Method getValuesSetDuringRequest() added to Drupal\Core\State\StateInterface

To provide a way to track state value changes within a request, the getValuesSetDuringRequest() method has been added to Drupal\Core\State\StateInterface. All classes that implement StateInterface but do not extend Drupal\Core\State\State will need to implement this new method.

Method signature:

/**
   * Returns the values if a key has been modified during the request.
   *
   * @param string $key
   *   The key to get the values for.
   *
   * @return array{value: mixed, original: mixed}|null
   *   An array containing:
   *     - value: The last value set during the request.
   *     - original: The initial value at the start of the request.
   *   If the key was not set, NULL is returned.
   */
  public function getValuesSetDuringRequest(string $key): ?array;

An example of how this method is used is in Drupal\Core\EventSubscriber\MaintenanceModeSubscriber, which now can log when maintenance mode has been enabled or disabled.

/**
   * Logs changes to maintenance mode.
   *
   * @param \Symfony\Component\HttpKernel\Event\TerminateEvent $event
   *   The event object.
   */
  public function onTerminate(TerminateEvent $event): void {
    $values = $this->state->getValuesSetDuringRequest('system.maintenance_mode');
    if ($values && $values['original'] !== $values['value']) {
      if ($values['value']) {
        $this->logger->info('Maintenance mode enabled.');
      }
      else {
        $this->logger->info('Maintenance mode disabled.');
      }
    }
  }