How do I know when a message has failed to reach the server?

Is there anyway to have a callback when I send a message?
Because I need to know whether the message reached the server or failed, otherwise I have to assume every message was delivered to the server even when there was an error. A good situation is clients with bad connections.

The current `onError(message)` function is never called if the user goes offline all of a sudden. And when it gets called sometimes, it just shows that "Server is not connected, please reconnect". It doesn't show which message failed. So if I sent many messages at once, and this function get called then I don't know from which message the fail started.

`onDisconnect(message)` as well doesn't get called right away when the user goes offline, at least if this was getting called I would have tried to monitor if the user is online before sending the message.

What can I do to know which messages failed and which ones reached the server successfully?