/**
* Renders collapsible toggle button.
* @return string the rendering toggle button.
*/
protected function renderToggleButton()
{
$bar = Html::tag('span', '', ['class' => 'icon-bar']);
$screenReader = "<span class=\"sr-only\">{$this->screenReaderToggleText}</span>";
return Html::button("{$screenReader}\n{$bar}\n{$bar}\n{$bar}", ['class' => 'navbar-toggle', 'data-toggle' => 'collapse', 'data-target' => "#{$this->containerOptions['id']}"]);
}
