Breaking: #69568 - FormEngine related classes 

Description 

The following classes have been removed:

• \TYPO3\CMS\Backend\Form\DataPreprocessor
• \TYPO3\CMS\Backend\Form\FormEngine
• \TYPO3\CMS\Backend\Form\FlexFormsHelper

The following hook has been removed:

• $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['t3lib/class.t3lib_tceforms.php']['getMainFieldsClass']

Impact 

Code trying to instantiate these classes will fatal.

Affected Installations 

A rather low number of extensions should be affected by this change. Searching for the above class names should reveal them.

Migration 

The methods and classes have been moved to different classes and solutions. Extensions needs adaption.

The hook getMainFieldsClass has been substituted with a much more fine grained and flexible API. Use FormDataProvider to change data given to the render engine of FormEngine from now on.