Magento 2 REST API – Get filterable attributes of a category before and after applying filter with multiselect

I am trying to achieve multiselect filter in custom script to be able to use it in custom REST API.

While I am able to get all filterable attributes specific to a category. But after applying filter through params (e.g. Size), the size attribute get removed from the filterable attribute list after apply (default magento behaviour).

Need filters to behave as multiselect. I tried to implement it using classes of some 3rd party modules but of no help.

Below is my code

<?php ini_set('display_errors', 1); ini_set('display_startup_errors', 1); error_reporting(E_ALL); // Copyright 2009, FedEx Corporation. All rights reserved. // Version 12.0.0 use Magento\Framework\App\Bootstrap; /** * If your external file is in root folder */ require __DIR__ . '/app/bootstrap.php'; $  params = $  _SERVER; $  bootstrap = Bootstrap::create(BP, $  params); ini_set('display_errors', 1); $  obj = $  bootstrap->getObjectManager(); $  state = $  obj->get('Magento\Framework\App\State'); $  state->setAreaCode('frontend');  $  objectManager = \Magento\Framework\App\ObjectManager::getInstance(); $  categoryFactory = $  objectManager->get('Magento\Catalog\Model\CategoryFactory'); $  storeManager = $  objectManager->get('Magento\Store\Model\StoreManagerInterface'); $  resource = $  objectManager->get('Magento\Framework\App\ResourceConnection'); $  request = $  objectManager->get('Magento\Framework\App\Request\Http'); $  params = $  request->getParams(); $  connection = $  resource->getConnection(); $  storeId = $  storeManager->getStore()->getId(); $  categoryId = 824;  $  category = $  categoryFactory->create()->setStoreId($  storeId)->load($  categoryId); $  storeManager->setCurrentStore($  storeId);  $  filterableAttributes = $  objectManager->get(\Magento\Catalog\Model\Layer\Category\FilterableAttributeList::class); $  appState = $  objectManager->get(\Magento\Framework\App\State::class); $  layerResolver = $  objectManager->get(\Magento\Catalog\Model\Layer\Resolver::class); $  filterList = $  objectManager->create(     \Magento\Catalog\Model\Layer\FilterList::class,     [         'filterableAttributes' => $  filterableAttributes     ] );  $  layer = $  layerResolver->get(); $  layer->setCurrentCategory($  category); $  allFilters = $  filterList->getFilters($  layer);  $  beforefinalFilters = []; foreach ($  allFilters as $  filter) {     if ($  filter->getName() != 'Category' && $  filter->getName() != 'Price'){         if ($  filter->getItemsCount()) {             $  output = [];             foreach ($  filter->getItems() as $  item) {                 if($  item->getFilter()->getData("attribute_model")){                     $  flag = false;                     $  code = $  item->getName();                     $  name = strip_tags($  item->getLabel());                      $  beforefinalFilters[$  code][] = array(                                                'name'=> $  name,                         'value'=> $  item->getValue(),                         'code'=> $  item->getFilter()->getAttributeModel()->getAttributeCode(),                         'isActive' => $  flag                     );                 }             }         }     } }  echo "Before filter: \n"; echo "<pre>"; print_r($  beforefinalFilters); echo "</pre>"; echo "----------------------------------------------------------------------------------"; echo "<br>";  $  appliedFilters = array(); foreach ($  allFilters as $  filter) {     if ($  filter->getName() != 'Category' && $  filter->getName() != 'Price' && $  filter->getItemsCount() && isset($  params[$  filter->getAttributeModel()->getAttributeCode()])) {         $  appliedFilters[$  filter->getAttributeModel()->getAttributeCode()]['label'] = $  filter->getAttributeModel()->getFrontendLabel();         $  appliedFilters[$  filter->getAttributeModel()->getAttributeCode()]['value'] = isset($  params[$  filter->getAttributeModel()->getAttributeCode()])?$  params[$  filter->getAttributeModel()->getAttributeCode()]:'';         $  appliedFilters[$  filter->getAttributeModel()->getAttributeCode()]['filter'] = $  filter;         $  request->setParam($  filter->getAttributeModel()->getAttributeCode(), $  params[$  filter->getAttributeModel()->getAttributeCode()]);     } }  foreach ($  appliedFilters as $  key => $  value) {     $  filter = $  value['filter'];     $  filterValue = $  value['value'];     if($  key == 'price' || $  key == 'category'){         continue;     }else{         //apply filter         //Magento\Catalog\Model\Layer\Filter\Attribute         $  filter->apply($  request);     } }  $  collection = $  layer->getProductCollection(); $  collection->getSelect()->group('e.entity_id');   //get filters after applying $  allFiltersAfter = $  filterList->getFilters($  layer);  $  afterfinalFilters = []; foreach ($  allFiltersAfter as $  filter) {     if ($  filter->getName() != 'Category' && $  filter->getName() != 'Price'){         if ($  filter->getItemsCount()) {             $  output = [];             foreach ($  filter->getItems() as $  item) {                 if($  item->getFilter()->getData("attribute_model")){                     $  flag = false;                     $  code = $  item->getName();                     $  name = strip_tags($  item->getLabel());                     $  afterfinalFilters[$  code][] = array(                                                 'name'=> $  name,                         'value'=> $  item->getValue(),                         'code'=> $  item->getFilter()->getAttributeModel()->getAttributeCode(),                         'isActive' => $  flag                     );                 }             }         }     } }  echo "\n After filter: \n"; echo "<pre>"; print_r($  afterfinalFilters); echo "</pre>";