Magento : how to change item price when adding it into the cart

第一种写法

After digging a bit into Magento’s core code, I found that you need to use $item->getProduct()->setIsSuperMode(true) in order to make $item->setCustomPrice() and $item->setOriginalPrice() work.

Here is some sample code you can use within an Observer that listens for the checkout_cart_product_add_after or checkout_cart_update_items_after events. The code is logically the same except checkout_cart_product_add_after is called for only one item and checkout_cart_update_items_after is called for all items in the cart. This code is separated/duplicated into 2 methods only as an example.

Event: checkout_cart_product_add_after
/**
 * @param Varien_Event_Observer $observer
 */
public function applyDiscount(Varien_Event_Observer $observer)
{
    /* @var $item Mage_Sales_Model_Quote_Item */
    $item = $observer->getQuoteItem();
    if ($item->getParentItem()) {
        $item = $item->getParentItem();
    }

    // Discounted 25% off
    $percentDiscount = 0.25; 

    // This makes sure the discount isn't applied over and over when refreshing
    $specialPrice = $item->getOriginalPrice() - ($item->getOriginalPrice() * $percentDiscount);

    // Make sure we don't have a negative
    if ($specialPrice > 0) {
        $item->setCustomPrice($specialPrice);
        $item->setOriginalCustomPrice($specialPrice);
        $item->getProduct()->setIsSuperMode(true);
    }
}

Event: checkout_cart_update_items_after

/**
 * @param Varien_Event_Observer $observer
 */
public function applyDiscounts(Varien_Event_Observer $observer)
{
    foreach ($observer->getCart()->getQuote()->getAllVisibleItems() as $item /* @var $item Mage_Sales_Model_Quote_Item */) {
         if ($item->getParentItem()) {
             $item = $item->getParentItem();
         }

         // Discounted 25% off
         $percentDiscount = 0.25; 

         // This makes sure the discount isn't applied over and over when refreshing
         $specialPrice = $item->getOriginalPrice() - ($item->getOriginalPrice() * $percentDiscount);

         // Make sure we don't have a negative
         if ($specialPrice > 0) {
             $item->setCustomPrice($specialPrice);
             $item->setOriginalCustomPrice($specialPrice);
             $item->getProduct()->setIsSuperMode(true);
         }
    }
}

调试

Mage::log($loginfo, null, "logfile.log");  

第二种写法

You can use an observer class to listen to checkout_cart_product_add_after, and use a product’s “Super Mode” to set custom prices against the quote item.

In your /app/code/local/{namespace}/{yourmodule}/etc/config.xml:

<config>
    ...
    <frontend>
        ...
        <events>
            <checkout_cart_product_add_after>
                <observers>
                    <unique_event_name>
                        <class>{{modulename}}/observer</class>
                        <method>modifyPrice</method>
                    </unique_event_name>
                </observers>
            </checkout_cart_product_add_after>
        </events>
        ...
    </frontend>
    ...
</config>

And then create an Observer class at /app/code/local/{namespace}/{yourmodule}/Model/Observer.php

<?php
    class <namespace>_<modulename>_Model_Observer
    {
        public function modifyPrice(Varien_Event_Observer $obs)
        {
            // Get the quote item
            $item = $obs->getQuoteItem();
            // Ensure we have the parent item, if it has one
            $item = ( $item->getParentItem() ? $item->getParentItem() : $item );
            // Load the custom price
            $price = $this->_getPriceByItem($item);
            // Set the custom price
            $item->setCustomPrice($price);
            $item->setOriginalCustomPrice($price);
            // Enable super mode on the product.
            $item->getProduct()->setIsSuperMode(true);
        }

        protected function _getPriceByItem(Mage_Sales_Model_Quote_Item $item)
        {
            $price;

            //use $item to determine your custom price.

            return $price;
        }

    }

参考及引用
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Magento : how to change item price when adding it into the cart
Programmatically add product to cart with price change

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