Firebase数据转Map类型错误：如何获取数据生成折线图？

从Firebase数据库获取数据生成折线图时，遇到类型转换错误：

'A value of type 'Object?' can't be assigned to a variable of type 'Map<dynamic, dynamic>?'

系统提示可修改变量类型，或将右侧强制转换为Map<dynamic, dynamic>?。尝试两段代码均未解决：

• 第一段给Map<dynamic, dynamic>?类型变量赋值databaseEvent.snapshot.value时触发错误；
• 第二段直接调用dataSnapshot.value!.forEach也报错。

尝试代码1 #

void fetchDataFromFirebase() {
  final databaseReference = FirebaseDatabase.instance
      .ref('1TJ5Srs0JsEvXaiehwPgCkJ8Ywqsoj5ERRCVcCFhGoDA/swh');

  databaseReference.once().then((DatabaseEvent databaseEvent) {
    if (databaseEvent.snapshot.value != null) {
      Map<dynamic, dynamic>? values = databaseEvent.snapshot.value;
      List<DataPoint> newDataPoints = [];

      values.forEach((key, value) {
        if (value['Num'] != null && value['Swh'] != null) {
          int? num = value['Num'] as int?;
          double? swh = value['Swh'] as double?;

          if (num != null && swh != null && num >= 1 && num <= 10) {
            newDataPoints.add(DataPoint(num.toDouble(), swh));
          }
        }
      });

      setState(() {
        dataPoints.clear();
        dataPoints.addAll(newDataPoints);
      });
    }
  });
}

尝试代码2 #

void fetchDataFromFirebase() {
  final databaseReference = FirebaseDatabase.instance
      .ref('1TJ5Srs0JsEvXaiehwPgCkJ8Ywqsoj5ERRCVcCFhGoDA/swh');

  databaseReference.once().then((DataSnapshot dataSnapshot) {
    if (dataSnapshot.value != null) {
      List<DataPoint> newDataPoints = [];

      dataSnapshot.value!.forEach((key, value) {
        if (value['Num'] != null && value['Swh'] != null) {
          int? num = value['Num'] as int?;
          double? swh = value['Swh'] as double?;

          if (num != null && swh != null && num >= 1 && num <= 10) {
            newDataPoints.add(DataPoint(num.toDouble(), swh));
          }
        }
      });

      setState(() {
        dataPoints.clear();
        dataPoints.addAll(newDataPoints);
      });
    }
  });
}

问题核心是Firebase返回的value类型为Object?，需显式做安全类型转换，同时增加类型检查避免运行时错误。修改后的代码如下：

void fetchDataFromFirebase() {
  final databaseReference = FirebaseDatabase.instance
      .ref('1TJ5Srs0JsEvXaiehwPgCkJ8Ywqsoj5ERRCVcCFhGoDA/swh');

  databaseReference.once().then((DatabaseEvent databaseEvent) {
    final snapshotValue = databaseEvent.snapshot.value;
    // 先判断类型再转换，避免强制转换失败
    if (snapshotValue is Map<dynamic, dynamic>) {
      List<DataPoint> newDataPoints = [];

      snapshotValue.forEach((key, value) {
        // 嵌套数据同样做类型检查，防止后续取值报错
        if (value is Map<dynamic, dynamic>) {
          final num = value['Num'] as int?;
          final swh = value['Swh'] as double?;

          if (num != null && swh != null && num >= 1 && num <= 10) {
            newDataPoints.add(DataPoint(num.toDouble(), swh));
          }
        }
      });

      setState(() {
        dataPoints.clear();
        dataPoints.addAll(newDataPoints);
      });
    }
  });
}

关键修改说明： #

1. 类型安全检查：用is关键字先确认snapshotValue是Map<dynamic, dynamic>类型，再进行后续操作，避免强制转换的风险；
2. 嵌套数据校验：遍历内部value时，同样检查是否为Map类型，防止取['Num']和['Swh']时出现类型错误；
3. 修正API调用错误：第二段代码的参数类型错误，databaseReference.once()返回的是Future<DatabaseEvent>，而非Future<DataSnapshot>，这也是其报错的原因之一。

内容的提问来源于stack exchange，提问作者Hasanmeors