/**
  * (string) actionCat : 分类
  * @param $slug
  * @param null $tag
  * @return string
  * @throws \yii\web\NotFoundHttpException
  */
 public function actionCat($slug, $tag = null)
 {
     $cat = Article::cat($slug);
     if (!$cat) {
         throw new \yii\web\NotFoundHttpException('Article category not found.');
     }
     return $this->render('cat', ['cat' => $cat, 'items' => $cat->items(['tags' => $tag, 'pagination' => ['pageSize' => 2]])]);
 }