code: public function init()
      {
          parent::init();
          $this->clientOptions = false;
          Html::addCssClass($this->options, 'navbar');
          if ($this->options['class'] === 'navbar') {
              Html::addCssClass($this->options, 'navbar-default');
          }
          
          if (empty($this->options['role'])) {
              $this->options['role'] = 'navigation';
          }
          $options = $this->options;
          $tag = ArrayHelper::remove($options, 'tag', 'nav');
          echo Html::beginTag($tag, $options);
          echo $this->renderToggleButton();        
          echo Html::beginTag('div', ['class' => 'navbar-right']);
      }
docs: Initializes the widget.