content/knitted-socks/product.txt

Title: Knitted socks
----
Price: 14.99
----
Tax: 19
----
Stock: 10

Check the availability of the products with the initializePayment:before and completePayment:before hooks. For each item in the cart check if the available stock is smaller than the quantity of the cart item. If that’s the case an exception is thrown.

In this example we use Kirby’s helpful Exception class to throw a nice error message.

site/config/config.php

<?php
use Kirby\Exception\Exception;

function checkStock($cart) {
  foreach($cart as $cartItem) {
    $cartItemPage = page($cartItem['id']);
    $availableStock = $cartItemPage->stock()->toInt();
    if ($availableStock < (int)$cartItem['quantity']) {
      throw new Exception([
        'httpCode' => 400,
        'fallback' => '“{productTitle}” is out of stock',
        'data' => [
          'productTitle' => $cartItemPage->title(),
        ],
      ]);
    }
  }
}

return [
  'hooks' => [
    'ww.merx.initializePayment:before' => function ($data, $cart) {
      checkStock($cart);
    },
    'ww.merx.completePayment:before' => function ($virtualOrderPage) {
      checkStock($virtualOrderPage->cart());
    },
  ],
];

It’s up to you were you print the exception / error message. For example you can use a checkout controller like this.

site/controllers/checkout.php

<?php
if (kirby()->request()->method() === 'POST') {
  try {
    $data = $_POST;
    $redirect = merx()->initializePayment($data);
    go($redirect);
  } catch (Exception $ex) {
    echo $ex->getMessage(); // e.g. “Knitted socks” is out of stock.
  }
}

Merx comes with a default success template which prints the exceptions of the completePayment method. You can/should use your own success template to handle completePayment exceptions.

site/plugins/merx/templates/success.php

<?php
try {
  $orderPage = merx()->completePayment($_GET);
  go($orderPage->url());
} catch (Exception $ex) {
  echo $ex->getMessage();
}

site/config/config.php

return [
  'hooks' => [
    …
    'ww.merx.completePayment:after' => function ($orderPage) {
      foreach($orderPage->cart() as $cartItem) {
        $cartItemPage = page($cartItem['id']);
        $newStock = $cartItemPage->stock()->toInt() - (int)$cartItem['quantity'];
        $cartItemPage->update([
          'stock' => (int)$newStock,
        ]);
      }
    },
  ],
];