class DisplayLink

class DisplayLink extends AreaPluginBase {
    
    /**
     * {@inheritdoc}
     */
    protected function defineOptions() {
        $options = parent::defineOptions();
        $options['display_id'] = [
            'default' => NULL,
        ];
        $options['label'] = [
            'default' => NULL,
        ];
        return $options;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildOptionsForm(&$form, FormStateInterface $form_state) {
        parent::buildOptionsForm($form, $form_state);
        $allowed_displays = [];
        $displays = $this->view->storage
            ->get('display');
        foreach ($displays as $display_id => $display) {
            if (!$this->isPathBasedDisplay($display_id)) {
                unset($displays[$display_id]);
                continue;
            }
            $allowed_displays[$display_id] = $display['display_title'];
        }
        $form['description'] = [
            [
                '#markup' => $this->t('To make sure the results are the same when switching to the other display, it is recommended to make sure the display:'),
            ],
            [
                '#theme' => 'item_list',
                '#items' => [
                    $this->t('Has a path.'),
                    $this->t('Has the same filter criteria.'),
                    $this->t('Has the same sort criteria.'),
                    $this->t('Has the same pager settings.'),
                    $this->t('Has the same contextual filters.'),
                ],
            ],
        ];
        if (!$allowed_displays) {
            $form['empty_message'] = [
                '#markup' => '<p><em>' . $this->t('There are no path-based displays available.') . '</em></p>',
            ];
        }
        else {
            $form['display_id'] = [
                '#title' => $this->t('Display'),
                '#type' => 'select',
                '#options' => $allowed_displays,
                '#default_value' => $this->options['display_id'],
                '#required' => TRUE,
            ];
            $form['label'] = [
                '#title' => $this->t('Label'),
                '#description' => $this->t('The text of the link.'),
                '#type' => 'textfield',
                '#default_value' => $this->options['label'],
                '#required' => TRUE,
            ];
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function validate() {
        $errors = parent::validate();
        // Do not add errors for the default display if it is not displayed in the
        // UI.
        if ($this->displayHandler
            ->isDefaultDisplay() && !\Drupal::config('views.settings')->get('ui.show.default_display')) {
            return $errors;
        }
        // Ajax errors can cause the plugin to be added without any settings.
        $linked_display_id = !empty($this->options['display_id']) ? $this->options['display_id'] : NULL;
        if (!$linked_display_id) {
            $errors[] = $this->t('%current_display: The link in the %area area has no configured display.', [
                '%current_display' => $this->displayHandler->display['display_title'],
                '%area' => $this->areaType,
            ]);
            return $errors;
        }
        // Check if the linked display hasn't been removed.
        if (!$this->view->displayHandlers
            ->get($linked_display_id)) {
            $errors[] = $this->t('%current_display: The link in the %area area points to the %linked_display display which no longer exists.', [
                '%current_display' => $this->displayHandler->display['display_title'],
                '%area' => $this->areaType,
                '%linked_display' => $this->options['display_id'],
            ]);
            return $errors;
        }
        // Check if the linked display is a path-based display.
        if (!$this->isPathBasedDisplay($linked_display_id)) {
            $errors[] = $this->t('%current_display: The link in the %area area points to the %linked_display display which does not have a path.', [
                '%current_display' => $this->displayHandler->display['display_title'],
                '%area' => $this->areaType,
                '%linked_display' => $this->view->displayHandlers
                    ->get($linked_display_id)->display['display_title'],
            ]);
            return $errors;
        }
        // Check if options of the linked display are equal to the options of the
        // current display. We "only" show a warning here, because even though we
        // recommend keeping the display options equal, we do not want to enforce
        // this.
        $unequal_options = [
            'filters' => $this->t('Filter criteria'),
            'sorts' => $this->t('Sort criteria'),
            'pager' => $this->t('Pager'),
            'arguments' => $this->t('Contextual filters'),
        ];
        foreach (array_keys($unequal_options) as $option) {
            if ($this->hasEqualOptions($linked_display_id, $option)) {
                unset($unequal_options[$option]);
            }
        }
        if ($unequal_options) {
            $warning = $this->t('%current_display: The link in the %area area points to the %linked_display display which uses different settings than the %current_display display for: %unequal_options. To make sure users see the exact same result when clicking the link, check that the settings are the same.', [
                '%current_display' => $this->displayHandler->display['display_title'],
                '%area' => $this->areaType,
                '%linked_display' => $this->view->displayHandlers
                    ->get($linked_display_id)->display['display_title'],
                '%unequal_options' => implode(', ', $unequal_options),
            ]);
            $this->messenger()
                ->addWarning($warning);
        }
        return $errors;
    }
    
    /**
     * {@inheritdoc}
     */
    public function render($empty = FALSE) {
        if ($empty && empty($this->options['empty']) || empty($this->options['display_id'])) {
            return [];
        }
        if (!$this->isPathBasedDisplay($this->options['display_id'])) {
            return [];
        }
        // Get query parameters from the exposed input and pager.
        $query = $this->view
            ->getExposedInput();
        if ($current_page = $this->view
            ->getCurrentPage()) {
            $query['page'] = $current_page;
        }
        // @todo Remove this parsing once these are removed from the request in
        //   https://www.drupal.org/node/2504709.
        foreach ([
            'view_name',
            'view_display_id',
            'view_args',
            'view_path',
            'view_dom_id',
            'pager_element',
            'view_base_path',
            AjaxResponseSubscriber::AJAX_REQUEST_PARAMETER,
            FormBuilderInterface::AJAX_FORM_REQUEST,
            MainContentViewSubscriber::WRAPPER_FORMAT,
        ] as $key) {
            unset($query[$key]);
        }
        // Set default classes.
        $classes = [
            'views-display-link',
            'views-display-link-' . $this->options['display_id'],
        ];
        if ($this->options['display_id'] === $this->view->current_display) {
            $classes[] = 'is-active';
        }
        return [
            '#type' => 'link',
            '#title' => $this->options['label'],
            '#url' => $this->view
                ->getUrl($this->view->args, $this->options['display_id'])
                ->setOptions([
                'query' => $query,
            ]),
            '#options' => [
                'view' => $this->view,
                'target_display_id' => $this->options['display_id'],
                'attributes' => [
                    'class' => $classes,
                ],
            ],
        ];
    }
    
    /**
     * Check if a views display is a path-based display.
     *
     * @param string $display_id
     *   The display ID to check.
     *
     * @return bool
     *   Whether the display ID is an allowed display or not.
     */
    protected function isPathBasedDisplay($display_id) {
        $loaded_display = $this->view->displayHandlers
            ->get($display_id);
        return $loaded_display instanceof PathPluginBase;
    }
    
    /**
     * Check if the options of a views display are equal to the current display.
     *
     * @param string $display_id
     *   The display ID to check.
     * @param string $option
     *   The option to check.
     *
     * @return bool
     *   Whether the option of the view display are equal to the current display
     *   or not.
     */
    protected function hasEqualOptions($display_id, $option) {
        $loaded_display = $this->view->displayHandlers
            ->get($display_id);
        return $loaded_display->getOption($option) === $this->displayHandler
            ->getOption($option);
    }

}