class MyModule_Model_Observer { // Event listener method public function myCustomMethod(Varien_Event_Observer $observer) { // Get the quote item that was changed $quoteItem = $observer->getQuoteItem(); // Perform some custom logic based on the change made if ($quoteItem->getQty() > 10 && $quoteItem->getProduct()->getTypeId() == "simple") { $quoteItem->setPrice($quoteItem->getProduct()->getPrice() * 0.9); } } }In this example, we define a class that includes a method called 'myCustomMethod'. This method is an event listener that will be triggered when the 'sales_quote_item_qty_set_after' event is fired. The event object is passed into the method as an argument. Within the method, we use the getQuoteItem() method to retrieve the quote item that was changed. We then perform some custom logic on the quote item - in this case, we check if the quantity is greater than 10 and the product type is 'simple', and if so, we apply a 10% discount to the item's price. The package library being used in this example is the Magento platform.