Magento 2 Registry – Get Current Category ID

Sort of.

If you obey the intents of the Magento system designers, you can't see what's in the registry. That's because Magento's core engineers have declared the array that stores the registry values on the Mage class as private

static private $_registry        = array();

Also, there's no method in Mage for fetching the raw registry or all its keys. This means under normal operating conditions, the only methods allowed to access $_registry are in the class Mage. Since you can't rewrite anything in Mage, you're out of luck.

If you're willing to use reflection (and have PHP 5.3+ installed), you can use the following to grab the array

$class = new ReflectionClass('Mage');
$prop  = $class->getProperty('_registry');
$prop->setAccessible(true);        
$registry = $prop->getValue();
var_dump(  
    array_keys($registry)
);

This instantiates a ReflectionClass class, passing in Mage to tell PHP it's the defined Mage class we wish to reflect into.

Then we grab a ReflectionProperty object with getProperty, passing in _registry which tells PHP we want to reflect into the _registry property on the Mage class.

Then we use the setAccessible method to change the property from private to public, and then grab its value with getValue.

My code to get the category list for the main category is:

public function __construct(
        Template\Context $context, 
        \Magento\Catalog\Model\Layer\Resolver $layerResolver, 
        \Magento\Framework\Registry $registry, 
        \Magento\Catalog\Helper\Category $categoryHelper, 
        \Magento\Catalog\Model\CategoryFactory  $categoryFactory,
        array $data = array()) 
    {
        parent::__construct($context, $layerResolver, $registry, $categoryHelper,$data);
        $this->_categoryFactory = $categoryFactory;
        $this->_collectionFactory = $collectionFactory;
     }

    public function getCategoryList()
    {
      $_category  = $this->getCurrentCategory();
      $collection = $this->_categoryFactory->create()->getCollection()->addAttributeToSelect('*')
              ->addAttributeToFilter('is_active', 1)
              ->setOrder('position', 'ASC')
              ->addIdFilter($_category->getChildren());
      return $collection;

    }