onAfterWrite function to auto incriment blocks

AutosortBlocks.php

public function onAfterWrite()
    {
        parent::onAfterWrite();
        if ($this->hasExtension('FilesystemPublisher')) {
            $this->republish($this);
        }
        $currentPage = Controller::curr()->getRequest()->requestVar('CMSMainCurrentPageID');

        // Check if Block is Used on Current Page and has a Sort of 0
        $query = new SQLSelect();
        $pageCount = $query->setFrom('SiteTree_Blocks')
            ->addWhere(array('BlockID' => $this->ID, 'SiteTreeID' => $currentPage, 'Sort' => 0))
            ->setSelect('COUNT("SiteTree_Blocks"."ID")')
            ->execute()->value();

        if($pageCount > 0) {
            //Update SiteTree_Blocks to increase count of current Block by One
            $max = DB::query('SELECT MAX(Sort) FROM SiteTree_Blocks WHERE SiteTreeID = '.$currentPage)->value();
            $update = SQLUpdate::create('SiteTree_Blocks')
                ->addWhere(array("SiteTreeID" => $currentPage, "BlockID" => $this->ID));
            $update->assign("Sort", $max + 1);
            $update->execute();
        }

    }