Flutter - Receive and then modify data from Stream

Question

I'm attempting to do the following:

Listen to a Firestore stream so when a new document is added, the StreamBuilder will receive it, modify it, and then present it.

The "modification" takes the Stream data, which includes a Firestore UID, gets the data from Firestore with that UID, and then the StreamBuilder is populated with that data.

So the flow is: New document added -> Stream gets document -> Function gets UID from that document -> Function uses that UID to get more data from Firestore -> Function returns to populate StreamBuilder with that new data.

My current set-up is as follows -- which works, but the FutureBuilder is obviously making the Firestore call each time the widget is rebuilt, and nobody wants that.


  Stream<QuerySnapshot> upperStream;

  void initState() {
    super.initState();
    upperStream = aStream();
}

  Stream<QuerySnapshot> aStream() {
    return Firestore.instance
        .collection('FirstLevel')
        .document(/*ownUID (not related to stream)*/)
        .collection('SecondLevel')
        .snapshots();
  }

  Future<List> processStream(List streamData) async {
    List futureData = List();

    for (var doc in streamData) {
      Map<String, dynamic> dataToReturn = Map<String, dynamic>();
      DocumentSnapshot userDoc = await Firestore.instance
          .collection('FirstLevel')
          .document(/*OTHER USER'S UID FROM STREAM*/)
          .get();

      dataToReturn['i'] = userDoc['i'];

      futureData.add(dataToReturn);
    }
    return futureData;
  }
...
...
//The actual widget
        Expanded(
          child: StreamBuilder(
              stream: upperStream,
              builder: (context, snapshot) {
                // Error/null handling
                return FutureBuilder(
                    future: processStream(snapshot.data.documents),
                    builder: (context, futureSnap) {
                     // Error/null handling
                      return ListView.builder(
                          shrinkWrap: true,
                          itemCount: futureSnap.data.length,
                          scrollDirection: Axis.vertical,
                          itemBuilder: (context, index) {
                           //Continuing with populating
                          });
                    });
              }),
        ),

What's the best way to handle a flow like this? Creating a method where the data from the Firestore stream is modified and then returned without needing ListView.builder at all?

Edit: I tried creating my own stream like this:

  Stream<Map<String, dynamic>> aStream2() async* {

    QuerySnapshot snap = await Firestore.instance
        .collection(FirstLevel)
        .document(/*OWN UID*/)
        .collection(SecondLevel)
        .getDocuments();

    for (var doc in snap.documents) {
      Map<String, dynamic> data = Map<String, dynamic>();
      DocumentSnapshot userDoc = await Firestore.instance
          .collection(FirstLevel)
          .document(/*OTHER USER'S UID RECEIVED FROM STREAM*/)
          .get();

      data['i'] = userDoc['i'];

      yield data;
    }
  }

However, the Stream is not triggered/updated when a new Document is added to the SecondLevel collection.

You can do this by creating a stream from scratch with async* and await for. You can read more about this here. — Christopher Moore
– Christopher Moore, Commented Jul 24, 2020 at 16:03
Thanks. I did try to change the stream to a custom and yield the modified result for the StreamBuilder, but I ran into issues with it not being triggered when a document was added to the Firestore collection (since the StreamBuilder was being fed by a Stream of List being returned from the custom stream, not from Firestore itself like in the example I provided). Am I looking at this flow wrong? — Sludge
– Sludge, Commented Jul 24, 2020 at 16:14

Sludge · Accepted Answer · 2020-07-24 21:31:19Z

Alright I think I found the path to the solution. I get the data from the stream, modify it, and then yield it to the StreamBuilder within one method and no longer need the FutureBuilder. The key to this, as Christopher Moore mentioned in the comment, is await for. The stream method looks like this:

  Stream<List> aStream() async* {

    List dataToReturn = List();

    Stream<QuerySnapshot> stream = Firestore.instance
        .collection(LevelOne)
        .document(OWN UID)
        .collection(LevelTwo)
        .snapshots();

    await for (QuerySnapshot q in stream){
      for (var doc in q.documents) {
        Map<String, dynamic> dataMap= Map<String, dynamic>();

        DocumentSnapshot userDoc = await Firestore.instance
            .collection('UserData')
            .document(doc['other user data var'])
            .get();

        dataMap['i'] = userDoc['i'];
        //...//
        dataToReturn.add(dataMap);
      }
      yield dataToReturn;
    }
  }

And then the StreamBuilder is populated with the modified data as I desired.

Emmett Deen · Accepted Answer · 2020-12-14 16:11:38Z

2

I found myself using this to implement a chat system using the Dash Chat package in my app. I think using the map function on a stream may be a little cleaner here is a sample:

Stream<List<ChatMessage>> getMessagesForConnection(
  String connectionId) {
return _db
    .collection('connections')
    .doc(connectionId)
    .collection('messages')
    .snapshots()
    .map<List<ChatMessage>>((event) {
  List<ChatMessage> messages = [];

  for (var doc in event.docs) {
    try {
      messages.add(ChatMessage.fromJson(doc.data()));
    } catch (e, stacktrace) {
      // do something with the error
    }
  }
  return messages;
});}

answered Dec 14, 2020 at 16:11

Emmett Deen

7516 silver badges18 bronze badges

Collectives™ on Stack Overflow

Flutter - Receive and then modify data from Stream

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related