public function of($name, $callback = null)
    {

        $type = $this->definition['name'];

        if (is_null($type) || !property_exists($this, $type)) {
            throw new RuntimeException('Not supported.');
        } elseif (!isset($this->keyMap[$name])) {
            throw new InvalidArgumentException("Name [{$name}] is not available.");
        }

        $id = $this->keyMap[$name];

        if (is_callable($callback)) {
            call_user_func($callback, $this->{$type}[$id]);
        }


        return $this->{$type}[$id];
    }