public function validate($data = null)
    {
        if ($data !== null) {
            $this->setData($data);
        }
        // data was already validated, return the results immediately
        if ($this->wasValidated === true) {
            return $this->wasValidated && count($this->messages) === 0;
        }
        foreach ($this->rules as $selector => $valueValidator) {
            foreach ($this->getDataWrapper()->getItemsBySelector($selector) as $valueIdentifier => $value) {
                /* @var $valueValidator \Sirius\Validation\ValueValidator */
                if (!$valueValidator->validate($value, $valueIdentifier, $this->getDataWrapper())) {
                    foreach ($valueValidator->getMessages() as $message) {
                        $this->addMessage($valueIdentifier, $message);
                    }
                }
            }
        }
        $this->wasValidated = true;

        return $this->wasValidated && count($this->messages) === 0;
    }