A Firebase User object represents the account of a user who has signed up to an app in your project. Apps usually have many registered users, and every app in a project shares a user database.
A Firebase User instance is independent from a Firebase Authentication instance. This means that you can have several references to different users within the same context and still call any of their methods.
A Firebase User has a fixed set of basic properties—a unique ID, a primary email address, a name and a photo URL—stored in the project's user database, that can be updated by the user (iOS, Android, web). You cannot add other properties to the Firebase User object directly; instead, you can store the additional properties in any other storage services like Google Cloud Firestore.
The first time a user signs up to your app, the user's profile data is populated using the available information:
- If the user signed up with an email address and password, only the primary email address property is populated
- If the user signed up with a federated identity provider, such as Google or Facebook, the account information made available by the provider is used to populate the Firebase User's profile
- If the user signed up with your custom auth system, you must explicitly add the information you want to the Firebase User's profile
Once a user account has been created, you can reload the user's information to incorporate any changes the user might have made on another device.
You can sign in users to your apps using several methods: email address and password, federated identity providers, and your custom auth system. You can associate more than one sign-in method with a user: for example, a user can sign in to the same account using an email address and a password, or using Google Sign-In.
A Firebase User instance keeps track of every provider linked to the user. This allows you to update empty profile's properties using the information given by a provider. See Managing Users (iOS, Android, web).
The current user
When a user signs up or signs in, that user becomes the current user of the Auth instance. The instance persists the user's state, so that refreshing the page (in a browser) or restarting the application doesn't lose the user's information.
When the user signs out, the Auth instance stops keeping a reference to the User object and no longer persists its state; there is no current user. However, the user instance continues to be completely functional: if you keep a reference to it, you can still access and update the user's data.
The user lifecycle
An Auth listener gets notified in the following situations:
- The Auth object finishes initializing and a user was already signed in from a previous session, or has been redirected from an identity provider's sign-in flow
- A user signs in (the current user is set)
- A user signs out (the current user becomes null)
- The current user's access token is refreshed. This case can happen in the
- The access token expires: this is a common situation. The refresh token is used to get a new valid set of tokens.
- The user changes their password: Firebase issues new access and refresh tokens and renders the old tokens expired. This automatically expires the user's token and/or signs out the user on every device, for security reasons.
- The user re-authenticates: some actions require that the user's credentials are recently issued; such actions include deleting an account, setting a primary email address, and changing a password. Instead of signing out the user and then signing in the user again, get new credentials from the user, and pass the new credentials to the reauthenticate method of the User object.
When you perform authentication with Firebase, there are three kinds of auth tokens you might encounter:
|Firebase ID tokens||Created by Firebase when a user signs in to an app. These tokens are signed JWTs that securely identify a user in a Firebase project. These tokens contain basic profile information for a user, including the user's ID string, which is unique to the Firebase project. Because the integrity of ID tokens can be verified, you can send them to a backend server to identify the currently signed-in user.|
|Identity provider tokens||Created by federated identity providers, such as Google and Facebook. These tokens can have different formats, but are often OAuth 2.0 access tokens. Apps use these tokens to verify that users have successfully authenticated with the identity provider, and then convert them into credentials usable by Firebase services.|
|Firebase custom tokens||Created by your custom auth system to allow users to sign in to an app using your auth system. Custom tokens are JWTs signed using a service account's private key. Apps use these tokens much like they use the tokens returned from federated identity providers.|