如何创建用户上次在线的“上次查看时间”？

import firebase from 'react-native-firebase';

// Fetch the current user's ID from Firebase Authentication.
const uid = firebase.auth().currentUser.uid;

// Create a reference to this user's specific status node.
// This is where we will store data about being online/offline.
const userStatusRef = firebase.database().ref('/status/' + uid);

// We'll create two constants which we will write to 
// the Realtime database when this device is offline
// or online.
const isOfflineForDatabase = {
    state: 'offline',
    last_changed: firebase.database.ServerValue.TIMESTAMP,
};

const isOnlineForDatabase = {
    state: 'online',
    last_changed: firebase.database.ServerValue.TIMESTAMP,
};

// Create a reference to the special '.info/connected' path in 
// Realtime Database. This path returns `true` when connected
// and `false` when disconnected.
firebase.database().ref('.info/connected').on('value', (snapshot) => {
    // If we're not currently connected, don't do anything.
    if (snapshot.val() == false) {
        return;
    };

    // If we are currently connected, then use the 'onDisconnect()' 
    // method to add a set which will only trigger once this 
    // client has disconnected by closing the app, 
    // losing internet, or any other means.
    userStatusRef.onDisconnect().set(isOfflineForDatabase).then(() => {
        // The promise returned from .onDisconnect().set() will
        // resolve as soon as the server acknowledges the onDisconnect() 
        // request, NOT once we've actually disconnected:
        // https://firebase.google.com/docs/reference/js/firebase.database.OnDisconnect

        // We can now safely set ourselves as 'online' knowing that the
        // server will mark us as offline once we lose connection.
        userStatusRef.set(isOnlineForDatabase);
    });
});