在CakePHP中,如何确定编辑操作中的字段是否已更改？

<?php

namespace App\Model\Behavior;

use Cake\Event\EventInterface;
use Cake\ORM\Behavior;
use Cake\ORM\Table;

class ArticleAttributeBehavior extends Behavior
{
    public function beforeSave($event, $entity)
    {
        if ($this->_isFieldDirty('title', $entity)) {
            $this->getEvent()->extra["field\_changed"] = 'title';
        } else if ($this->_isFieldDirty('body', $entity)) {
            $this->getEvent()->extra["field\_changed"] = 'body';
        }

        parent::beforeSave($event, $entity);
    }

    private function _isFieldDirty($field, $entity)
    {
        $oldData = $entity->original($field);
        $newData = $entity->{$field};

        return $oldData !== $newData;
    }
}

use Cake\ORM\Table;
use Cake\Datasource\EntityInterface;

class ArticlesTable extends Table
{