How to add items to a StreamController in flutter

Question

I want to return a list of members via a StreamController.

batches collection contains batch details and ids of members assigned to the batch.

So, in-order to get the list of members in a batch, have to loop through batch collection and get the ids of members, then match with members collection and return the matching member data as stream.

  final CollectionReference _batchCollectionReference =
      Firestore.instance.collection('batches');
  final CollectionReference _membersCollectionReference =
      Firestore.instance.collection('members');

  final StreamController<List<Member>> _membersController =
      StreamController<List<Member>>.broadcast();

  Stream getMembers(String batchId) { //passing a batch id
    _batchCollectionReference
        .document(batchId)
        .snapshots()
        .map((batchSnapshot) => Batch.fromData( //return as Batch type
            data: batchSnapshot.data, batchId: batchSnapshot.documentID))
        .listen((snapshot) {

      List<String> members = snapshot.members; //list of members

      members.forEach((member) {
        var data = _membersCollectionReference
            .document(member)
            .snapshots()
            .map((memberData) => Member.fromData(data: memberData.data)); //return as Member type

        _membersController.add(data);
      });
    });

    return _membersController.stream;
  }
}

The problem is I couldn't able to push the member data to the StreamContoller.

It says,

The argument type 'Stream<Member>' can't be assigned to the parameter type 'List<Member>'

The stream should contains instance of members; Ex: [[instance of 'Member'], [instance of 'Member'], [instance of 'Member']]

If I got the data like this way, it would be easy to loop and do the other stuff.

I couldn't able fix this issue. Any help would be appreciated.

you dont need StreamContoller (and Stream.listen()) at all: just use Stream API like map / asyncMAp / expand (or the most generic transform) etc — pskink
– pskink, Commented May 19, 2020 at 16:15
can you provide any examples? or do you have any video tutorial? — Thanooshan
– Thanooshan, Commented May 19, 2020 at 16:19

Luis Miguel Mantilla · Accepted Answer · 2020-05-19 15:49:47Z

1

Firstable when you need to add a list to the stream so convert your map data to a list, just adding toList() at the end of you map as follows:

members.forEach((member) {
        var data = _membersCollectionReference
            .document(member)
            .snapshots()
            .map((memberData) => Member.fromData(data: memberData.data)).toList();

And to push the data in the Stream, you need to use sink.add() this can be an example of a function to push data into the stream and the other one to get the values:

final StreamController<List<Member>> _membersController = StreamController<List<Member>>.broadcast();

  /// Inputs
  Function(List<Member>) get changeMembers => _membersController.sink.add;

  /// Getters
  String get members => _membersController.value;

In your case you can do it directly in this way:

_membersController.sink.add(data);

Hope it helps, for more info please check this video or the documentation about streams in dart.

edited May 19, 2020 at 15:49

answered May 19, 2020 at 15:44

Luis Miguel Mantilla

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3 Comments

Thanooshan Over a year ago

Thanks for the quick response. I updated my code as per your suggestion. But now I'm getting this error . The argument type 'Stream<Member>' can't be assigned to the parameter type 'List<Member>'. What should I do?

Thanooshan Over a year ago

I updated the StreamController<List<Member>> to StreamController<Stream<Member>> then it worked. Thanks dude.

Zaid Soomro Over a year ago

You can also return stream from the StreamController. StreamController<Stream<List<Member>>>.stream

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