Tier price calculation on Special Price

Question

I need to change the logic of the tier price calculation. I have special prices on my site and their calculation depends on a product base price, but I need to calculate it depending on the special price. I found a partial solution in How to override magento2 product tierprice? and it works only in a PDP when I visit the cart - it doesn't change the price with changing qty, the price for a is always special price... My code is:

di.xml

<type name="Magento\Catalog\Model\Product">
    <plugin name="after_final_price" type="Vendor\TierPriceOnSpecialPrice\Plugin\ChangeTierPrice" sortOrder="999" disabled="false" />
</type>
<type name="Magento\Catalog\Pricing\Price\FinalPrice">
    <plugin name="after_final_price_info" type="Vendor\TierPriceOnSpecialPrice\Plugin\ChangeTierPriceInfo" sortOrder="999" disabled="false" />
</type>

Plugins code the same like below:

public function beforeGetFinalPrice($subject, $qty = null)
{
    $specialPrice = $subject->getSpecialPrice();
    $oldTierPrices = $subject->getData('tier_price');
    $newTierPrices = [];

    if ($oldTierPrices) {
        foreach ($oldTierPrices as $tierPriceName => $tierPriceData) {
            $calculatedTierPrice = $specialPrice - ($tierPriceData['percentage_value'] / 100) * $specialPrice;
            $newTierPrices[$tierPriceName] = [
                'price_id' => $tierPriceData['price_id'],
                'website_id' => $tierPriceData['website_id'],
                'all_groups' => $tierPriceData['all_groups'],
                'cust_group' => $tierPriceData['cust_group'],
                'price' => $calculatedTierPrice,
                'price_qty' => $tierPriceData['price_qty'],
                'percentage_value' => $tierPriceData['percentage_value'],
                'product_id' => $tierPriceData['product_id'],
                'website_price' => $calculatedTierPrice
            ];
        }
    }

    //You need to create your own array or modify the default tier price here and set in to product object
    $subject->setData('tier_price', $newTierPrices);
}

Answer 1

You need to subscribe to the checkout_cart_product_add_after event. To do this, in the etc folder of your module, you need to create an events.xml file with something like this.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_cart_product_add_after">
        <observer name="custom_cart_item_price" instance="Vendor\ModuleName\Observer\CustomCartItemPrice" />
    </event>
</config>

Then you need to create the observer file itself, where you describe the price logic you need on the cart.

<?php

namespace Vendor\ModuleName\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class CustomCartItemPrice implements ObserverInterface
{
    public function execute(Observer $observer): void
    {
        $item = $observer->getEvent()->getData('quote_item');
        $newCustomPrice = 100; //For example set new item price to 100
        $item->setCustomPrice($newCustomPrice);
        $item->setOriginalCustomPrice($newCustomPrice);
        $item->getProduct()->setIsSuperMode(true);
    }
}

Answer 2

As for me, the best way is to create preference for \Magento\Catalog\Model\Product\Attribute\Backend\Tierprice class.

In your di.xml:

<preference for="Magento\Catalog\Model\Product\Attribute\Backend\Tierprice"
            type="Company\Module\Rewrite\Magento\Catalog\Model\Product\Attribute\Backend\Tierprice"/>

And \Company\Module\Rewrite\Magento\Catalog\Model\Product\Attribute\Backend\Tierprice:

    <?php

namespace Company\Module\Rewrite\Magento\Catalog\Model\Product\Attribute\Backend;

use Magento\Catalog\Model\Product;

/**
 * Class Tierprice
 */
class Tierprice extends \Magento\Catalog\Model\Product\Attribute\Backend\Tierprice
{
    protected function modifyPriceData($object, $data)
    {
        /** @var Product $object */
        $data = parent::modifyPriceData($object, $data);
        foreach ($data as $key => $tierPrice) {
            $percentageValue = $this->getPercentage($tierPrice);
            if ($percentageValue) {
                // Implement your logic to get correct base price here:
                $price = $this->yourCustomFunctionForPrice();

                $data[$key]['price'] = $price * (1 - $percentageValue / 100);
                $data[$key]['website_price'] = $data[$key]['price'];
            }
        }
        return $data;
    }
}

I hope this will help.