Login “autenticar” um Cliente Programaticamente – Magento 2

Como realizar o login “autenticar um cliente” de um cliente programaticamente no Magento 2

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
// Load customer
$customer = $objectManager->create('Magento\Customer\Model\Customer')->load(2); //2 is Customer ID

// Load customer session
$customerSession = $objectManager->create('Magento\Customer\Model\Session');
$customerSession->setCustomerAsLoggedIn($customer);

if($customerSession->isLoggedIn()) {
    echo "Customer Logged in";
}else{
    echo "customer is Not Logged in";
}

Como proceder, da forma correta em um Controller

protected $_customerFactory;
protected $_sessionFactory;

public function __construct(
    ...
    \Magento\Customer\Model\CustomerFactory $customerFactory,
    \Magento\Customer\Model\SessionFactory $sessionFactory,
    ...
)
{
    ...
    $this->_customerFactory = $customerFactory;
    $this->_sessionFactory = $sessionFactory;
    ...
}

Na função Execute:

public function execute()
{
    ...
    $customer = $this->_customerFactory->create()->load($id) //$id is the 
    customer id you want to load
    $sessionManager = $this->_sessionFactory->create();
    $sessionManager->setCustomerAsLoggedIn($customer);
    ...
}