Realtime Database provides the ability to trigger events on the Firebase servers whenever a device is disconnected. This could be whenever a user has no access to a network or when they quit the app.

One use-case using this functionality is to build a simple presence detection system, whereby we can hold a realtime list of users currently online within our application. This could be useful when building a chat application and you wish to view which of your users are currently online.

Setup

To get started, we first need a location to store our online users.

To keep things simple we'll assume the user is authenticated (e.g. with Firebase Authentication) so we can use their unique user identifier.

Whenever the application opens, write a new value on a reference node (e.g. /online/:userId):

import React, { useEffect } from 'react';
import auth from '@react-native-firebase/auth';
import database from '@react-native-firebase/database';

function App() {
  useEffect(() => {
    // Assuming user is logged in
    const userId = auth().currentUser.uid;

    const reference = database().ref(`/online/${userId}`);

    // Set the /users/:userId value to true
    reference.set(true).then(() => console.log('Online presence set'));
  }, []);
}

Whenever the application boots and can connect to the remote server, the value will be written to the database indicating the user is online.

On Disconnect

Next we need to remove the value when our user disconnects. Typically you would execute this functionality from the device itself, however this presents a problem.

If the device suddenly goes offline, or is quit, the app can no longer execute code or connect to Firebase. Luckily, the Realtime Database API provides a way to execute code on the Firebase servers whenever the connection between an app & server is lost.

The onDisconnect method on a reference returns a new OnDisconnect instance. This instance provides functionality to remove or set data whenever a client disconnects. Using the remove method we can remove the node on the database if the client disconnects:

import React, { useEffect } from 'react';
import auth from '@react-native-firebase/auth';
import database from '@react-native-firebase/database';

function App() {
  useEffect(() => {
    // Assuming user is logged in
    const userId = auth().currentUser.uid;

    const reference = database().ref(`/online/${userId}`);

    // Set the /users/:userId value to true
    reference.set(true).then(() => console.log('Online presence set'));

    // Remove the node whenever the client disconnects
    reference
      .onDisconnect()
      .remove()
      .then(() => console.log('On disconnect function configured.'));
  }, []);
}

The above code demonstrates a very simple but powerful way to track which users are currently online.