Series 1 of 2 — Part 3 of 5

Building a Flutter Shopping App — Part 3: Cloud Firestore and Push Notifications

Series: Building a Flutter Shopping App — From Zero to Architecture

By this point the app had a working UI, Riverpod state management, and Firebase Auth. The next step was connecting it to a real backend — Firestore for user data persistence and Firebase Cloud Messaging for push notifications.

Integrating Cloud Firestore

I added cloud_firestore to pubspec.yaml and updated the plugin registrations. I used Firestore to persist user profile data and cart items server-side. The data structure was a top-level users collection, keyed by Firebase Auth uid, with a cart subcollection.

await FirebaseFirestore.instance
  .collection('users')
  .doc(currentUser.uid)
  .collection('cart')
  .doc(product.id)
  .set(cartItem.toJson());

One thing that caught me out was plugin registration. After adding Firestore and other Firebase plugins, I had to make sure they were all properly registered for both platforms. Missing a registration does not always throw an obvious error — sometimes it silently fails to initialise.

Setting up push notifications

I integrated both Firebase Cloud Messaging and flutter_local_notifications. FCM handles server-sent notifications. flutter_local_notifications handles displaying them when the app is in the foreground, since FCM does not show a notification UI when the app is active.

There are three scenarios to handle separately: app in foreground, app in background, and app terminated. Each has a different handler.

FirebaseMessaging.onMessage.listen((RemoteMessage message) {
  // App foregrounded — show local notification
});

FirebaseMessaging.onMessageOpenedApp.listen((RemoteMessage message) {
  // App backgrounded — user tapped notification
});

Saving and refreshing the FCM token

For targeted notifications you need the device's FCM token saved in Firestore. I fetched and saved it after every login, and set up a token refresh listener because FCM tokens can change. If you do not update the stored token when it refreshes, notifications will eventually stop delivering.

What I learned

• Verify plugin registration: Register all Firebase plugins explicitly — silent failures are harder to debug than crashes.
• Handle all notification states: Push notifications have three separate states (foreground, background, terminated) and each needs its own handler.
• Token lifecycle: Save and refresh FCM tokens on every login, not just the first one.

Next: Part 4 — The Big Refactor: Migrating to MVVM with Repositories and ViewModels