hook_flag_flag() is not working I am working in one of the project using the Flag module. I want to add custom event when user flag any content. I am trying to use hook_flag_flag() but it is not working.
This is the code I am using in a module file.

use Drupal\Core\Form\FormStateInterface;
use Drupal\node\Entity\NodeType;
use Drupal\flag\Plugin\Flag\EntityFlagType;
use Drupal\flag\FlagInterface;
use Drupal\flag\Event\FlagEvents;
use Drupal\flag\Event\FlaggingEvent;
use Drupal\Core\Session\AccountInterface;

function nppe_custom_flag_flag( $flag, $entity_id, $account, $flagging) {
  drupal_set_message("This is custom message after Flag.");
}


Is there anything I am missing?
To add some custom event on flag/unflag actions in Drupal 8 you need to use Event subscriber:

public static function getSubscribedEvents() {
    $events = [];
    $events[FlagEvents::ENTITY_FLAGGED][] = ['onFlag'];
    $events[FlagEvents::ENTITY_UNFLAGGED][] = ['onUnflag'];
    return $events;
}


And then in onFlag() and onUnflag() methods you can implement your custom logic.

I just wrote a blog post with some code examples: http://dev.studiopresent.com/blog/back-end/perform-actions-flag-unflag-drupal-8