When testing our code in the previous section, we were forced to reauthenticate ourselves every time we ran the app. The process of reauthenticating over and over again is tedious and makes us less likely to run our code often! We want to make it fast and easy to test new features.

From the perspective of a user who downloaded your app, it would be annoying to have to remember my email/password and type it out each time I open your app. Imagine if you had to login yourself every time you opened your favorite app?

For both reasons, once a user is authenticated into your app, we want their authentication to persist between app uses. They should stay logged in and authenticated until they delete the app or log out. To implement this functionality, we'll need to use local persistance to store some data on the user's device that tells us whether the user has previously authenticated.

Let's learn about UserDefaults and how it can help us do this!

What is UserDefaults?

UserDefaults is a quick way to store small amounts of non-sensitive data on the user's phone. It is typically used to store flags such as whether the user has logged in.

  • UserDefaults is not for storing large amounts of data on the user's device. Use CoreData or Realm instead.
  • UserDefaults is not for storing important, sensitive information like passwords or auth tokens. Use Keychain instead.

Using UserDefaults to store data is really easy. To write data, access the UserDefaults singleton and use the provided instance methods to store various types of information like so:

// write data
UserDefaults.standard.set(false, forKey: Constants.UserDefaults.isFirstTimeUser)

To read information from UserDefaults:

// read data
let isFirstTimeUser = UserDefaults.standard.bool(forKey: Constants.UserDefaults.isFirstTimeUser)

If a user removes your app from their phone, all content stored within UserDefaults will also be deleted.

Persisting Our User

To persist authentication, we'll use UserDefaults to store our User singleton between sessions. Storing a custom class in UserDefaults requires a little additional setup. We'll need to use NSKeyedArchiver to convert our class from type User to the Data type.

To use NSKeyedArchiver to archive our User, we'll need for:

  1. User must be a subclass NSObject
  2. User must implement the NSCoding protocol

Subclassing NSObject

Add the NSObject superclass to your User class. We'll also need to add super.init() to our initializers to be explicit about calling init from NSObject.

import UIKit
import FirebaseDatabase.FIRDataSnapshot

class User: NSObject {

    // ...

    init?(snapshot: DataSnapshot) {
        // ...


    init(uid: String, username: String) {
        // ...


Implementing the NSCoding Protocol

Next we'll need to implementing the NSCoding protocol so the user object can properly be encoded as Data. Add the following extension to the bottom of the User.swift source file:

extension User: NSCoding {
    func encode(with aCoder: NSCoder) {
        aCoder.encode(uid, forKey: Constants.UserDefaults.uid)
        aCoder.encode(username, forKey: Constants.UserDefaults.username)

You'll notice that we're using our constants files to manage stringly-typed keys. This is the last time we'll bring up creating constants in this tutorial! Make sure to update your constants file with the following:

struct Constants {
    // ...

    struct UserDefaults {
        static let currentUser = "currentUser"
        static let uid = "uid"
        static let username = "username"

We're using the uid and username keys to store each respective property of the user object. We'll later use currentUser to store our current user.

If you try to build the app right now, the compiler with throw an error saying our user object doesn't conform to the NSCoding protocol. This is because we need to implement init?(coder:) in our User class. This allows users to be decoded from data. Add the following alongside our other initializers:

required init?(coder aDecoder: NSCoder) {
    guard let uid = aDecoder.decodeObject(forKey: Constants.UserDefaults.uid) as? String,
        let username = aDecoder.decodeObject(forKey: Constants.UserDefaults.username) as? String
        else { return nil }

    self.uid = uid
    self.username = username


Great! Now we've successfully implemented the NSCoding protocol. Now we can store our current user in UserDefaults.

Storing Our Current User in User Defaults

We'll create an option in our setCurrent(_:) method to persist the current user to UserDefaults. In our user class, change the class method for setCurrent(_:) to the following:

// 1
class func setCurrent(_ user: User, writeToUserDefaults: Bool = false) {
    // 2
    if writeToUserDefaults {
        // 3
        let data = NSKeyedArchiver.archivedData(withRootObject: user)

        // 4
        UserDefaults.standard.set(data, forKey: Constants.UserDefaults.currentUser)

    _current = user

Let's break this down:

  1. We add another parameter that takes a Bool on whether the user should be written to UserDefaults. We give this value a default value of false.
  2. We check if the boolean value for writeToUserDefaults is true. If so, we write the user object to UserDefaults.
  3. We use NSKeyedArchiver to turn our user object into Data. We needed to implement the NSCoding protocol and inherit from NSObject to use NSKeyedArchiver.
  4. We store the data for our current user with the correct key in UserDefaults.

Great, now we can use this method to store our current user in UserDefaults. Let's go to our LoginViewController to make use of this method when an existing user logs in.

Open LoginViewController and modify authUI(_:didSignInWith:error:) to the following:

func authUI(_ authUI: FUIAuth, didSignInWith user: FIRUser?, error: Error?) {
    // ...

    UserService.show(forUID: user.uid) { (user) in
        if let user = user {
            // handle existing user
            User.setCurrent(user, writeToUserDefaults: true)

            let initialViewController = UIStoryboard.initialViewController(for: .main)
            self.view.window?.rootViewController = initialViewController
        } else {
            // handle new user
            self.performSegue(withIdentifier: Constants.Segue.toCreateUsername, sender: self)

Next, navigate to the CreateUsernameViewController and do the same in nextButtonTapped(_:):

@IBAction func nextButtonTapped(_ sender: UIButton) {
    // ...

    UserService.create(firUser, username: username) { (user) in
        guard let user = user else {
            // handle error

        User.setCurrent(user, writeToUserDefaults: true)

        let initialViewController = UIStoryboard.initialViewController(for: .main)
        self.view.window?.rootViewController = initialViewController

Now whenever a user signs up or logs in, the user will be stored in UserDefaults.

Keeping Users Logged In on Launch

To finish up, we need to add some logic that checks UserDefaults for the currentUser key when the app first launches. If the the data exists, we'll know that the user has been previously authenticated and set the rootViewController accordingly.

In our AppDelegate add the following code to the bottom of the file:

extension AppDelegate {
    func configureInitialRootViewController(for window: UIWindow?) {
        let defaults = UserDefaults.standard
        let initialViewController: UIViewController

        if Auth.auth().currentUser != nil,
            let userData = defaults.object(forKey: Constants.UserDefaults.currentUser) as? Data,
            let user = NSKeyedUnarchiver.unarchiveObject(with: userData) as? User {


            initialViewController = UIStoryboard.initialViewController(for: .main)
        } else {
            initialViewController = UIStoryboard.initialViewController(for: .login)

        window?.rootViewController = initialViewController

In our new method, we determine which storyboard's initial view controller should be set as the rootViewController of the window.

If the FIRUser singleton already exists and we unarchive data for the currentUser key from UserDefaults, we know the user has previously been authenticated on the current device. This allows us to skip the login flow.

Now we can change our application(_:didFinishLaunchingWithOptions:) method to look like this:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

    configureInitialRootViewController(for: window)

    return true

Run the app and go through the login flow. Terminate the app and run it again. You'll notice that we're now directed to the appropriate initial view controller based on whether we previously authenticated with Firebase.

Currently, we haven't implemented a way to log out, switch users, and get back to the LoginViewController. However, an easy hack around this is deleting the app and installing it again on your phone. Remember each user's UserDefaults will be cleared when the app is deleted from their phone.


If you have feedback on this tutorial or find any mistakes, please open issues on the GitHub Repository or comment below.

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