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