class EntityAccessControlHandler

class EntityAccessControlHandler extends EntityHandlerBase implements EntityAccessControlHandlerInterface {
    
    /**
     * Stores calculated access check results.
     *
     * @var array
     */
    protected $accessCache = [];
    
    /**
     * The entity type ID of the access control handler instance.
     *
     * @var string
     */
    protected $entityTypeId;
    
    /**
     * Information about the entity type.
     *
     * @var \Drupal\Core\Entity\EntityTypeInterface
     */
    protected $entityType;
    
    /**
     * Allows to grant access to just the labels.
     *
     * By default, the "view label" operation falls back to "view". Set this to
     * TRUE to allow returning different access when just listing entity labels.
     *
     * @var bool
     */
    protected $viewLabelOperation = FALSE;
    
    /**
     * Constructs an access control handler instance.
     *
     * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
     *   The entity type definition.
     */
    public function __construct(EntityTypeInterface $entity_type) {
        $this->entityTypeId = $entity_type->id();
        $this->entityType = $entity_type;
    }
    
    /**
     * {@inheritdoc}
     */
    public function access(EntityInterface $entity, $operation, AccountInterface $account = NULL, $return_as_object = FALSE) {
        $account = $this->prepareUser($account);
        $langcode = $entity->language()
            ->getId();
        if ($operation === 'view label' && $this->viewLabelOperation == FALSE) {
            $operation = 'view';
        }
        // If an entity does not have a UUID, either from not being set or from not
        // having them, use the 'entity type:ID' pattern as the cache $cid.
        $cid = $entity->uuid() ?: $entity->getEntityTypeId() . ':' . $entity->id();
        // If the entity is revisionable, then append the revision ID to allow
        // individual revisions to have specific access control and be cached
        // separately.
        if ($entity instanceof RevisionableInterface) {
            
            /** @var $entity \Drupal\Core\Entity\RevisionableInterface */
            $cid .= ':' . $entity->getRevisionId();
        }
        if (($return = $this->getCache($cid, $operation, $langcode, $account)) !== NULL) {
            // Cache hit, no work necessary.
            return $return_as_object ? $return : $return->isAllowed();
        }
        // Invoke hook_entity_access() and hook_ENTITY_TYPE_access(). Hook results
        // take precedence over overridden implementations of
        // EntityAccessControlHandler::checkAccess(). Entities that have checks that
        // need to be done before the hook is invoked should do so by overriding
        // this method.
        // We grant access to the entity if both of these conditions are met:
        // - No modules say to deny access.
        // - At least one module says to grant access.
        $access = array_merge($this->moduleHandler()
            ->invokeAll('entity_access', [
            $entity,
            $operation,
            $account,
        ]), $this->moduleHandler()
            ->invokeAll($entity->getEntityTypeId() . '_access', [
            $entity,
            $operation,
            $account,
        ]));
        $return = $this->processAccessHookResults($access);
        // Also execute the default access check except when the access result is
        // already forbidden, as in that case, it can not be anything else.
        if (!$return->isForbidden()) {
            $return = $return->orIf($this->checkAccess($entity, $operation, $account));
        }
        $result = $this->setCache($return, $cid, $operation, $langcode, $account);
        return $return_as_object ? $result : $result->isAllowed();
    }
    
    /**
     * We grant access to the entity if both of these conditions are met:
     * - No modules say to deny access.
     * - At least one module says to grant access.
     *
     * @param \Drupal\Core\Access\AccessResultInterface[] $access
     *   An array of access results of the fired access hook.
     *
     * @return \Drupal\Core\Access\AccessResultInterface
     *   The combined result of the various access checks' results. All their
     *   cacheability metadata is merged as well.
     *
     * @see \Drupal\Core\Access\AccessResultInterface::orIf()
     */
    protected function processAccessHookResults(array $access) {
        // No results means no opinion.
        if (empty($access)) {
            return AccessResult::neutral();
        }
        
        /** @var \Drupal\Core\Access\AccessResultInterface $result */
        $result = array_shift($access);
        foreach ($access as $other) {
            $result = $result->orIf($other);
        }
        return $result;
    }
    
