Dart/Flutter - “yield” inside a callback function

问题

I need to yield a list for a function; however, I want to yield the list from within a callback function, which itself is inside the main function - this results in the yield statement not executing for the main function, but rather for the callback function.

My problem is very similar to the problem that was solved here: Dart Component: How to return result of asynchronous callback? but I cannot use a Completer because I need to yield and not return.

The code below should describe the problem better:

Stream<List<EventModel>> fetchEvents() async* { //function [1]
    Firestore.instance
        .collection('events')
        .getDocuments()
        .asStream()
        .listen((snapshot) async* { //function [2]
      List<EventModel> list = List();
      snapshot.documents.forEach((document) {
        list.add(EventModel.fromJson(document.data));
      });

      yield list; //This is where my problem lies - I need to yield for function [1] not [2]
    });
  }

回答1:

Instead of .listen which handles events inside another function you can use await for to handle events inside the outer function.

Separately - you might want to reconsider the pattern when you yield List instances that are still getting populated inside an inner stream callback...

Stream<List<EventModel>> fetchEvents() async* {
  final snapshots =
      Firestore.instance.collection('events').getDocuments().asStream();
  await for (final snapshot in snapshots) {
    // The `await .toList()` ensures the full list is ready
    // before yielding on the Stream
    final events = await snapshot.documents
        .map((document) => EventModel.fromJson(document.data))
        .toList();
    yield events;
  }
}

来源：https://stackoverflow.com/questions/55939917/dart-flutter-yield-inside-a-callback-function