Magento 2 get current category Id using registry

If you need the category Id in Magento2 ,then you can get the values using the following steps to be followed

1.Include the use Magento\Framework\Registry in your class file.

<?php
/**
 * class file
 */

namespace Vendor\Module\Model;

use Magento\Framework\Registry;

...

2.Create an object for that using object Manager or else if your using it in the controller then assign in your __construct() function as \Magento\Framework\Registry $registry:

...

/**
 * @var Registry
 */

class BlueLine
{
    ...
    private $registry;
    ...

    public function __construct(Registry $registry) 
    {
        $this->registry = $registry;
    }

    ...

3.Then you can simply use it with the class as:

$category = $this->registry->registry('current_category');

echo $category->getId();

For the further Reference in Magento2 Implementation of this concept refer the class file and function called public function _initCategory(). In this method they are registering the current category.

Try this code. this will definitely help you.

<?php 
   $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
   $category = $objectManager->get('Magento\Framework\Registry')->registry('current_category');//get current category
    echo $category->getId();
    echo $category->getName();
?>

The above to seem correct, but I think that jumping straight to the Registry is not the best approach. Magento provides a Layer Resolver that already encapsulates that functionality. (See the TopMenu Block in the Catalog Plugins)

I suggest injecting the \Magento\Catalog\Model\Layer\Resolver class and using that to get the current category. Here is the code :

<?php

namespace FooBar\Demo\Block;

class Demo extends \Magento\Framework\View\Element\Template
{
    private $layerResolver;

    public function __construct(
        \Magento\Framework\View\Element\Template\Context $context,
        \Magento\Catalog\Model\Layer\Resolver $layerResolver,
        array $data = []
    ) {
        parent::__construct($context, $data);

        $this->layerResolver = $layerResolver;
    }

    public function getCurrentCategory()
    {
        return $this->layerResolver->get()->getCurrentCategory();
    }

    public function getCurrentCategoryId()
    {
        return $this->getCurrentCategory()->getId();
    }
}

Here is what the actual getCurrentCategory() method does in the Resolver Class.

public function getCurrentCategory()
{
    $category = $this->getData('current_category');
    if ($category === null) {
        $category = $this->registry->registry('current_category');
        if ($category) {
            $this->setData('current_category', $category);
        } else {
            $category = $this->categoryRepository->get($this->getCurrentStore()->getRootCategoryId());
            $this->setData('current_category', $category);
        }
    }

    return $category;
}

As you can see, it does still use the registry but it provides a fallback in case that fails.