    /**
     * Performs access checks.
     *
     * This method is supposed to be overwritten by extending classes that
     * do their own custom access checking.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity for which to check access.
     * @param string $operation
     *   The entity operation. Usually one of 'view', 'view label', 'update' or
     *   'delete'.
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The user for which to check access.
     *
     * @return \Drupal\Core\Access\AccessResultInterface
     *   The access result.
     */
    protected function checkAccess(EntityInterface $entity, $operation, AccountInterface $account) {
        if ($operation == 'delete' && $entity->isNew()) {
            return AccessResult::forbidden()->addCacheableDependency($entity);
        }
        if ($admin_permission = $this->entityType
            ->getAdminPermission()) {
            return AccessResult::allowedIfHasPermission($account, $admin_permission);
        }
        else {
            // No opinion.
            return AccessResult::neutral();
        }
    }
    
    /**
     * Tries to retrieve a previously cached access value from the static cache.
     *
     * @param string $cid
     *   Unique string identifier for the entity/operation, for example the
     *   entity UUID or a custom string.
     * @param string $operation
     *   The entity operation. Usually one of 'view', 'update', 'create' or
     *   'delete'.
     * @param string $langcode
     *   The language code for which to check access.
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The user for which to check access.
     *
     * @return \Drupal\Core\Access\AccessResultInterface|null
     *   The cached AccessResult, or NULL if there is no record for the given
     *   user, operation, langcode and entity in the cache.
     */
    protected function getCache($cid, $operation, $langcode, AccountInterface $account) {
        // Return from cache if a value has been set for it previously.
        if (isset($this->accessCache[$account->id()][$cid][$langcode][$operation])) {
            return $this->accessCache[$account->id()][$cid][$langcode][$operation];
        }
    }
    
    /**
     * Statically caches whether the given user has access.
     *
     * @param \Drupal\Core\Access\AccessResultInterface $access
     *   The access result.
     * @param string $cid
     *   Unique string identifier for the entity/operation, for example the
     *   entity UUID or a custom string.
     * @param string $operation
     *   The entity operation. Usually one of 'view', 'update', 'create' or
     *   'delete'.
     * @param string $langcode
     *   The language code for which to check access.
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The user for which to check access.
     *
     * @return \Drupal\Core\Access\AccessResultInterface
     *   Whether the user has access, plus cacheability metadata.
     */
    protected function setCache($access, $cid, $operation, $langcode, AccountInterface $account) {
        // Save the given value in the static cache and directly return it.
        return $this->accessCache[$account->id()][$cid][$langcode][$operation] = $access;
    }
    
    /**
     * {@inheritdoc}
     */
    public function resetCache() {
        $this->accessCache = [];
    }
    
    /**
     * {@inheritdoc}
     */
    public function createAccess($entity_bundle = NULL, AccountInterface $account = NULL, array $context = [], $return_as_object = FALSE) {
        $account = $this->prepareUser($account);
        $context += [
            'entity_type_id' => $this->entityTypeId,
            'langcode' => LanguageInterface::LANGCODE_DEFAULT,
        ];
        $cid = $entity_bundle ? 'create:' . $entity_bundle : 'create';
        if (($access = $this->getCache($cid, 'create', $context['langcode'], $account)) !== NULL) {
            // Cache hit, no work necessary.
            return $return_as_object ? $access : $access->isAllowed();
        }
        // Invoke hook_entity_create_access() and hook_ENTITY_TYPE_create_access().
        // Hook results take precedence over overridden implementations of
        // EntityAccessControlHandler::checkCreateAccess(). Entities that have
        // checks that need to be done before the hook is invoked should do so by
        // overriding this method.
        // We grant access to the entity if both of these conditions are met:
        // - No modules say to deny access.
        // - At least one module says to grant access.
        $access = array_merge($this->moduleHandler()
            ->invokeAll('entity_create_access', [
            $account,
            $context,
            $entity_bundle,
        ]), $this->moduleHandler()
            ->invokeAll($this->entityTypeId . '_create_access', [
            $account,
            $context,
            $entity_bundle,
        ]));
        $return = $this->processAccessHookResults($access);
        // Also execute the default access check except when the access result is
        // already forbidden, as in that case, it can not be anything else.
        if (!$return->isForbidden()) {
            $return = $return->orIf($this->checkCreateAccess($account, $context, $entity_bundle));
        }
        $result = $this->setCache($return, $cid, 'create', $context['langcode'], $account);
        return $return_as_object ? $result : $result->isAllowed();
    }
    
