Feature: #67293 - Dependency ordering service

See Issue #67293


In many cases it is necessary to establish a sorted list of items from a set of "dependencies". The ordered list is then used to execute actions in the given order.

Some examples from the Core are: - Hook execution order - Extension loading order - Listing of menu items

The dependencies are therefore specified in a relative manner, outlining that an item has to be executed/loaded/listed "before" or "after" some other item.

Typical use case:

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['someExt']['someHook'][<some id>] = [
        'handler' => someClass::class,
        'runBefore' => [ <some other ID> ],
        'runAfter' => [ ... ],

In order to evaluate such relative dependencies to finally have a sorted list for ['someHook'], we introduced a new helper class \TYPO3\CMS\Core\Service\DependencyOrderingService, which does the evaluation work for you.

Example usage:

$hooks = $GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['someExt']['someHook'];
$sortedHooks = GeneralUtility:makeInstance(DependencyOrderingService::class)->orderByDependencies($hooks , 'runBefore', 'runAfter');

$sortedHooks will then contain the content of $hooks, but sorted according to the dependencies.

The DependencyOrderingService class also detects cycles in the dependencies and will throw an Exception in case conflicting dependencies have been defined.

In case the initial list does not specify a dependency for an item, those items will be put last in the final sorted list.