Class yii\bootstrap4\Tabs

                protected function activateFirstVisibleTab()
{
    foreach ($this->items as $i => $item) {
        $active = ArrayHelper::getValue($item, 'active', null);
        $visible = ArrayHelper::getValue($item, 'visible', true);
        $disabled = ArrayHelper::getValue($item, 'disabled', false);
        if ($visible && $active !== false && $disabled !== true) {
            $this->items[$i]['active'] = true;
            return;
        }
    }
}

            

                protected function hasActiveTab()
{
    foreach ($this->items as $item) {
        if (isset($item['active']) && $item['active'] === true) {
            return true;
        }
    }
    return false;
}

            

                public function init()
{
    parent::init();
    Html::addCssClass($this->options, ['widget' => 'nav', $this->navType]);
    Html::addCssClass($this->tabContentOptions, ['panel' => 'tab-content']);
}

            

                protected function prepareItems(&$items, $prefix = '')
{
    if (!$this->hasActiveTab()) {
        $this->activateFirstVisibleTab();
    }
    foreach ($items as $n => $item) {
        $options = array_merge($this->itemOptions, ArrayHelper::getValue($item, 'options', []));
        $options['id'] = ArrayHelper::getValue($options, 'id', $this->options['id'] . $prefix . '-tab' . $n);
        unset($items[$n]['options']['id']); // @see https://github.com/yiisoft/yii2-bootstrap4/issues/108#issuecomment-465219339
        if (!ArrayHelper::remove($item, 'visible', true)) {
            continue;
        }
        if (!array_key_exists('label', $item)) {
            throw new InvalidConfigException("The 'label' option is required.");
        }
        $selected = ArrayHelper::getValue($item, 'active', false);
        $disabled = ArrayHelper::getValue($item, 'disabled', false);
        $headerOptions = ArrayHelper::getValue($item, 'headerOptions', $this->headerOptions);
        if (isset($item['items'])) {
            $this->prepareItems($items[$n]['items'], '-dd' . $n);
            continue;
        } else {
            ArrayHelper::setValue($items[$n], 'options', $headerOptions);
            if (!isset($item['url'])) {
                ArrayHelper::setValue($items[$n], 'url', '#' . $options['id']);
                ArrayHelper::setValue($items[$n], 'linkOptions.data.toggle', 'tab');
                ArrayHelper::setValue($items[$n], 'linkOptions.role', 'tab');
                ArrayHelper::setValue($items[$n], 'linkOptions.aria-controls', $options['id']);
                if (!$disabled) {
                    ArrayHelper::setValue($items[$n], 'linkOptions.aria-selected', $selected ? 'true' : 'false');
                }
            } else {
                continue;
            }
        }
        Html::addCssClass($options, ['widget' => 'tab-pane']);
        if ($selected) {
            Html::addCssClass($options, ['activate' => 'active']);
        }
        if ($this->renderTabContent) {
            $tag = ArrayHelper::remove($options, 'tag', 'div');
            $this->panes[] = Html::tag($tag, isset($item['content']) ? $item['content'] : '', $options);
        }
    }
}

            

                public function renderPanes($panes)
{
    return $this->renderTabContent ? "\n" . Html::tag('div', implode("\n", $panes), $this->tabContentOptions) : '';
}

            

                public function run()
{
    $this->registerPlugin('tab');
    $this->prepareItems($this->items);
    return Nav::widget([
            'dropdownClass' => $this->dropdownClass,
            'options' => ArrayHelper::merge(['role' => 'tablist'], $this->options),
            'items' => $this->items,
            'encodeLabels' => $this->encodeLabels,
        ]) . $this->renderPanes($this->panes);
}