flutter firestore查询listen-无法引用listen中的变量

我有一个查询来查找指定条形码的文档ID,如下所示:

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,我想引用函数之外的idofbarcodevalue。我试图找到一种方法将其传递给另一个函数以传递到simpledialog。

现在,函数之外的任何东西都无法识别它。它真的起作用了,指纹验证了。

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');
        }
      }