hook to track changes on save ready

HelperHooks.module

<?php

/**
 *  Helper Module
 *  Example to track a change before page is saved
 *
 */



class HelperHooks extends WireData implements Module {

    /**
     * getModuleInfo is a method required by all modules to tell ProcessWire about them
     * @return array
     */
    public static function getModuleInfo() {

        return array(
            'title' => 'Helper Hooks',
            'version' => 100,
            'summary' => '',
            'href' => '',
            'author' => 'Philipp Urlich',
            'singular' => true,
            'autoload' => true,
            );
    }

    /**
     * Initialize the module
     *
     * ProcessWire calls this when the module is loaded. For 'autoload' modules, this will be called
     * when ProcessWire's API is ready. As a result, this is a good place to attach hooks.
     *
     */
    public function init() {

        $this->addHookAfter('Pages::saveReady', $this, 'hookPagesSave');

    }


    public function hookPagesSave(HookEvent $event) {
        // Modified page, already contains changes in memory, not yet in DB
        $page = $event->arguments('page');

        // If there is a change in your field
        if ($page->isChanged('select_categories')) {
            wire("log")->save("trackchanged", $page->select_categories);
        }
    }

}