    /**
     * Performs create access checks.
     *
     * This method is supposed to be overwritten by extending classes that
     * do their own custom access checking.
     *
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The user for which to check access.
     * @param array $context
     *   An array of key-value pairs to pass additional context when needed.
     * @param string|null $entity_bundle
     *   (optional) The bundle of the entity. Required if the entity supports
     *   bundles, defaults to NULL otherwise.
     *
     * @return \Drupal\Core\Access\AccessResultInterface
     *   The access result.
     */
    protected function checkCreateAccess(AccountInterface $account, array $context, $entity_bundle = NULL) {
        if ($admin_permission = $this->entityType
            ->getAdminPermission()) {
            return AccessResult::allowedIfHasPermission($account, $admin_permission);
        }
        else {
            // No opinion.
            return AccessResult::neutral();
        }
    }
    
    /**
     * Loads the current account object, if it does not exist yet.
     *
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The account interface instance.
     *
     * @return \Drupal\Core\Session\AccountInterface
     *   Returns the current account object.
     */
    protected function prepareUser(AccountInterface $account = NULL) {
        if (!$account) {
            $account = \Drupal::currentUser();
        }
        return $account;
    }
    
    /**
     * {@inheritdoc}
     */
    public function fieldAccess($operation, FieldDefinitionInterface $field_definition, AccountInterface $account = NULL, FieldItemListInterface $items = NULL, $return_as_object = FALSE) {
        $account = $this->prepareUser($account);
        // Get the default access restriction that lives within this field.
        $default = $items ? $items->defaultAccess($operation, $account) : AccessResult::allowed();
        // Explicitly disallow changing the entity ID and entity UUID.
        $entity = $items ? $items->getEntity() : NULL;
        if ($operation === 'edit' && $entity) {
            if ($field_definition->getName() === $this->entityType
                ->getKey('id')) {
                // String IDs can be set when creating the entity.
                if (!($entity->isNew() && $field_definition->getType() === 'string')) {
                    return $return_as_object ? AccessResult::forbidden('The entity ID cannot be changed.')->addCacheableDependency($entity) : FALSE;
                }
            }
            elseif ($field_definition->getName() === $this->entityType
                ->getKey('uuid')) {
                // UUIDs can be set when creating an entity.
                if (!$entity->isNew()) {
                    return $return_as_object ? AccessResult::forbidden('The entity UUID cannot be changed.')->addCacheableDependency($entity) : FALSE;
                }
            }
        }
        // Get the default access restriction as specified by the access control
        // handler.
        $entity_default = $this->checkFieldAccess($operation, $field_definition, $account, $items);
        // Combine default access, denying access wins.
        $default = $default->andIf($entity_default);
        // Invoke hook and collect grants/denies for field access from other
        // modules. Our default access flag is masked under the ':default' key.
        $grants = [
            ':default' => $default,
        ];
        $hook_implementations = $this->moduleHandler()
            ->getImplementations('entity_field_access');
        foreach ($hook_implementations as $module) {
            $grants = array_merge($grants, [
                $module => $this->moduleHandler()
                    ->invoke($module, 'entity_field_access', [
                    $operation,
                    $field_definition,
                    $account,
                    $items,
                ]),
            ]);
        }
        // Also allow modules to alter the returned grants/denies.
        $context = [
            'operation' => $operation,
            'field_definition' => $field_definition,
            'items' => $items,
            'account' => $account,
        ];
        $this->moduleHandler()
            ->alter('entity_field_access', $grants, $context);
        $result = $this->processAccessHookResults($grants);
        return $return_as_object ? $result : $result->isAllowed();
    }
    
    /**
     * Default field access as determined by this access control handler.
     *
     * @param string $operation
     *   The operation access should be checked for.
     *   Usually one of "view" or "edit".
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The field definition.
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The user session for which to check access.
     * @param \Drupal\Core\Field\FieldItemListInterface $items
     *   (optional) The field values for which to check access, or NULL if access
     *   is checked for the field definition, without any specific value
     *   available. Defaults to NULL.
     *
     * @return \Drupal\Core\Access\AccessResultInterface
     *   The access result.
     */
    protected function checkFieldAccess($operation, FieldDefinitionInterface $field_definition, AccountInterface $account, FieldItemListInterface $items = NULL) {
        return AccessResult::allowed();
    }

}