class EntityViewBuilder

class EntityViewBuilder extends EntityHandlerBase implements EntityHandlerInterface, EntityViewBuilderInterface, TrustedCallbackInterface {
    
    /**
     * The type of entities for which this view builder is instantiated.
     *
     * @var string
     */
    protected $entityTypeId;
    
    /**
     * Information about the entity type.
     *
     * @var \Drupal\Core\Entity\EntityTypeInterface
     */
    protected $entityType;
    
    /**
     * The entity repository service.
     *
     * @var \Drupal\Core\Entity\EntityRepositoryInterface
     */
    protected $entityRepository;
    
    /**
     * The entity display repository.
     *
     * @var \Drupal\Core\Entity\EntityDisplayRepositoryInterface
     */
    protected $entityDisplayRepository;
    
    /**
     * The cache bin used to store the render cache.
     *
     * @var string
     */
    protected $cacheBin = 'render';
    
    /**
     * The language manager.
     *
     * @var \Drupal\Core\Language\LanguageManagerInterface
     */
    protected $languageManager;
    
    /**
     * The theme registry.
     *
     * @var \Drupal\Core\Theme\Registry
     */
    protected $themeRegistry;
    
    /**
     * The EntityViewDisplay objects created for individual field rendering.
     *
     * @var \Drupal\Core\Entity\Display\EntityViewDisplayInterface[]
     *
     * @see \Drupal\Core\Entity\EntityViewBuilder::getSingleFieldDisplay()
     */
    protected $singleFieldDisplays;
    
    /**
     * Constructs a new EntityViewBuilder.
     *
     * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
     *   The entity type definition.
     * @param \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository
     *   The entity repository service.
     * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
     *   The language manager.
     * @param \Drupal\Core\Theme\Registry $theme_registry
     *   The theme registry.
     * @param \Drupal\Core\Entity\EntityDisplayRepositoryInterface $entity_display_repository
     *   The entity display repository.
     */
    public function __construct(EntityTypeInterface $entity_type, EntityRepositoryInterface $entity_repository, LanguageManagerInterface $language_manager, Registry $theme_registry, EntityDisplayRepositoryInterface $entity_display_repository) {
        $this->entityTypeId = $entity_type->id();
        $this->entityType = $entity_type;
        $this->entityRepository = $entity_repository;
        $this->languageManager = $language_manager;
        $this->themeRegistry = $theme_registry;
        $this->entityDisplayRepository = $entity_display_repository;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function createInstance(ContainerInterface $container, EntityTypeInterface $entity_type) {
        return new static($entity_type, $container->get('entity.repository'), $container->get('language_manager'), $container->get('theme.registry'), $container->get('entity_display.repository'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function view(EntityInterface $entity, $view_mode = 'full', $langcode = NULL) {
        $build_list = $this->viewMultiple([
            $entity,
        ], $view_mode, $langcode);
        // The default ::buildMultiple() #pre_render callback won't run, because we
        // extract a child element of the default renderable array. Thus we must
        // assign an alternative #pre_render callback that applies the necessary
        // transformations and then still calls ::buildMultiple().
        $build = $build_list[0];
        $build['#pre_render'][] = [
            $this,
            'build',
        ];
        return $build;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function trustedCallbacks() {
        return [
            'build',
            'buildMultiple',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewMultiple(array $entities = [], $view_mode = 'full', $langcode = NULL) {
        $build_list = [
            '#sorted' => TRUE,
            '#pre_render' => [
                [
                    $this,
                    'buildMultiple',
                ],
            ],
        ];
        $weight = 0;
        foreach ($entities as $key => $entity) {
            // Ensure that from now on we are dealing with the proper translation
            // object.
            $entity = $this->entityRepository
                ->getTranslationFromContext($entity, $langcode);
            // Set build defaults.
            $build_list[$key] = $this->getBuildDefaults($entity, $view_mode);
            $entityType = $this->entityTypeId;
            $this->moduleHandler()
                ->alter([
                $entityType . '_build_defaults',
                'entity_build_defaults',
            ], $build_list[$key], $entity, $view_mode);
            $build_list[$key]['#weight'] = $weight++;
        }
        return $build_list;
    }
    
