Overwriting the Magento\Checkout\Controller\Cart\Add::getBackUrl with Plugin

Question

I've been trying to overwrite the getBackUrl method in the Cart Add controller of the checkout. With a preference I managed without an issue but plugins are a bit more of a challenge.

Here's what I have so far.

di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Checkout\Controller\Cart\Add">
        <plugin name="[namespace]_[module]_checkout_controller_cart_add_before" type="[Namespace]\[Module]\Controller\Checkout\Cart\Plugin" sortOrder="1" />
    </type>
</config>

Controller/Checkout/Cart/Plugin.php

namespace [Namespace]\[Module]\Controller\Checkout\Cart;

class Plugin
{

    /**
     * @var \Magento\Framework\App\Config\ScopeConfigInterface
     */
    protected $_config;

    /**
     * @var \Magento\Framework\Url
     */
    protected $_url;

    /**
     * @param \Magento\Framework\App\Config\ScopeConfigInterface $config
     * @param \Magento\Framework\Url $url
     */
    public function __construct(
        \Magento\Framework\App\Config\ScopeConfigInterface $config,
        \Magento\Framework\Url $url
    ) {
        $this->_config = $config;
        $this->_url = $url;
    }

    /**
     * Get resolved back url, rewritten to return checkout URL instead of cart url
     *
     * @param \Magento\Checkout\Controller\Cart\Add $subject
     * @return string
     */
    protected function beforeGetBackUrl(\Magento\Checkout\Controller\Cart\Add $subject)
    {
        return $this->_url->getUrl('some/custom/url');
    }
}

Whatever I try I can't get it to reach the beforeGetBackUrl method.

Answer 1

It is possible to plug in to the public non-static methods only: http://devdocs.magento.com/guides/v2.0/extension-dev-guide/plugins.html#plugin-limit
It is impossible to plug in to the protected, private, static methods.
Also it is impossible to plug in to the public __construct method (constructor).
https://github.com/magento/magento2/issues/2265 https://github.com/magento/magento2/issues/2367 https://github.com/magento/magento2/issues/2202#issuecomment-151599087

Answer 2

You cannot plugginize getBackUrl, but you can getUrl.

For this you need:

1. Create virtual type myUrlBulder from \Magento\Framework\Url
2. Use it as argument for \Magento\Checkout\Controller\Cart\Add
3. Declare plugin for myUrlBulder virtual type