I use Flutter and Firebase, just ask myself how to allow users to add items to cart when they're not logged in yet and keep the cart when they logging in, anyone have an idea for this ?

Solution 1: Frank van Puffelen

On Firebase you'd typically start the user off with an anonymous authentication account. With that sort of account, Firebase generates a user ID (UID) for the user, without them having to enter any credentials. You then associate the cart/items with the UID, and then when you/they are ready to identify themselves, you can sign them in with another provider, and link that to the anonymous account.

I've provided links to the documentation for Android developers above, but the same functionality is available in the FlutterFire libraries too. For example, anonymous sign-in on Flutter is as simple as:

UserCredential userCredential = await FirebaseAuth.instance.signInAnonymously();

Solution 2: abdalla ebrahim

you can store this data in a separate database SQL lite as an example