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“对象？”类型的值无法分配给“Map？”类型的变量。尝试更改变量的类型，或将右侧类型转换为“Map” “对象？”类型的值无法分配给“Map？”类型的变量。尝试更改变量的类型，或者将右侧类型转换为“Map？”。我想将firebase数据库中的点转换为x点来制作折线图。 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); }); } }); } 那么如何使其成为可能呢？在此之前我这样做但仍然出现“foreach”错误 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); }); } }); } 您可以通过强制转换为 Map<dynamic,dynamic>? 来避免此问题 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 as Map<dynamic, dynamic>?; // Explicit cast if (values != null) { // you shpuld check is it null or not null 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); }); } } }); }

flutter firebase dart foreach linegraph

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