TYPO3 Core

The userFunc should get handed the following informations:
if new:
-tablename
-fieldname
-fieldconfig
-currentPageId

if already saved once:
-tablename
-fieldname
-fieldconfig
-currentPageId
-currentRecordData

Return Value would be boolean as its just a decision if the field should be displayed or not.
The userFunc should be called with TYPO3\CMS\Core\Utility\GeneralUtility::callUserFunction to have the same handling like every other userfunc

Example for a configuration would be
'displayCond' => 'USER:\Example\Namespace\UserClass->functionName:REQ:true',

Want to implement this? Would be great!

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/34728

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/34728

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/34728

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/34728

As an additional feature, I suggest extending this to sheets.
The solution seems to be simple, just adding the case 'USER': in the switch on method getSingleField_typeFlex at TYPO3\CMS\Backend\Form\FormEngine.php