    /**
     * Provides entity-specific defaults to the build process.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity for which the defaults should be provided.
     * @param string $view_mode
     *   The view mode that should be used.
     *
     * @return array
     */
    protected function getBuildDefaults(EntityInterface $entity, $view_mode) {
        // Allow modules to change the view mode.
        $context = [];
        $this->moduleHandler()
            ->alter('entity_view_mode', $view_mode, $entity, $context);
        $build = [
            "#{$this->entityTypeId}" => $entity,
            '#view_mode' => $view_mode,
            // Collect cache defaults for this entity.
'#cache' => [
                'tags' => Cache::mergeTags($this->getCacheTags(), $entity->getCacheTags()),
                'contexts' => $entity->getCacheContexts(),
                'max-age' => $entity->getCacheMaxAge(),
            ],
        ];
        // Add the default #theme key if a template exists for it.
        if ($this->themeRegistry
            ->getRuntime()
            ->has($this->entityTypeId)) {
            $build['#theme'] = $this->entityTypeId;
        }
        // Cache the rendered output if permitted by the view mode and global entity
        // type configuration.
        if ($this->isViewModeCacheable($view_mode) && !$entity->isNew() && $entity->isDefaultRevision() && $this->entityType
            ->isRenderCacheable()) {
            $build['#cache'] += [
                'keys' => [
                    'entity_view',
                    $this->entityTypeId,
                    $entity->id(),
                    $view_mode,
                ],
                'bin' => $this->cacheBin,
            ];
            if ($entity instanceof TranslatableDataInterface && count($entity->getTranslationLanguages()) > 1) {
                $build['#cache']['keys'][] = $entity->language()
                    ->getId();
            }
        }
        return $build;
    }
    
    /**
     * Builds an entity's view; augments entity defaults.
     *
     * This function is assigned as a #pre_render callback in ::view().
     *
     * It transforms the renderable array for a single entity to the same
     * structure as if we were rendering multiple entities, and then calls the
     * default ::buildMultiple() #pre_render callback.
     *
     * @param array $build
     *   A renderable array containing build information and context for an entity
     *   view.
     *
     * @return array
     *   The updated renderable array.
     *
     * @see \Drupal\Core\Render\RendererInterface::render()
     */
    public function build(array $build) {
        $build_list = [
            $build,
        ];
        $build_list = $this->buildMultiple($build_list);
        return $build_list[0];
    }
    
