Flutter/Firestore - 错误：“列表”类型的值<string> Function(QuerySnapshot)' 不能分配给 'List' 类型的变量<dynamic> '</dynamic></string>

Question

I am trying to create a dynamic list view which displays document ids from Firebase. I have created the list of events from snapshot:

    List<String> _eventListFromSnapshot(QuerySnapshot snapshot) {
  return snapshot.documents.map((doc) {
    return doc.documentID == null ? doc.documentID : ' ';
  }).toList();
}

But when I come to build the stream using this code:

    Stream<List<String>> get events {
  return DatabaseService().eventsCollection.snapshots().map((event) => _eventListFromSnapshot);
}

I get the following error:

Error: A value of type 'List<String> Function(QuerySnapshot)' can't be assigned to a variable of type 'List<String>'.

Answer 1

You forgot calling function _eventListFromSnapshot . Fix like this

Stream<List<String>> get events {
  return DatabaseService()
    .eventsCollection
    .snapshots()
    .map((snapshot) => _eventListFromSnapshot(snapshot));
}

or simpler:

Stream<List<String>> get events {
  return DatabaseService()
    .eventsCollection
    .snapshots().map(_eventListFromSnapshot);
}

Answer 2

I think we have to do it explicitly.
Try something like this in your method.

List<String> _eventListFromSnapshot(QuerySnapshot snapshot) {
    List<String> docIDs = [];
    for(int i = 0; i < snapshot.documents.length; i++){
        String value = snapshot.documents[i].documentID ?? " "; 
        docIDs.add(value);
    }
    return docIDs;
}

or else, we might have an issue in this line,

Stream<List<String>> get events {
  return DatabaseService().eventsCollection.snapshots().map((event) => _eventListFromSnapshot(event));
}

We have to call the function, instead of assigning it maybe.

Hope this solves your issue.