code: protected function getDefaultValue($event)
      {
          if ($this->defaultValue instanceof \Closure || (is_array($this->defaultValue) && is_callable($this->defaultValue))) {
              return call_user_func($this->defaultValue, $event);
          }
  
          return $this->defaultValue;
      }
docs: Get default value
  @param \yii\base\Event $event
  @return array|mixed
  @since 2.0.14