Using Auth0 with Expo in the Managed Workflow

Here I am, once again writing something about Expo! Today I want to cover an interesting argument, which is the integration with Auth0.

What is Auth0

Auth0 is an authentication (and authorization) platform providing a lot of very useful features to allow users to signup and login into your application. Whether you need a full oAuth2 authentication flow, or a simple username/password login with Auth0 you will have it implemented in just a few lines of code.

Personally I'm quite positive using these technologies while building a new application, especially when you have very sensible data. Managing the users and their credentials is always a delicate subject and most of the time much more expensive than a monthly subscription.

Compatibility with Expo

When in our team decided to use Auth0 and Expo the first thing I did was check for the compatibility. Expo is awesome to build applications using React Native, but it's a bit of a walled garden: features like the Cloud Building and over-the-air updates are specific for the Managed Workflow and will work out of the box just using supported Expo modules. When we need to eject an Expo app to use a particular React Native library or because we need to go a bit more "low level" we are talking about the Bare Workflow (here you can find more details about this topic).

With the new EAS Build this requirement has been removed. We can build a React Native application in the cloud and have over-the-air updates even in Bare Workflows, although it's still a preview feature.

Turns out that Auth0 is not effectively supported by Expo. When there's not an expo-something module and the library to integrate needs a pod install (here the link to the Auth0 React Native SDK) usually we can conclude that we'll have to eject our application from the magical Expo garden.

Luckily this was not the case! 🥳

The integration

Following the tutorial I linked before, and more specifically the Github repo we can see that the Objective-C binding described is just needed to open a browser page:

#import <React/RCTLinkingManager.h>

- (BOOL)application:(UIApplication *)app openURL:(NSURL *)url
            options:(NSDictionary<UIApplicationOpenURLOptionsKey, id> *)options
{
  return [RCTLinkingManager application:app openURL:url options:options];
}

The rest of the stuff in the tutorial are just changes to the Info.plist file, and they can be done easily without ejecting our application through the app.config.ts (or app.json) modifying the ios.infoPlist value:

const appConfig: ExpoConfig = {
  // ...

  ios: {
    infoPlist: {
      CFBundleURLTypes: [
        {
          CFBundleTypeRole: 'None',
          CFBundleURLName: 'auth0',
          CFBundleURLSchemes: ['$(PRODUCT_BUNDLE_IDENTIFIER)'],
        },
      ],
    },
  },

  // ...
}

We don't really need to add these lines though because using the SDK without linking it with the React Native ecosystem through pod install will allow us to only use the functions that do not need to open a browser window or handle deep links, like passwordRealm:

auth0.auth.passwordRealm({
  username: '[e-mail]',
  password: 'best_password',
  realm: 'UsernamePasswordConnection',
})

Using Auth0 web-based authentication

Even if we can't use the SDK for a web-based authentication, we can still leverage the Auth0 API and the AuthSession component from Expo. With AuthSession we can let the user do a complete authentication flow in the browser (such as oAuth). The module will start a browser session and will listen for the user to come back into the application through a Deep Link.

To start using Auth0 with AuthSession we first need to setup a Social Login on the Auth0 dashboard. After that we need to start an authentication session pointing to https://your-domain.auth0.com/authorize with the correct query parameters:

const returnUrl = AuthSession.makeRedirectUri()

const queryParams = toQueryString({
  response_type: 'token',
  client_id: 'client_id',
  connection: 'connection',
  redirect_uri: returnUrl,
})

const authUrl = `https://${domain}/authorize?${queryParams}`

AuthSession.startAsync({
  returnUrl,
  authUrl,
})
  .then(console.log)
  .error(console.error)

This code will open a browser page to the Auth0 endpoint, redirecting the user to the specific authentication provider based on the connection parameter. The returnUrl variable contains the deep link URL of our application and AuthSession will be waiting for the user to be redirected back there signaling the authentication flow's end.

That's all! ✌️