Bugfix for unused elements message in backend (introduced in TYPO3 9)

[schoeppe]

Implemented record_is_used Hook.

See
https://docs.typo3.org/c/typo3/cms-core/master/en-us/Changelog/9.0/Feature-67884-DisplayUnusedCEs.html

[IchHabRecht]

Hi @schoeppe,

Thank you for your pull request. Instead of implementing an own solution I would suggest to use a default extension like https://extensions.typo3.org/extension/hide_used_content/

[schoeppe]

Hi,
thanks for pointing me to this extension, didn't know about it.

My implementation is based on the code I found in the flux-Extension. In my opinion this should be handled by the extension itself - the extension just knows best which content elements are used in the backend context.

[IchHabRecht]

Hi @schoeppe,

To be honest I was against the core implementation because I don't understand why every extension should ship its own hook. Everything is set up as TCA information. This is why I wrote the hide_used_content extension. IHMO this solution should be implemented core wise.

[joximu]

Just want to tell:
I installed "hide_used_content" but the message about unused content didn't disappear.
With the patch/bugfix from schoeppe it's fine.