Webiny\Component\Entity\Attribute\AbstractAttribute::onSet PHP Method

    public function onSet($value, $callable = null)
    {
        if (is_callable($value) || is_string($value) && $callable == null) {
            $callable = $value;
            $value = null;
        }
        $this->onSetValue = $value;
        $this->onSetCallback = $callable;
        return $this;
    }