Document::getFieldNames

protected function getFieldNames(): array
{
	$fieldNames = [];
	foreach($this->getDefaultFields() as $placeholder => $field)
	{
		$fieldNames[$placeholder] = $placeholder;
	}
	foreach($this->externalFields as $placeholder => $field)
	{
		$fieldNames[$placeholder] = $placeholder;
	}
	if(!$this->body)
	{
		$this->result->addError(new Error('no body'));
	}
	else
	{
		$fieldNames = array_merge($this->body->getFieldNames(), $this->fieldNames, $fieldNames);
	}

	return $fieldNames;
}