public function renderControls()
    {
        if (isset($this->controls[0], $this->controls[1])) {
            return Html::a($this->controls[0], '#' . $this->options['id'], [
                'class' => 'left carousel-control',
                'data-slide' => 'prev',
            ]) . "\n"
            . Html::a($this->controls[1], '#' . $this->options['id'], [
                'class' => 'right carousel-control',
                'data-slide' => 'next',
            ]);
        } elseif ($this->controls === false) {
            return '';
        } else {
            throw new InvalidConfigException('The "controls" property must be either false or an array of two elements.');
        }
    }