public function isValid()
    {
        $valid = true;
        $messages = array();

        /** @var Input $input */
        foreach ($this->_inputs as $input) {
            if (!$input->isValid()) {
                $valid = false;
                $messages[$input->name] = $input->getMessages();
            }
        }
        $this->_messages = $messages;
        return $valid;
    }