The onDisconnect class allows you to write or clear data when your client disconnects from the Firebase servers. These updates occur whether your client disconnects cleanly or not, so you can rely on them to clean up data even if a connection is dropped or a client crashes.
The onDisconnect class is most commonly used to manage presence in applications where it is useful to detect how many clients are connected and when other clients disconnect. See Offline Capabilities for more information.
Note that these functions should be called before any data is written to avoid problems if a connection is dropped before the requests can be transferred to the Firebase servers.
Note that onDisconnect operations are only triggered once. If you want an operation to occur each time a disconnect occurs, you'll need to re-establish the onDisconnect operations each time.