Flutter从Firebase取数据报错NoSuchMethodError，求解决方案

问题说明 #

从Firebase服务器拉取商品数据时，登录阶段出现错误：Unhandled Exception: NoSuchMethodError: The method '[]' was called on null。此前通过以下代码处理空值问题，但删除服务器上的用户收藏数据后错误复现，推测与空安全逻辑有关但无法自行修复：

bool? _isFavorite =
    favoriteData[key] != null || favoriteData[key] == 'true'
        ? true
        : false;

相关代码 #

Fetch Data 函数 #

Future<void> fetchAndSetProducts([bool filterByUser = false]) async {
  final filterString =
      filterByUser ? 'orderBy="creatorId"&equalTo="$userId"' : '';
  var url =
      'https://ecommerce-test-753ad-default-rtdb.firebaseio.com/products.json?auth=$authToken&$filterString';
  try {
    final response = await http.get(Uri.parse(url));
    final extractedData = json.decode(response.body) as Map<String, dynamic>;
    if (extractedData == {}) {
      return;
    }
    url =
        'https://ecommerce-test-753ad-default-rtdb.firebaseio.com/userFavorites/$userId.json?auth=$authToken';
    final favoriteResponse = await http.get(Uri.parse(url));
    final favoriteData = json.decode(favoriteResponse.body);
    print(favoriteData);
    print(extractedData);
    final List<Product> loadedProduct = [];
    extractedData.forEach((key, value) {
      bool? _isFavorite =
          favoriteData[key] != null || favoriteData[key] == 'true'
              ? true
              : false;
      loadedProduct.add(
        Product(
          id: key,
          title: value['title'],
          price: value['price'],
          imageUrl: value['imageUrl'],
          description: value['description'],
          isFavorite: _isFavorite,
        ),
      );
    });
    _productItems = loadedProduct;
    notifyListeners();
  } catch (e) {
    rethrow;
  }
}

Product Model 类 #

class Product with ChangeNotifier {
  final String id;
  final String title;
  final double price;
  final String imageUrl;
  final String description;
  bool isFavorite;

  Product({
    required this.id,
    required this.title,
    required this.price,
    required this.imageUrl,
    required this.description,
    this.isFavorite = false,
  });

  void _setFavValue(bool newValue) {
    isFavorite = newValue;
    notifyListeners();
  }

  Future<void> toggleFavoriteStatus(String authToken, String userId) async {
    final oldStatus = isFavorite;
    isFavorite = !isFavorite;
    notifyListeners();
    final url =
        'https://ecommerce-test-753ad-default-rtdb.firebaseio.com/userFavorites/$userId/$id.json?auth=$authToken';
    try {
      final response = await http.put(
        Uri.parse(url),
        body: json.encode(isFavorite),
      );
      if (response.statusCode >= 400) {
        _setFavValue(oldStatus);
      }
    } catch (error) {
      _setFavValue(oldStatus);
    }
  }
}

Firebase 规则配置 #

"products":{
  ".indexOn":["creatorId"]
}

解决方案 #

错误核心原因：当服务器上的用户收藏数据被删除后，favoriteData本身会变为null，此时直接调用favoriteData[key]就会触发[]方法调用在null对象上的错误。同时原代码的逻辑判断存在漏洞（用||会导致favoriteData[key]为false时仍返回true）。

修复方案1：先判断favoriteData是否为null #

修改_isFavorite的赋值逻辑：

bool _isFavorite = false;
if (favoriteData != null) {
  final favValue = favoriteData[key];
  _isFavorite = favValue == true || favValue == 'true';
}

修复方案2：使用Dart空安全语法简化判断 #

bool _isFavorite = (favoriteData?[key] == true || favoriteData?[key] == 'true') ?? false;

额外优化：提前处理favoriteData的null情况 #

在解析收藏数据时，直接给null情况赋值为空Map，避免后续所有判断都要处理null：

final favoriteData = json.decode(favoriteResponse.body) ?? <String, dynamic>{};

之后直接调用favoriteData[key]只会返回null，不会触发方法调用错误，再按正常逻辑判断即可。

内容的提问来源于stack exchange，提问作者Muhammad Tawil