不确定在 Flutter 或 Dart 中 await 关键字需要应用于哪个语句

Question

我在 await 关键字下方看到绿色的漩涡线。

当我将鼠标悬停在绿色漩涡线上时，它显示 'await' applied to 'Query', which is not a 'Future'。

我不确定 await 关键字需要应用于哪个语句。

如果不将 await/async 和 Future 应用于方法，调用方法不会等待此方法完成并继续执行其余代码，因此返回空值。

请在下面找到有问题的代码：

Future<List<ContData>> readCC(String tID) async {
    List<ContData> conList = [];
    try {
      final conCollection = await FirebaseFirestore.instance
          .collection('CC')
          .where('conID', isEqualTo: tID);

      await conCollection.snapshots().listen((snapshot) {
        var tc = snapshot.docs[0].data()['con'];
        ContData con;
        for (var t in tc) {
          con = ContData.fromMap(data: t);
          print(con);
          final conType = con.conType;
          final conText = con.conText;
          final conCodeCaption = con.conCodeCaption;
          final conCodeText = con.conCodeText;
          final conCodeOutput = con.conCodeOutput;
          final conImageFilename = con.conImageFilename;
          final conImageCaption = con.conImageCaption;
          final conImageCredit = con.conImageCredit;
          final conImageCreditLink = con.conImageCreditLink;
          final conAnchorText = con.conAnchorText;
          final conAnchorLink = con.conAnchorLink;
          final ContData = ContData(
              conType: conType,
              conText: conText,
              conCodeCaption: conCodeCaption,
              conCodeText: conCodeText,
              conCodeOutput: conCodeOutput,
              conImageFilename: conImageFilename,
              conImageCaption: conImageCaption,
              conImageCredit: conImageCredit,
              conImageCreditLink: conImageCreditLink,
              conAnchorText: conAnchorText,
              conAnchorLink: conAnchorLink);
        }
      });
    } catch (e) {
      print(e.toString());
    }
    return conList;
  }

非常感谢您的宝贵时间和提前帮助！

Answer 1

您只能将await添加到returns一个Future的方法中，例如：

final conCollection = await FirebaseFirestore.instance
          .collection('CC')
          .where('conID', isEqualTo: tID).get();

方法 get() returns a Future<QuerySnapshot> 因此，而不是使用 then() ，它接受一个回调，它将在未来完成时被调用。

阅读以下指南：

https://dart.dev/codelabs/async-await

不确定在 Flutter 或 Dart 中 await 关键字需要应用于哪个语句

Not sure to which statement the await keyword needs to be applied in Flutter or Dart

dart

firebase

flutter

google-cloud-firestore

flutter-futurebuilder