Condition.php

namespace My\Site\Userfunc;

/**
 * This file is part of the TYPO3 CMS project.
 *
 * It is free software; you can redistribute it and/or modify it under
 * the terms of the GNU General Public License, either version 2
 * of the License, or any later version.
 *
 * For the full copyright and license information, please read the
 * LICENSE.txt file that was distributed with this source code.
 *
 * The TYPO3 project - inspiring people to share!
 */


class Condition {

	/**
	 * Custom condition to check if the current page has access-restricted content elements
	 *
	 * 	Usage in TypoScript:
	 *	[userFunc = My\Site\Userfunc\Condition::pageHasRestrictedContent()]
	 *
	 * @return bool
	 */
	public function pageHasRestrictedContent() {
	
	        // get pid
		$pid = $GLOBALS['TSFE']->id;
		
		// get array of all usergroups of logged in user
		// does this return all, and as an array?
		$loggedInGroups = $GLOBALS['TSFE']->fe_user->user[usergroup];
		
		// get array of all CE uids on page
		// should I use $GLOBALS['TYPO3_DB']->exec_SELECTquery and loop through it with $GLOBALS['TYPO3_DB']->sql_num_rows afterwards? 
		// Or something else?
		// And do I have to pass "not hidden, not deleted, not expired" manually?
		$cEs = xxx
		
        	// loop through all content elements
		foreach ($cEs as $cE) {
			// get array of all group IDs for that CE
			// same question as above: how to fetch them properly?
			$cEGroups = xxx
			foreach ($cEGroups as $cEGroup){
				// if this element has a fe_groups restriction, but the currently logged in user doesn't belong to at least one of those groups, we have a match: there is content on the page the user can't access
				if (!empty($cEGroup) && !array_key_exists($cEGroup, $loggedInGroups)) {
					return TRUE;
				}
			}
		}
		return FALSE;
	}

}

Supposing one person can only have one fe user, the function could return false already after detecting a logged in fe user: no need to display a login form.

Then again, all that detecting loggedInGroups is unneccessary too.. aaah