Stop listening to snapshot updates in cloud firestore in flutter

The listen method returns a Subscription

This class is used to cancel the listening.

You should store inside your state that object to cancel subscription on dispose.

Your listener is of type StreamSubscription, so you can call some helpful methods on your listener such as cancel()

    CollectionReference reference = Firestore.instance.collection('Events');
StreamSubscription<QuerySnapshot> streamSub = reference.snapshots().listen((querySnapshot) {
  querySnapshot.documentChanges.forEach((change) {
    // Do something with change
  });
});
//somewhere
streamSub.cancel();

Very late answer, but I thought I'd complete the previous answers with a code sample as it might be useful to others.

class EventsScreen extends StatefulWidget {
  EventsScreen({Key key}) : super(key: key);

  @override
  _EventsScreenState createState() => _EventsScreenState();
}

class _EventsScreenState extends State<EventsScreen> {
  StreamSubscription<QuerySnapshot> _eventsSubscription;

  @override
  void initState() {
    // Initialise your stream subscription once
    CollectionReference eventsReference = Firestore.instance.collection('Events');
    _eventsSubscription = eventsReference.snapshots().listen((snapshot) => _onEventsSnapshot);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    // Build your widget here
    return Container();
  }

  void _onEventsSnapshot(QuerySnapshot snapshot) {
    // Remove the setState() call if you don't want to refresh your screen whenever you get a fresh snapshot
    setState(() {
      snapshot?.documentChanges?.forEach(
        (docChange) => {
          // If you need to do something for each document change, do it here.
        },
      );
      // Anything you might do every time you get a fresh snapshot can be done here.
    });
  }

  @override
  void dispose() {
    // Cancel your subscription when the screen is disposed
    _eventsSubscription?.cancel();
    super.dispose();
  }
}

This approach leverages the StatefulWidget's state to handle documents changes.

A better approach would be to use a Provider or the BLoC pattern so that the subscription to your Firestore is not created and handled in the UI.