search - Flutter Firestore Query Listen - 无法引用 Listen 中的变量

Question

我有一个查询来查找指定条形码的文档 ID:

Future findBarcode() async {
  String searchBarcode = await BarcodeScanner.scan();

Firestore.instance.collection('${newUser.userLocation}').where('barcode', isEqualTo: '${searchBarcode}').snapshots().listen(
(data) { 
  String idOfBarcodeValue = data.documents[0].documentID;
  print(idOfBarcodeValue);
     }
  );  
}    

但是，我想在函数外部引用 idOfBarcodeValue。我正在尝试找到一种方法将其传递给另一个函数以传递给 SimpleDialog。

目前，函数之外的任何东西都无法识别它。它确实有效，打印验证。

下面是正在执行的扫描功能:

Future scan() async {
   try {
    String barcode = await BarcodeScanner.scan();
    setState(() => this.barcode = barcode);
        } on PlatformException catch (e) {
          if (e.code == BarcodeScanner.CameraAccessDenied) {
            setState(() {
              this.barcode = 'The user did not grant the camera permission!';
            });
          } else {
            setState(() => this.barcode = 'Unknown error: $e');
          }
        } on FormatException{
          setState(() => this.barcode = 'null (User returned using the "back"-button before scanning anything. Result)');
        } catch (e) {
          setState(() => this.barcode = 'Unknown error: $e');
        }
      }

Answer 1

snapshots()返回 Stream<Query> .以与您相同的方式 await返回 Future 的函数, 你可以 await for Stream 的所有值(可以是零、一个或多个)产生，例如:

Future findBarcode() async {
  String searchBarcode = await BarcodeScanner.scan();

  String idOfBarcodeValue;
  Stream<Query> stream = Firestore.instance
      .collection('${newUser.userLocation}')
      .where('barcode', isEqualTo: '${searchBarcode}')
      .snapshots();
  await for (Query q in stream) {
    idOfBarcodeValue = q.documents[0].documentID;
  }

  print(idOfBarcodeValue);
  // if the stream had no results, this will be null
  // if the stream has one or more results, this will be the last result
  return idOfBarcodeValue;
}