Plugin always breaks the core Flatrate Shipping method

Question

I have created a plugin to customize the Flatrate shipping(per order) when a guest checks out any product.

Below are my relevant module-plugin files which I supposedly wrote as per Magento 2.x's coding standard(Magento 2.3.4 installed in Windows 10 with some minor path related tweaks):

app/code/MyLearning/CustomFlatrate/registration.php:

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE, 'MyLearning_CustomFlatrate', __DIR__
);

app/code/MyLearning/CustomFlatrate/Plugin/Flatrate.php:

<?php

namespace MyLearning\CustomFlatrate\Plugin;

class Flatrate
{
    protected $_customerSession;

    public function __construct(\Magento\Customer\Model\Session $customerSession)
    {
        $this->_customerSession = $customerSession;
    }   

    /**
     * Returns shipping price
     * @param string $shippingPrice
     * @return string
     */
    public function afterCollectRates($result)
    {
        if (!$this->_customerSession->isLoggedIn()) {
            $this->debugLog($result->debug());
        }
        return $result;
    }

    public function debugLog($string)
    {
        $writer = new \Zend\Log\Writer\Stream(BP . '/var/log/test.log');
        $logger = new \Zend\Log\Logger();
        $logger->addWriter($writer);
        return $logger->info($string);
    }
}

app/code/MyLearning/CustomFlatrate/etc/di.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\OfflineShipping\Model\Carrier\Flatrate">
        <plugin name="customFlatrate" type="MyLearning\CustomFlatrate\Plugin\Flatrate"/>
    </type>
</config>

app/code/MyLearning/CustomFlatrate/etc/module.xml:

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="MyLearning_CustomFlatrate" setup_version="1.1.0">
        <sequence>
            <module name="Magento_OfflineShipping"/>
        </sequence>
    </module>
</config>

It has been quite sometime, I restarted working in Magento 2, so I can't get my head around, what's going on when I enable my plugin and add product to cart and apply the necessary param-values like state, zipcode, country for getting flatrate shipping rates, but if my plugin is not disabled, then it doesn't load flatrates and asa(as soon as) I disable my plugin, it loads the flatrate.

Can anyone help out understanding where the $result's value is going wrong, such that it doesn't bring the flatrates at all ?

Answer 1

Update your plugin file using this below code. afterCollectRates function missing one arguement.

<?php

namespace MyLearning\CustomFlatrate\Plugin;

class Flatrate
{
    protected $_customerSession;
    protected $_rateMethodFactory;

    public function __construct(
        \Magento\Customer\Model\Session $customerSession,
        \Magento\Quote\Model\Quote\Address\RateResult\MethodFactory $rateMethodFactory
    )
    {
        $this->_customerSession = $customerSession;
        $this->_rateMethodFactory = $rateMethodFactory;
    }   

    public function afterCollectRates(\Magento\OfflineShipping\Model\Carrier\Flatrate $flatRate, $result)
    {
        /** @var \Magento\Shipping\Model\Rate\Result $result */
        $updateResult = $result->getResult();

        if (!$this->_customerSession->isLoggedIn()) {
            /** @var \Magento\Quote\Model\Quote\Address\RateResult\Method $method */
            $method = $this->_rateMethodFactory->create();

            $method->setCarrier('shipping');
            $method->setCarrierTitle('Title');
            $method->setMethod('shipping');
            $method->setMethodTitle('Method Title');
            $amount = 100; // Set your custom price
            $method->setPrice($amount);
            $method->setCost($amount);
            $updateResult->append($method);
            $this->myCustomLog("Your Text");
        }
        return $result;
    }

    private function myCustomLog($string)
    {
        $writer = new \Zend\Log\Writer\Stream(BP . '/var/log/test.log');
        $logger = new \Zend\Log\Logger();
        $logger->addWriter($writer);
        return $logger->info($string);
    }
}

Clean cache and check it.