Adding extension attribute to category api response

Question

The default response from magento 2 category api endpoint /rest/V1/categories/3 is

{
"id": 3,
"parent_id": 2,
"name": "Gear",
"is_active": true,
"position": 4,
"level": 2,
"children": "4,5,6",
"created_at": "2018-08-10 08:32:49",
"updated_at": "2018-08-24 08:02:27",
"path": "1/2/3",
"available_sort_by": [],
"include_in_menu": true,
"custom_attributes": [
    {
        "attribute_code": "description",
        "value": "<p>Gear Category Static Contents.</p>"
    },
    {
        "attribute_code": "display_mode",
        "value": "PAGE"
    },
    {
        "attribute_code": "landing_page",
        "value": "11"
    },
    {
        "attribute_code": "is_anchor",
        "value": "0"
    },
    {
        "attribute_code": "path",
        "value": "1/2/3"
    },
    {
        "attribute_code": "custom_layout_update",
        "value": "<referenceContainer name=\"catalog.leftnav\" remove=\"true\"/>"
    },
    {
        "attribute_code": "children_count",
        "value": "3"
    },
    {
        "attribute_code": "custom_use_parent_settings",
        "value": "0"
    },
    {
        "attribute_code": "custom_apply_to_products",
        "value": "0"
    },
    {
        "attribute_code": "url_key",
        "value": "gear"
    },
    {
        "attribute_code": "url_path",
        "value": "gear"
    }
]}

I would like to add a custom value to be passed in response, which is not derived from database, but generated based on certain calculation.

So my response should also contain

{
...
   {
    "attribute_code": "my_custom_flag",
    "value": "1"
   }
...
}

I tried the plugin approach and setting custom data in "afterGet" method, however it never shows up in response.

Appreciate any help on how to achieve this.

UPDATE: Basis Sairam's suggestion ...

Added di.xml as

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">    
<type name="Magento\Catalog\Api\CategoryRepositoryInterface">
    <plugin name="my_category_plugin" type="My\Catalog\Plugin\CategoryPlugin"/>
</type>

Added extension_attributes.xml as

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
<extension_attributes for="Magento\Catalog\Api\Data\CategoryInterface">
    <attribute code="my_custom_flag" type="string"/>
</extension_attributes>

And the plugin file

<?php
namespace My\Catalog\Plugin;

use Magento\Catalog\Api\Data\CategoryExtensionFactory;
use Magento\Catalog\Api\Data\CategoryExtensionInterface;
use Magento\Catalog\Api\Data\CategoryInterface;
use Magento\Catalog\Api\CategoryRepositoryInterface;

class CategoryPlugin
{

protected $extensionFactory;

/**
 * @param \Magento\Catalog\Api\Data\CategoryExtensionFactory 
 * $extensionFactory
 */
public function __construct(\Magento\Catalog\Api\Data\CategoryExtensionFactory $extensionFactory)
{
    $this->extensionFactory = $extensionFactory;
}

/**
 * @param CategoryRepositoryInterface $subject
 * @param CaetgoryInterface $category
 *
 * @return CaetgoryInterface
 */
public function afterGet(\Magento\Catalog\Api\CategoryRepositoryInterface $subject, \Magento\Catalog\Api\Data\CategoryInterface $category)
{
    $extensionAttributes = $category->getExtensionAttributes();
    $extensionAttributes = $extensionAttributes ? $extensionAttributes : $this->extensionFactory->create();

    // set custom flag
    $extensionAttributes->setMyCustomFlag('1');

    // $extensionAttributes->setExtensionAttribute('my_custom_flag', '1');

    $category->setExtensionAttributes($extensionAttributes);

    return $category;
 }
}

However, the response from the API call never changes, it's still missing "my_custom_flag". It remains unaffected.

Answer 1

Create a extension attribute for the required datainterface here I'm providing sample:

<extension_attributes for="Magento\Store\Api\Data\StoreConfigInterface">
    <attribute code="sample" type="string"/>
</extension_attributes>

Have a plugin on the interface di.xml

<type name="Magento\Store\Api\StoreConfigManagerInterface">
    <plugin name="nameofplugin"
            type="Namespace\Modulename\Plugin\filename"/>
</type>

Plugin class

    <?php

      namespace Emc\App\Plugin;

      use Magento\Store\Api\Data\StoreConfigExtensionInterface;
      use Magento\Store\Api\Data\StoreConfigInterface;
      use Magento\Store\Api\Data\StoreConfigExtensionFactory;
      use Magento\Setup\Exception;
      use Magento\Directory\Model\Currency;
      use Magento\Store\Model\StoreManagerInterface;


/**
 * Class classname
 */
class classname
{
    protected $storeConfigExtensionFactory;
    protected $storeConfigInterface;
    protected $storeConfigExtensionInterface;
    protected $scopeConfig;

    /**
     * Product constructor.
     * @param \Magento\Catalog\Model\ProductRepository $productRepository
     */
    // objects need to be changed based on your requirement 
    public function __construct(
    Currency $currencyFactory, 
    StoreManagerInterface $storeManager, 
    StoreConfigExtensionFactory $storeConfigExtensionFactory, 
    StoreConfigInterface $storeConfigInterface, 
    \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
    ) {
            $this->storeConfigExtensionFactory = $storeConfigExtensionFactory;
            $this->storeConfigInterface = $storeConfigInterface;
            $this->scopeConfig = $scopeConfig;
            $this->currencyFactory = $currencyFactory;
            $this->storeManager = $storeManager;

     }


    /**
     * @param string[] $storeCodes list of stores by store codes, will return all if storeCodes is not set
     * @return \Magento\Store\Api\Data\StoreConfigInterface[]
     */
    public function aroundGetStoreConfigs(\Magento\Store\Model\Service\StoreConfigManager $storeConfigManager, \Closure $proceed, array $storeCodes = null)
    {

        $result = $proceed($storeCodes);
        foreach ($result as $storevalues) {
            $storeconfigExtension = $this->storeConfigExtensionFactory->create();
->getCurrencySymbol());
            $storeconfigExtension->setSample($storedKey); //attribute code

            $storevalues->setExtensionAttributes($storeconfigExtension);
        }
        return $result;
    }

}