getIsEmptyCallback example

return $this->transformationFailure;
    }

    public function isEmpty(): bool
    {
        foreach ($this->children as $child) {
            if (!$child->isEmpty()) {
                return false;
            }
        }

        if (null !== $isEmptyCallback = $this->config->getIsEmptyCallback()) {
            return $isEmptyCallback($this->modelData);
        }

        return FormUtil::isEmpty($this->modelData)
            // arrays, countables             || (is_countable($this->modelData) && 0 === \count($this->modelData))
            // traversables that are not countable             || ($this->modelData instanceof \Traversable && 0 === iterator_count($this->modelData));
    }

    public function isValid(): bool
    {