty($errors[$id])) {
                // Always show the tab.
                $tabs[$id]['#access'] = TRUE;
                // Add a class to mark the error and a title to make a hover tip.
                $tabs[$id]['#link']['localized_options']['attributes']['class'][] = 'error';
                $tabs[$id]['#link']['localized_options']['attributes']['title'] = $this->t('This display has one or more validation errors.');
            }
        }
        return $tabs;
    }
    
    /**
     * Controls whether or not the default display should have its own tab on edit.
     */
    public function isDefaultDisplayShown(ViewUI $view) {
        // Always show the default display for advanced users who prefer that mode.
        $advanced_mode = \Drupal::config('views.settings')->get('ui.show.default_display');
        // For other users, show the default display only if there are no others, and
        // hide it if there's at least one "real" display.
        $additional_displays = count($view->getExecutable()->displayHandlers) == 1;
        return $advanced_mode || $additional_displays;
    }
    
    /**
     * Placeholder function for overriding $display['display_title'].
     *
     * @todo Remove this function once editing the display title is possible.
     */
    public function getDisplayLabel(ViewUI $view, $display_id, $check_changed = TRUE) {
        $display = $view->get('display');
        $title = $display_id == 'default' ? $this->t('Default') : $display[$display_id]['display_title'];
        $title = Unicode::truncate($title, 25, FALSE, TRUE);
        if ($check_changed && !empty($view->changed_display[$display_id])) {
            $changed = '*';
            $title = $title . $changed;
        }
        return $title;
    }

}