Triggered before a product quantity in the cart is changed.
Inside the event handler you can trigger an event to prevent changing the quantity.
Usage example:
public function subscribeEvents()
{
Backend::$events->addEvent('shop:onBeforeSetCartQuantity', $this, 'before_set_cart_quantity');
}
public function before_set_cart_quantity($cart_name, $key, $value)
{
$item = Shop_Cart::find_item($key, $cart_name);
if ($item)
traceLog('New quantity value for '.$item->product->name.': '.$value);
}