Using a Firebase Stream as an input for another Stream in Flutter?

Question

Context: I've got two Firebase Streams that work correctly, and they fetch i) a list of user profiles ('users' collection), and ii) a list of locations belonging to each user profile ('locations' collection), and then map them over to a custom User and Location model.

Users stream:

class DatabaseService {

final String uid;
final String friendUid;
final String locationId;
DatabaseService({ this.uid, this.locationId, this.friendUid });

// collection reference for users
final CollectionReference userCollection = FirebaseFirestore.instance.collection('users');

// get users stream
Stream<List<CustomUserModel>> get users {

final FirebaseAuth auth = FirebaseAuth.instance;
final User user = auth.currentUser;
final uid = user.uid;

List<CustomUserModel> userList = [];

List<CustomUserModel> _streamMapper(DocumentSnapshot snapshot) {
  CustomUserModel individualUser = CustomUserModel(
    uid: snapshot.id,
    name: snapshot.data()['name'],
    username: snapshot.data()['username'],
    email: snapshot.data()['email'],
  );
  userList.add(individualUser);
  return userList;
}

  return userCollection.doc(uid).snapshots().map(_streamMapper);
}

and the Location Stream:

  // collection reference for location
   final CollectionReference locationCollection = 
   FirebaseFirestore.instance.collection('locations');

  Stream<List<Location>> get locations {

  final FirebaseAuth auth = FirebaseAuth.instance;
  final User user = auth.currentUser;
  final uid = user.uid;

  List<Location> _locationListFromSnapshot(QuerySnapshot snapshot) {
   List<Location> locationList = [];
    snapshot.docs.forEach((element) {
     Location individualLocation = Location(
       locationId: element.id,
       locationName: element.data()['locationName'],
       city: element.data()['city'],
     );
    locationList.add(individualLocation);
  });
   return locationList;
 }

  return userLocationCollection.doc(uid).collection('locations').snapshots()
   .map(_locationListFromSnapshot);
 }

What I want to do is to generate a custom Stream which outputs all the locations for all users - in other words to use the users stream as an input for the locations stream.

I'm not sure what approach works here - I considered adding the users stream as an input parameter to the locations stream and then creating a for-loop, something like this:

Stream<List<Location>> allLocations(Stream<List<CustomUserModel>> users) {

final FirebaseAuth auth = FirebaseAuth.instance;
final User user = auth.currentUser;
final uid = user.uid;

List<Location> locationList = [];

users.forEach((element) {

// append user's locations to empty list
locationList.add(locationCollection.doc(element.first.uid).collection('locations')
.snapshots().map(SOME FUNCTION TO MAP A DOCUMENT SNAPSHOT TO THE CUSTOM LOCATION MODEL)

}

return locationList;

but of course I get an error as this returns a list, not a stream. So I've no idea how to proceed...

Answer 1

I hear your pain. I have been there. You were pretty close. Let me explain how I like to do it.

First of all, some clean up:

It seemed like you were not using these in the allLocations functions, so I deleted them

final FirebaseAuth auth = FirebaseAuth.instance;
final User user = auth.currentUser;
final uid = user.uid;

Second, I changed the return type of the function from Stream<List<Location>> to Stream<Map<String, List<Location>> where the key of the map would be the userId. I find this type useful, because you don't have to worry about the order of users being in sync with the stream.

Third, when you are creating streams, you cannot return, but have to yield from a function. You also have to mark the function async* (* is not a typo).

With this, I propose you use something like this for your allLocations function:

class DataService {
  List<Location> convertToLocations(QuerySnapshot snap) {
    // This is the function to convert QuerySnapshot into List<Location>
    return [Location()];
  }

  Stream<Map<String, List<Location>>> allLocations(
      Stream<List<CustomUserModel>> usersStream) async* {
    Map<String, List<Location>> locationsMap = {};

    await for (List<CustomUserModel> users in usersStream) {
      for (CustomUserModel user in users) {
        final Stream<List<Location>> locationsStream = locationCollection
            .doc(user.uid)
            .collection('locations')
            .snapshots()
            .map(convertToLocations);
        await for (List<Location> locations in locationsStream) {
          locationsMap[user.uid] = locations;
          yield locationsMap;
        }
      }
    }
  }
}

I hope you like this method. Please let me know if something is not what you want. I can make adjustments.