Functional replacement for App\Action\IndexAction from http://www.sitepoint.com/build-nasa-photo-gallery-zend-expressive/

index_action.php

<?php
function index_action(TemplateRendererInterface $templateRenderer) {  
  return function (ServerRequestInterface $request,
                   ResponseInterface $response,
                   callable $next = null) use ($templateRenderer) {
    $html = templateRenderer->render('app::index');
    $response->getBody()->write($html);
    return $response->withHeader('Content-Type', 'text/html');
  }
}