mutable object

class MutableDomainObject extends ImmutableDomainObject implements IObservable
{
    protected $modified = false;
    protected $observable;

    protected function __construct()
    {
        $this->observable = new Observable();
        $this->observable->registerEvents(array('invalidate','modified'));
    }

    function __call($method, array $args)
    {
        if ( substr($method,0,3) == 'set' )
        {
            $propname = substr($method, 3);
            if ( sizeof($args) <= 0 )
            {
                throw new DomainObjectException
                    ("Mutator $method requires an argument");
            }
            $validator = "validate$propname";
            if ( method_exists($this, $validator) )
            {
                if ( !$this->$validator($args[0]) )
                {
                    $this->observable->fireEvent('invalidate',
                        $propname, $args[0], $this);
                    return false;
                }
            }
            $this->$propname = $args[0];
            $this->observable->fireEvent('modified',$propname,$args[0],$this);
            return $this->modified = true;
        }
        return parent::__call($method, $args);
    }

    function on($eventId,$callback,$context=null,$addArg=null)
    {
        $this->observable->on($eventId,$callback,$context,$addArg);
    }

    function un($eventId,$callback,$context=null,$addArg=null)
    {
        $this->observable->un($eventId,$callback,$context,$addArg);
    }

    function isModified()
    {
        return $this->modified;
    }

}