问Flutter -无法检索Firebase集合中的文档列表

EN

StreamBuilder<QuerySnapshot>(
    stream: Firestore.instance
        .collection('collectionName')
        .snapshots(),
    builder: (context, snapshot) {
        print(
            "Number of documents: ${snapshot.data.documents.length}, 
            ${snapshot.data.documents.first.documentID}");
        return _DropdownWithLabel(
            'Label', 
             snapshot.data.documents.map((doc) => doc.documentID).toList());
    }) //StreamBuilder

class _DropdownWithLabel extends StatefulWidget {
  final String label;
  final List<String> dropdownItems;
  _DropdownWithLabel(this.label, this.dropdownItems);
  @override
  State<StatefulWidget> createState() =>
      _DropdownWithLabelState(label, dropdownItems);
}

class _DropdownWithLabelState extends State {
  final String label;
  final List<String> dropdownItems;
  String selectedItem;
  _DropdownWithLabelState(this.label, this.dropdownItems) {
    selectedItem = dropdownItems.first;
  }
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceBetween,
      children: <Widget>[
        Text(label),
        Container(
          width: 150,
          child: DropdownButton<String>(
              value: selectedItem,
              onChanged: (newValue) {
                setState(() => selectedItem = newValue);
              },
              items: dropdownItems.map((value) {
                return DropdownMenuItem(
                  value: value,
                  child: Text(value),
                );
              }).toList()),
        )
      ],
    );
  }
}