code: public function getLogger()
      {
          if ($this->_logger === null) {
              $this->setLogger(Yii::getLogger());
          }
  
          return $this->_logger;
      }
docs: Gets the connected logger.
  If not set, [[\Yii::getLogger()]] will be used.
  @property Logger the logger. If not set, [[\Yii::getLogger()]] will be used.
  @return Logger the logger.