Implementing the Notify ChangeTracking Policy

Section author: Roman Borschel (roman@code-factory.org)

The NOTIFY change-tracking policy is the most effective change-tracking policy provided by Doctrine but it requires some boilerplate code. This recipe will show you how this boilerplate code should look like. We will implement it on a Layer Supertype for all our domain objects.

Implementing NotifyPropertyChanged

The NOTIFY policy is based on the assumption that the entities notify interested listeners of changes to their properties. For that purpose, a class that wants to use this policy needs to implement the NotifyPropertyChanged interface from the Doctrine\Common namespace.

<?php

use Doctrine\Common\NotifyPropertyChanged,
    Doctrine\Common\PropertyChangedListener;

abstract class DomainObject implements NotifyPropertyChanged
{
    private $_listeners = array();

    public function addPropertyChangedListener(PropertyChangedListener $listener)
    {
        $this->_listeners[] = $listener;
    }

    /** Notifies listeners of a change. */
    protected function _onPropertyChanged($propName, $oldValue, $newValue)
    {
        if ($this->_listeners) {
            foreach ($this->_listeners as $listener) {
                $listener->propertyChanged($this, $propName, $oldValue, $newValue);
            }
        }
    }
}

Then, in each property setter of concrete, derived domain classes, you need to invoke _onPropertyChanged as follows to notify listeners:

<?php

// Mapping not shown, either in annotations, xml or yaml as usual
class MyEntity extends DomainObject
{
    private $data;
    // ... other fields as usual

    public function setData($data)
    {
        if ($data != $this->data) { // check: is it actually modified?
            $this->_onPropertyChanged('data', $this->data, $data);
            $this->data = $data;
        }
    }
}

The check whether the new value is different from the old one is not mandatory but recommended. That way you can avoid unnecessary updates and also have full control over when you consider a property changed.