Resource tracking ================= resource:notify_when_destroyed(Pid, Message) -> NifRef. notify_when_destroyed creates a nif resource that can be used as a reference to any kind of external data. When resource is destroyed, beacuse it is not referenced any where, the destructor callback will send the message to Pid. For example, it could be used for wrapping file handles. The file is then closed automatically when there are no one referencing the file, thereby garbage collect files.