Working of discount coupons on the checkout page in Magento 2.2.2

Question

I am trying to understand the working of discount coupon codes on the checkout page.

When we apply any coupon code, then the discount is added to cart without reloading the page.

I have checked and found some below points:

• "discount" totals segment is added by Magento_SalesRule module.

When we apply any coupon code then, using vendor\magento\module-sales-rule\view\frontend\web\js\action\set-coupon-code.js, an ajax request is send to rest/default/V1/carts/mine/coupons/:coupon.

Code from vendor\magento\module-sales-rule\view\frontend\web\js\action\set-coupon-code.js :

 return storage.put(
            url,
            {},
            false
        ).done(function (response) {
            var deferred;

            if (response) {
                deferred = $.Deferred();

                isApplied(true);
                totals.isLoading(true);
                getPaymentInformationAction(deferred);
                $.when(deferred).done(function () 
                {
                    fullScreenLoader.stopLoader();
                    totals.isLoading(false);
                });
                messageContainer.addSuccessMessage({
                    'message': message
                });
            }
        }).fail(function (response) {
            fullScreenLoader.stopLoader();
            totals.isLoading(false);
            errorProcessor.process(response, messageContainer);
        });

• After the above ajax call, Summary total section gets refresh and Discount appears in it below image.

• When we click on the cancel coupon button then code from vendor\magento\module-sales-rule\view\frontend\web\js\action\cancel-coupon.js runs and "Discount" disappears from the summary total section.

How does the total_segments add or remove "discount"?

Answer 1

1) For customer account: /V1/carts/mine/coupons/:couponCode

I just explain how sale rules work for customer logged

a) Set coupon:

vendor/magento/module-quote/etc/webapi.xml

<route url="/V1/carts/mine/coupons/:couponCode" method="PUT">
    <service class="Magento\Quote\Api\CouponManagementInterface" method="set"/>
    <resources>
        <resource ref="Magento_Cart::manage" />
    </resources>
</route>

So, the service class interface class Magento\Quote\Api\CouponManagementInterface::set() will be used and Magento\Quote\Model\CouponManagement::set will be called.

$quote->setCouponCode($couponCode);
$this->quoteRepository->save($quote->collectTotals());

When quote collects totals: $quote->collectTotals()

\Magento\SalesRule\Model\Quote\Discount::collect() will be called. It will calculate the coupon logic here.

b) Remove coupon:

vendor/magento/module-quote/etc/webapi.xml

<route url="/V1/carts/mine/coupons" method="DELETE">
    <service class="Magento\Quote\Api\CouponManagementInterface" method="remove"/>
    <resources>
        <resource ref="self" />
    </resources>
    <data>
        <parameter name="cartId" force="true">%cart_id%</parameter>
    </data>
</route>

Magento\Quote\Model\CouponManagement::remove() will be called.

c) Total Segment: V1/carts/mine/payment-information

vendor/magento/module-checkout/etc/webapi.xml

<route url="/V1/carts/mine/payment-information" method="GET">
    <service class="Magento\Checkout\Api\PaymentInformationManagementInterface" method="getPaymentInformation"/>
    <resources>
        <resource ref="self" />
    </resources>
    <data>
        <parameter name="cartId" force="true">%cart_id%</parameter>
    </data>
</route>

vendor/magento/module-checkout/Model/PaymentInformationManagement.php

/**
 * {@inheritDoc}
 */
public function getPaymentInformation($cartId)
{
    ...
    $paymentDetails->setTotals($this->cartTotalsRepository->get($cartId));
    ...
}

\Magento\Quote\Model\Cart\CartTotalRepository::get()

 if ($quote->isVirtual()) {
        $addressTotalsData = $quote->getBillingAddress()->getData();
        $addressTotals = $quote->getBillingAddress()->getTotals();
    } else {
        $addressTotalsData = $quote->getShippingAddress()->getData();
        $addressTotals = $quote->getShippingAddress()->getTotals();
    }
......

$calculatedTotals = $this->totalsConverter->process($addressTotals);
$quoteTotals->setTotalSegments($calculatedTotals);

This will get the totals data and set the total segments.

\Magento\Quote\Model\Quote\TotalsReader::fetch()

foreach ($this->collectorList->getCollectors($quote->getStoreId()) as $reader) {
        $data = $reader->fetch($quote, $total);

2) For guest account, we can see the apis:

vendor/magento/module-quote/etc/webapi.xml

<!-- Managing Guest Cart Coupons -->
<route url="/V1/guest-carts/:cartId/coupons" method="GET">
    <service class="Magento\Quote\Api\GuestCouponManagementInterface" method="get"/>
    <resources>
        <resource ref="anonymous" />
    </resources>
</route>
<route url="/V1/guest-carts/:cartId/coupons/:couponCode" method="PUT">
    <service class="Magento\Quote\Api\GuestCouponManagementInterface" method="set"/>
    <resources>
        <resource ref="anonymous" />
    </resources>
</route>
<route url="/V1/guest-carts/:cartId/coupons" method="DELETE">
    <service class="Magento\Quote\Api\GuestCouponManagementInterface" method="remove"/>
    <resources>
        <resource ref="anonymous" />
    </resources>
</route>