    /**
     * Builds multiple entities' views; augments entity defaults.
     *
     * This function is assigned as a #pre_render callback in ::viewMultiple().
     *
     * By delaying the building of an entity until the #pre_render processing in
     * \Drupal::service('renderer')->render(), the processing cost of assembling
     * an entity's renderable array is saved on cache-hit requests.
     *
     * @param array $build_list
     *   A renderable  array containing build information and context for an
     *   entity view.
     *
     * @return array
     *   The updated renderable array.
     *
     * @see \Drupal\Core\Render\RendererInterface::render()
     */
    public function buildMultiple(array $build_list) {
        // Build the view modes and display objects.
        $view_modes = [];
        $entity_type_key = "#{$this->entityTypeId}";
        $view_hook = "{$this->entityTypeId}_view";
        // Find the keys for the ContentEntities in the build; Store entities for
        // rendering by view_mode.
        $children = Element::children($build_list);
        foreach ($children as $key) {
            if (isset($build_list[$key][$entity_type_key])) {
                $entity = $build_list[$key][$entity_type_key];
                if ($entity instanceof FieldableEntityInterface) {
                    $view_modes[$build_list[$key]['#view_mode']][$key] = $entity;
                }
            }
        }
        // Build content for the displays represented by the entities.
        foreach ($view_modes as $view_mode => $view_mode_entities) {
            $displays = EntityViewDisplay::collectRenderDisplays($view_mode_entities, $view_mode);
            $this->buildComponents($build_list, $view_mode_entities, $displays, $view_mode);
            foreach (array_keys($view_mode_entities) as $key) {
                // Allow for alterations while building, before rendering.
                $entity = $build_list[$key][$entity_type_key];
                $display = $displays[$entity->bundle()];
                $this->moduleHandler()
                    ->invokeAll($view_hook, [
                    &$build_list[$key],
                    $entity,
                    $display,
                    $view_mode,
                ]);
                $this->moduleHandler()
                    ->invokeAll('entity_view', [
                    &$build_list[$key],
                    $entity,
                    $display,
                    $view_mode,
                ]);
                $this->addContextualLinks($build_list[$key], $entity);
                $this->alterBuild($build_list[$key], $entity, $display, $view_mode);
                // Assign the weights configured in the display.
                // @todo Once https://www.drupal.org/node/1875974 provides the missing
                //   API, only do it for 'extra fields', since other components have
                //   been taken care of in EntityViewDisplay::buildMultiple().
                foreach ($display->getComponents() as $name => $options) {
                    if (isset($build_list[$key][$name])) {
                        $build_list[$key][$name]['#weight'] = $options['weight'];
                    }
                }
                // Allow modules to modify the render array.
                $this->moduleHandler()
                    ->alter([
                    $view_hook,
                    'entity_view',
                ], $build_list[$key], $entity, $display);
            }
        }
        return $build_list;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildComponents(array &$build, array $entities, array $displays, $view_mode) {
        $entities_by_bundle = [];
        foreach ($entities as $id => $entity) {
            // Initialize the field item attributes for the fields being displayed.
            // The entity can include fields that are not displayed, and the display
            // can include components that are not fields, so we want to act on the
            // intersection. However, the entity can have many more fields than are
            // displayed, so we avoid the cost of calling $entity->getProperties()
            // by iterating the intersection as follows.
            foreach ($displays[$entity->bundle()]
                ->getComponents() as $name => $options) {
                if ($entity->hasField($name)) {
                    foreach ($entity->get($name) as $item) {
                        $item->_attributes = [];
                    }
                }
            }
            // Group the entities by bundle.
            $entities_by_bundle[$entity->bundle()][$id] = $entity;
        }
        // Invoke hook_entity_prepare_view().
        $this->moduleHandler()
            ->invokeAll('entity_prepare_view', [
            $this->entityTypeId,
            $entities,
            $displays,
            $view_mode,
        ]);
        // Let the displays build their render arrays.
        foreach ($entities_by_bundle as $bundle => $bundle_entities) {
            $display_build = $displays[$bundle]->buildMultiple($bundle_entities);
            foreach ($bundle_entities as $id => $entity) {
                $build[$id] += $display_build[$id];
            }
        }
    }
    
    /**
     * Add contextual links.
     *
     * @param array $build
     *   The render array that is being created.
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity to be prepared.
     */
    protected function addContextualLinks(array &$build, EntityInterface $entity) {
        if ($entity->isNew()) {
            return;
        }
        $key = $entity->getEntityTypeId();
        $rel = 'canonical';
        if ($entity instanceof ContentEntityInterface && !$entity->isDefaultRevision()) {
            $rel = 'revision';
            $key .= '_revision';
        }
        if ($entity->hasLinkTemplate($rel) && $entity->toUrl($rel)
            ->isRouted()) {
            $build['#contextual_links'][$key] = [
                'route_parameters' => $entity->toUrl($rel)
                    ->getRouteParameters(),
            ];
            if ($entity instanceof EntityChangedInterface) {
                $build['#contextual_links'][$key]['metadata'] = [
                    'changed' => $entity->getChangedTime(),
                ];
            }
        }
    }
    
    /**
     * Specific per-entity building.
     *
     * @param array $build
     *   The render array that is being created.
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity to be prepared.
     * @param \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display
     *   The entity view display holding the display options configured for the
     *   entity components.
     * @param string $view_mode
     *   The view mode that should be used to prepare the entity.
     */
    protected function alterBuild(array &$build, EntityInterface $entity, EntityViewDisplayInterface $display, $view_mode) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheTags() {
        return [
            $this->entityTypeId . '_view',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function resetCache(array $entities = NULL) {
        // If no set of specific entities is provided, invalidate the entity view
        // builder's cache tag. This will invalidate all entities rendered by this
        // view builder.
        // Otherwise, if a set of specific entities is provided, invalidate those
        // specific entities only, plus their list cache tags, because any lists in
        // which these entities are rendered, must be invalidated as well. However,
        // even in this case, we might invalidate more cache items than necessary.
        // When we have a way to invalidate only those cache items that have both
        // the individual entity's cache tag and the view builder's cache tag, we'll
        // be able to optimize this further.
        if (isset($entities)) {
            $tags = [];
            foreach ($entities as $entity) {
                $tags = Cache::mergeTags($tags, $entity->getCacheTags());
                $tags = Cache::mergeTags($tags, $entity->getEntityType()
                    ->getListCacheTags());
            }
            Cache::invalidateTags($tags);
        }
        else {
            Cache::invalidateTags($this->getCacheTags());
        }
    }
    
