Retrieving the BackendUser from BackendUserAuthentication - typo3

Question

I'm trying to develop an extension that adds a Button to the ClearCache menu in the TYPO3 Backend. In a large installation with multiple domains, non-admin users need a button to clear the page cache for their domain, but only of those pages that they have access to. The default options.clearCache.pages = 1 instead flushes the whole Frontend Cache of the installation.
I've gotten so far as to calling a method in a custom class ClearCacheHook, that implements \TYPO3\CMS\Backend\Toolbar\ClearCacheActionsHookInterface.
I next need to get a list of all page uids the BackendUser has access to, which is done with $backendUser->getDbMountPoints(). All the docs speak of a global variable $BE_USER, but this isn't set for me. I have a $GLOBALS['BE_USER'], but that is of the class BackendUserAuthentication.
I can't figure out how to resolve the BackendUser from the BackendUserAuthentication. Theres the BackendUser uid in the object so I tried initializing a TYPO3\\CMS\\Beuser\\Domain\\Repository\\BackendUserRepository via the ObjectManager, but that fails.

Answer 1

I'll focus on the more specific tasks: Create an instance of BackendUserRepository and create an instance of BackendUser from uid.
You might have a look at the UsernameViewHelper.php class of be_log in the TYPO3 core.
specifically this:
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Extbase\Domain\Repository\BackendUserRepository;
use TYPO3\CMS\Extbase\Object\ObjectManager;
...
$objectManager = GeneralUtility::makeInstance(ObjectManager::class);
$backendUserRepository = $objectManager->get(BackendUserRepository::class);
/** #var \TYPO3\CMS\Extbase\Domain\Model\BackendUser $user */
$user = $backendUserRepository->findByUid($uid);
If there is something, I don't find in the documentation, I sometimes look at existing extensions. A good candidate is the news extensions. Other good candidates are, of course, the TYPO3 source code.