TYPO3 Core

Every Formelement run through some lifecycle callbacks which make it possible to manipulate the element on runtime.
It would be usefull to execute this callback on toplevel composit elements aka. pages.
This make it possible to dynamically add and remove elements on runtime.
At the moment, this callback is not triggered for pages.

Use this example to test the behavior
https://code.tritum.de/TYPO3.CMS/ext_form_example1480338786

This example provide a predefined form definition called "conditional elements example".
Install this Extension, include the statics, create a new "Form" Contentelement and select this form.
If you check the checkbox "Subscribe to newsletter" on the first page and continue to the next page,
then a new formelement "Name" will be shown on the second page. Additionaly a NotEmpty validator will be added to the "Email" formelement.

Without this patch, the formelement "Name" will not be created.