    /**
     * Determines whether the view mode is cacheable.
     *
     * @param string $view_mode
     *   Name of the view mode that should be rendered.
     *
     * @return bool
     *   TRUE if the view mode can be cached, FALSE otherwise.
     */
    protected function isViewModeCacheable($view_mode) {
        if ($view_mode == 'default') {
            // The 'default' is not an actual view mode.
            return TRUE;
        }
        $view_modes_info = $this->entityDisplayRepository
            ->getViewModes($this->entityTypeId);
        return !empty($view_modes_info[$view_mode]['cache']);
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewField(FieldItemListInterface $items, $display_options = []) {
        
        /** @var \Drupal\Core\Entity\FieldableEntityInterface $entity */
        $entity = $items->getEntity();
        // If the field is not translatable and the entity is, then the field item
        // list always points to the default translation of the entity. Attempt to
        // fetch it in the current content language.
        if (!$items->getFieldDefinition()
            ->isTranslatable() && $entity->isTranslatable()) {
            $entity = $this->entityRepository
                ->getTranslationFromContext($entity);
        }
        $field_name = $items->getFieldDefinition()
            ->getName();
        $display = $this->getSingleFieldDisplay($entity, $field_name, $display_options);
        $output = [];
        $build = $display->build($entity);
        if (isset($build[$field_name])) {
            $output = $build[$field_name];
        }
        return $output;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewFieldItem(FieldItemInterface $item, $display = []) {
        $entity = $item->getEntity();
        $field_name = $item->getFieldDefinition()
            ->getName();
        // Clone the entity since we are going to modify field values.
        $clone = clone $entity;
        // Push the item as the single value for the field, and defer to viewField()
        // to build the render array for the whole list.
        $clone->{$field_name}
            ->setValue([
            $item->getValue(),
        ]);
        $elements = $this->viewField($clone->{$field_name}, $display);
        // Extract the part of the render array we need.
        $output = $elements[0] ?? [];
        if (isset($elements['#access'])) {
            $output['#access'] = $elements['#access'];
        }
        return $output;
    }
    
    /**
     * Gets an EntityViewDisplay for rendering an individual field.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity.
     * @param string $field_name
     *   The field name.
     * @param string|array $display_options
     *   The display options passed to the viewField() method.
     *
     * @return \Drupal\Core\Entity\Display\EntityViewDisplayInterface
     */
    protected function getSingleFieldDisplay($entity, $field_name, $display_options) {
        if (is_string($display_options)) {
            // View mode: use the Display configured for the view mode.
            $view_mode = $display_options;
            $display = EntityViewDisplay::collectRenderDisplay($entity, $view_mode);
            // Hide all fields except the current one.
            foreach (array_keys($entity->getFieldDefinitions()) as $name) {
                if ($name != $field_name) {
                    $display->removeComponent($name);
                }
            }
        }
        else {
            // Array of custom display options: use a runtime Display for the
            // '_custom' view mode. Persist the displays created, to reduce the number
            // of objects (displays and formatter plugins) created when rendering a
            // series of fields individually for cases such as views tables.
            $entity_type_id = $entity->getEntityTypeId();
            $bundle = $entity->bundle();
            $key = $entity_type_id . ':' . $bundle . ':' . $field_name . ':' . Crypt::hashBase64(serialize($display_options));
            if (!isset($this->singleFieldDisplays[$key])) {
                $this->singleFieldDisplays[$key] = EntityViewDisplay::create([
                    'targetEntityType' => $entity_type_id,
                    'bundle' => $bundle,
                    'status' => TRUE,
                ])->setComponent($field_name, $display_options);
            }
            $display = $this->singleFieldDisplays[$key];
        }
        return $display;
    }

}