public function getCallback($prefix)
    {
        if ($this->isRegistered($prefix) && !$this->_callbacks[$prefix]['is_fallback']) {
            return $this->_callbacks[$prefix]['callback'];
        }

        if ($callback = $this->findUnregisteredCallback($prefix)) {
            return $callback;
        }

        if ($this->isRegistered($prefix)) {
            return $this->_callbacks[$prefix]['callback'];
        }

        return null;
    }