Magento 2 - Add class active to the current category

Question

I use the following code to get the categories in the left side of my store :

function categoryLoop($id, $is_sub = false){
    $objectManager = \Magento\Framework\App\ObjectManager::getInstance();

$categories = $objectManager->create('Magento\Catalog\Model\Category')->load($id);
if($categories->hasChildren()){
echo '<ul class="leftcat"' . (($is_sub) ? ' class="category_children category-'.$categories->getId().'"' : '') . '>';
        $subcategories = explode(',', $categories->getChildren());
        foreach ($subcategories as $category) {
            $subcategory = $objectManager->create('Magento\Catalog\Model\Category')->load($category);
              echo '<li><a href="'.$subcategory->getUrl().'">';
            echo $subcategory->getName();
            echo "</a></li>";
            if($subcategory->hasChildren()){ categoryLoop($category, true); }
        }
    echo "</ul>";
    }
}
categoryLoop(3); 

How I can add a class "active" to the current / selected category?

Thank you

Answer 1

You can use Registry to get current Category ID:

$registry = $objectManager->get('Magento\Framework\Registry');

$category = $registry->registry('current_category');
$catId = 0;
if($category) {
   $catId = $category->getId(); 
}

Now by comparing Category ID you can add active class.

Updated Code:

function categoryLoop($id, $currentCatId, $is_sub = false){

   $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
   $categories = $objectManager->create('Magento\Catalog\Model\Category')->load($id);

   if($categories->hasChildren()){
       echo '<ul class="leftcat"' . (($is_sub) ? ' class="category_children category-'.$categories->getId().'"' : '') . '>';

       $subcategories = explode(',', $categories->getChildren());
        foreach ($subcategories as $category) {
            $subcategory = $objectManager->create('Magento\Catalog\Model\Category')->load($category);
            $class = ($currentCatId == $subcategory->getId())?'active':'';
            echo '<li class="'.$class.'"><a href="'.$subcategory->getUrl().'">';
            echo $subcategory->getName();
            echo "</a></li>";
            if($subcategory->hasChildren()){ categoryLoop($category, true); }
        }
      echo "</ul>";
    }
}

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$registry = $objectManager->get('Magento\Framework\Registry');

$category = $registry->registry('current_category');
$currentCatId = 0;
if($category) {
   $currentCatId = $category->getId(); 
}
categoryLoop(3, $currentCatId);