SomeDataObject.php

<?php
class SomeDataObject extends DataObject 
{

    public function updateQuantity($newQuantity)
    {
        $beforeUpdateQuantity = $this->extend('onBeforeUpdateQuantity', $this, $newQuantity);
        if ($beforeUpdateQuantity !== null) {
            return $beforeUpdateQuantity;
        }

        $oldQuantity = $this->Quantity;

        $this->extend('onUpdateQuantity', $this, $oldQuantity, $newQuantity);

        if (!$newQuantity) {
            return $this;
        }


        $this->Quantity = $newQuantity;
        $this->write();

        $this->extend('onAfterUpdateQuantity', $this, $oldQuantity, $newQuantity);

        return $this;
    }
}