为什么在 Flutter wih Firestore 中使用 foreach 方法后得到 null？

Question

I was trying to get a user document and its sub-collection, simultaneously. But I found that it is not possible within a query. So I have decided to approach the problem from another perspective.

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What do we have?

I have a getRelative() getter and it returns Stream<Relative> .

  Stream<Relative> get getRelative async* {
    List<Patient> patientList;

    patientList = await getPatientCollection
        .forEach((element) => element.patients)
        .then((value) => value);

      yield* usersCollection
          .document(_userEmail)
          .snapshots()
          .map((snapshot) => _relativeDataFromSnapshot(snapshot, patientList));
    
  }

getPatientCollection() is another getter method and it returns Stream<Pat.netList> . I am not gonna give detail of the getter. Because it works fine.

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What is the problem?

The problem is when I get Stream<PatientList> , I am getting a Stream . So, just to convert it to Future , I have used forEach method. And it also works fine. I can see element.patients is not empty. I can see inside of it and its return type List<Patient> as it should be.

But when I was debugging it, after forEach() has done its job, I have realized that it return null. And patientList become null. That causes a problem.

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What am I asking?

1. What does it cause this problem? And how can I solve it?
2. Is there any better way to get subcollection without using another Stream to get patient subcollection ( getPatientCollection )?

Answer 1

Stream.forEach is the Stream equivalent of List.forEach (which returns void ). It's for doing some operation on each element, not for returning a new collection. If you want to convert a Stream<T> to a List<T> , you can doawait stream.toList() .