code: protected function getValue($event)
      {
          if ($this->value instanceof Closure || (is_array($this->value) && is_callable($this->value))) {
              return call_user_func($this->value, $event);
          }
  
          return $this->value;
      }
docs: Returns the value for the current attributes.
  This method is called by [[evaluateAttributes()]]. Its return value will be assigned
  to the attributes corresponding to the triggering event.
  @param Event $event the event that triggers the current attribute updating.
  @return mixed the attribute value