public function actionDropFocus()
 {
     $courseId = Yii::$app->request->post('courseId');
     $userId = Yii::$app->user->id;
     if (UserCourse::isFocus($userId, $courseId)) {
         if (UserCourse::deleteFocus($userId, $courseId, UserCourse::TYPE_FOCUS)) {
             echo true;
             return;
         }
     } else {
         echo true;
         return;
     }
     echo false;
 }