Magento 2.2 override extension file

Question

I have a third-party module with a file I want to override so I don't loss my changes when upgrading the third-party extension.

Vendor\Module\Plugin\Quote\UpdateItem.php

namespace Vendor\Module\Plugin\Quote;

use Vendor\Module\Model\Product\Type\GiftCard;

class UpdateItem {

    /**
     * @var \Magento\Catalog\Model\ProductRepository
     */
    private $productRepository;

    public function __construct(\Magento\Catalog\Model\ProductRepository $productRepository)
    {
        $this->productRepository = $productRepository;
    }

    /**
     * save
     *
     * @param \Magento\Quote\Model\Quote $subject
     * @param callable $proceed
     * @param int $itemId
     * @param \Magento\Framework\DataObject $buyRequest
     * @param null|array|\Magento\Framework\DataObject $params
     * @return \Magento\Quote\Model\Quote\Item
     */
    public function aroundUpdateItem($subject, callable $proceed, $item, $buyRequest, $params = null)
    {
        // some code

        return $proceed($itemId, $buyRequest, $params = null);
    }
}

Vendor\Module\etc\di.xml

<type name="Magento\Quote\Model\Quote">
    <plugin name="Vendor_Module::UpdateItem" type="Vendor\Module\Plugin\Quote\UpdateItem"/>
</type>

I tried to add this file to my theme but it doesn't override the third-party file:

\app\design\frontend\Mytheme\custom\Vendor_Module\Plugin\Quote\UpdateItem.php

UPDATE

Vendor\Module and Vendor_Module is the third-party module name. So above code is the code I want to override.

UPDATE 2

The accepted answer got me on the right track. I did the following:

Myvendor\Mymodule and Myvendor_Mymodule is a custom module I created.

Myvendor\Mymodule\etc\di.xml

<type name="Magento\Quote\Model\Quote">
    <plugin name="Vendor_Module::UpdateItem" type="Vendor\Module\Plugin\Quote\UpdateItem" disabled="true" />
</type>
<type name="Magento\Quote\Model\Quote">
    <plugin name="Myvendor_Mymodule::UpdateItem" type="Myvendor\Mymodule\Plugin\Quote\UpdateItem"/>
</type>

Myvendor\Mymodule\Plugin\Quote\UpdateItem.php

namespace Myvendor\Mymodule\Plugin\Quote;

use Vendor\Module\Model\Product\Type\GiftCard;

class UpdateItem {

    /**
     * @var \Magento\Catalog\Model\ProductRepository
     */
    private $productRepository;

    public function __construct(\Magento\Catalog\Model\ProductRepository $productRepository)
    {
        $this->productRepository = $productRepository;
    }

    /**
     * save
     *
     * @param \Magento\Quote\Model\Quote $subject
     * @param callable $proceed
     * @param int $itemId
     * @param \Magento\Framework\DataObject $buyRequest
     * @param null|array|\Magento\Framework\DataObject $params
     * @return \Magento\Quote\Model\Quote\Item
     */
    public function aroundUpdateItem($subject, callable $proceed, $item, $buyRequest, $params = null)
    {
        // some code

        return $proceed($itemId, $buyRequest, $params = null);
    }
}

Answer 1

Your custom Plugin class file needs to be located at -

$MAGENTO_HOME\app\code\Vendor\Module\Plugin\Quote\UpdateItem.php

Note: I'm assuming Vendor\Module is your custom-module's namespace and not the 3rd party vendors namespace.

Additionally, you could try using the sortOrder attribute to ensure your custom Plugin executes before/after (as desired, per your specific requirements) the Vendor's code.

• This StackExchange answer - https://magento.stackexchange.com/a/115117/42207 - should give you a better idea on how the sortOrder param is interpreted by Magento's Plugin system.

• I'd highly recommend reading Alan Storm's very detailed and informative article on Magento 2x's Plugin System - http://alanstorm.com/magento_2_object_manager_plugin_system/

Good Luck! :)