class ExtraFieldBlock

class ExtraFieldBlock extends BlockBase implements ContextAwarePluginInterface, ContainerFactoryPluginInterface {
    
    /**
     * The entity field manager.
     *
     * @var \Drupal\Core\Entity\EntityFieldManagerInterface
     */
    protected $entityFieldManager;
    
    /**
     * The field name.
     *
     * @var string
     */
    protected $fieldName;
    
    /**
     * The entity type manager.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * Constructs a new ExtraFieldBlock.
     *
     * @param array $configuration
     *   A configuration array containing information about the plugin instance.
     * @param string $plugin_id
     *   The plugin ID for the plugin instance.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager.
     * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
     *   The entity field manager.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager) {
        $this->entityTypeManager = $entity_type_manager;
        $this->entityFieldManager = $entity_field_manager;
        // Get field name from the plugin ID.
        [
            ,
            ,
            ,
            $field_name,
        ] = explode(static::DERIVATIVE_SEPARATOR, $plugin_id, 4);
        assert(!empty($field_name));
        $this->fieldName = $field_name;
        parent::__construct($configuration, $plugin_id, $plugin_definition);
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'label_display' => FALSE,
            'formatter' => [
                'settings' => [],
                'third_party_settings' => [],
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('entity_type.manager'), $container->get('entity_field.manager'));
    }
    
    /**
     * Gets the entity that has the field.
     *
     * @return \Drupal\Core\Entity\FieldableEntityInterface
     *   The entity.
     */
    protected function getEntity() {
        return $this->getContextValue('entity');
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        $entity = $this->getEntity();
        // Add a placeholder to replace after the entity view is built.
        // @see layout_builder_entity_view_alter().
        $extra_fields = $this->entityFieldManager
            ->getExtraFields($entity->getEntityTypeId(), $entity->bundle());
        if (!isset($extra_fields['display'][$this->fieldName])) {
            $build = [];
        }
        else {
            $build = [
                '#extra_field_placeholder_field_name' => $this->fieldName,
                // Always provide a placeholder. The Layout Builder will NOT invoke
                // hook_entity_view_alter() so extra fields will not be added to the
                // render array. If the hook is invoked the placeholder will be
                // replaced.
                // @see ::replaceFieldPlaceholder()
'#markup' => $this->t('Placeholder for the @preview_fallback', [
                    '@preview_fallback' => $this->getPreviewFallbackString(),
                ]),
            ];
        }
        CacheableMetadata::createFromObject($this)->applyTo($build);
        return $build;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getPreviewFallbackString() {
        $entity = $this->getEntity();
        $extra_fields = $this->entityFieldManager
            ->getExtraFields($entity->getEntityTypeId(), $entity->bundle());
        return new TranslatableMarkup('"@field" field', [
            '@field' => $extra_fields['display'][$this->fieldName]['label'],
        ]);
    }
    
    /**
     * Replaces all placeholders for a given field.
     *
     * @param array $build
     *   The built render array for the elements.
     * @param array $built_field
     *   The render array to replace the placeholder.
     * @param string $field_name
     *   The field name.
     *
     * @see ::build()
     */
    public static function replaceFieldPlaceholder(array &$build, array $built_field, $field_name) {
        foreach (Element::children($build) as $child) {
            if (isset($build[$child]['#extra_field_placeholder_field_name']) && $build[$child]['#extra_field_placeholder_field_name'] === $field_name) {
                $placeholder_cache = CacheableMetadata::createFromRenderArray($build[$child]);
                $built_cache = CacheableMetadata::createFromRenderArray($built_field);
                $merged_cache = $placeholder_cache->merge($built_cache);
                $build[$child] = $built_field;
                $merged_cache->applyTo($build);
            }
            else {
                static::replaceFieldPlaceholder($build[$child], $built_field, $field_name);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    protected function blockAccess(AccountInterface $account) {
        return $this->getEntity()
            ->access('view', $account, TRUE);
    }

}