protected function renderProgress()
    {
        if (empty($this->bars)) {
            $config = ['percent' => $this->percent, 'animated' => $this->animated, 'striped' => $this->striped];
            return $this->renderBar($config, $this->label, $this->barOptions);
        }
        $bars = [];
        foreach ($this->bars as $bar) {
            $label = ArrayHelper::getValue($bar, 'label', '');
            if (!isset($bar['percent'])) {
                throw new InvalidConfigException("The 'percent' option is required.");
            }
            $options = ArrayHelper::getValue($bar, 'options', []);
            $bars[] = $this->renderBar($bar, $label, $options);
        }

        return implode("\n", $bars);
    }