Message::render() 

\nn\t3::Message()->render($queueID = NULL); 

Renders the Flash messages in the queue Simple example:

\nn\t3::Message()->OK('Yes', 'No');
echo \nn\t3::Message()->render();
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Example with a queue ID:

\nn\t3::Message()->setId('above')->OK('Yes', 'No');
echo \nn\t3::Message()->render('above');
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Output in the fluid via the ViewHelper:

{nnt3:flashMessages()}
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| @return string

Source Code 

public function render( $queueID = null ) {
	if (!($messages = $this->flush($queueID))) return '';
	$html = GeneralUtility::makeInstance(FlashMessageRendererResolver::class)->resolve()->render($messages);
	return $html;
}
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