飞镖/颤振-“产量”在回调函数中 [英] Dart/Flutter - "yield" inside a callback function

问题描述

我需要产生一个函数列表；但是，我想从回调函数中产生列表，该回调函数本身在主函数内部-这导致yield语句不是针对主函数执行，而是针对回调函数执行。

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.

我的问题与此处解决的问题非常相似： Dart组件：如何返回异步回调的结果？，但是我不能使用Completer，因为我需要屈服而不返回。

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.

以下代码应该更好地描述问题：

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]
    });
  }

推荐答案

而不是 .listen 处理另一个函数内部的事件，您可以使用 await for 处理外部函数内部的事件。

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

另外-当生成 List 实例仍在内部流回调中填充时，您可能想重新考虑该模式...

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;
  }
}

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