升级Cloud Firestore依赖后出现Object类型未定义[]操作符错误如何解决

cloud_firestore 0.14.0及之后的版本引入了类型安全变更，DocumentSnapshot.data()和QueryDocumentSnapshot.data()方法的返回值从原来的Map<String, dynamic>调整为Object?类型，Dart无法直接对Object类型使用[]下标运算符访问属性，因此触发报错。

最简单的修复方式是显式将data()返回的结果强制转换为Map<String, dynamic>类型，各文件修改后的代码如下：

model.dart 修复代码 #

Product.fromSnapshot(DocumentSnapshot snapshot){
  // 先将返回数据转换为Map类型
  final data = snapshot.data() as Map<String, dynamic>;
  _featured = data[FEATURED];
  _brand = data[BRAND];
  _category = data[CATEGORY];
}

update.dart 修复代码 #

FutureBuilder<QuerySnapshot>(
  future: FirebaseFirestore.instance
      .collection("items")
      .where("shortInfo", whereIn: (snapshots.data.docs[index].data() as Map<String, dynamic>)[EcommerceApp.productID])
      .get(),
  builder: (c,snap)
  {
    if(!snap.hasData) return Center(child: circularProgress());
    // 提前转换当前订单数据为Map，避免重复强转
    final orderData = snapshots.data.docs[index].data() as Map<String, dynamic>;
    return AdminOrderCard(
      itemCount: snap.data!.docs.length,
      data:snap.data!.docs,
      orderId: snapshots.data.docs[index].id,
      orderBy: orderData["orderBy"],
      addressID: orderData["addressID"],
    );
  },
);

List<DropdownMenuItem<String>> getCategoriesDropdown(){
  List<DropdownMenuItem<String>> items = [];
  for(int i = 0; i < categories.length; i++){
    setState(() {
      print(i);
      final catData = categories[i].data() as Map<String, dynamic>;
      items.insert(0, DropdownMenuItem(child: Text(catData['category']),
          value: catData['category']));
    });
  }
  return items;
}

_currentCategory = (categories[0].data() as Map<String, dynamic>)['category'];

pro.dart 修复代码 #

Future<void> getMobileandTabIconData() async {
  List<CategoryIcon> newList = [];
  QuerySnapshot dressSnapShot = await FirebaseFirestore.instance
      .collection("categoryicon")
      .doc("MhZyArOf2zrOmPO7R3y6")
      .collection("mobile&tablets")
      .get();
  for (var element in dressSnapShot.docs) {
    final iconData = element.data() as Map<String, dynamic>;
    mobileandtabiconData = CategoryIcon(image: iconData["image"]);
    newList.add(mobileandtabiconData);
  }
  mobileandtabIcon = newList;
  notifyListeners();
}

如果不想每次调用data()都手动强转，可以在查询时通过withConverter提前指定数据类型，示例如下：

final productRef = FirebaseFirestore.instance
    .collection("products")
    .withConverter<Map<String, dynamic>>(
      fromFirestore: (snapshot, _) => snapshot.data()!,
      toFirestore: (data, _) => data,
    );

后续通过这个引用查询得到的文档，data()返回值直接为Map<String, dynamic>类型，无需再次强转。

如果担心数据为空或类型不匹配，可以用as?替代as并配合默认值避免崩溃，示例：

final data = snapshot.data() as Map<String, dynamic>?;
_featured = data?[FEATURED] ?? false;

内容的提问来源于stack exchange，提问作者Aakib Algohar