升级Flutter3后Map类型转换错误的解决方法咨询

将Flutter升级至3版本，同时把Firebase及数据集更新到最新版本后，控制台出现错误提示：

type '_Map<Object?, Object?>' is not a subtype of type 'Map<String?, dynamic>' in type cast

错误出现在代码行：

itemId = (event.snapshot.value as Chat).itemId;

相关代码如下：

Chat类代码 #

class Chat extends MyObject<Chat?> {
  Chat({
    this.itemId,
    this.receiverId,
    this.senderId,
  });

  String? itemId;
  String? receiverId;
  String? senderId;

  @override
  bool operator ==(dynamic other) => other is Chat && itemId == other.itemId;

  @override
  int get hashCode => hash2(itemId.hashCode, itemId.hashCode);

  @override
  String? getPrimaryKey() {
    return itemId;
  }

  @override
  List<Chat?> fromMapList(List<dynamic> dynamicDataList) {
    final List<Chat?> subCategoryList = <Chat?>[];
    for (dynamic dynamicData in dynamicDataList) {
      if (dynamicData != null) {
        subCategoryList.add(fromMap(dynamicData));
      }
    }
    return subCategoryList;
  }

  @override
  Chat? fromMap(dynamic dynamicData) {
    if (dynamicData != null) {
      return Chat(
        itemId: dynamicData['itemId'],
        receiverId: dynamicData['receiver_id'],
        senderId: dynamicData['sender_id'],
      );
    } else {
      return null;
    }
  }

  @override
  List<Map<String, dynamic>?> toMapList(List<Chat?> objectList) {
    final List<Map<String, dynamic>?> mapList = <Map<String, dynamic>?>[];
    for (Chat? data in objectList) {
      if (data != null) {
        mapList.add(toMap(data));
      }
    }
    return mapList;
  }

  @override
  Map<String, dynamic>? toMap(Chat? object) {
    if (object != null) {
      final Map<String, dynamic> data = <String, dynamic>{};
      data['itemId'] = object.itemId;
      data['receiver_id'] = object.receiverId;
      data['sender_id'] = object.senderId;
      return data;
    } else {
      return null;
    }
  }
}

MyObject抽象类代码 #

abstract class MyObject<T> {
  String? key = '0';

  String? getPrimaryKey();

  T fromMap(dynamic dynamicData);

  Map<String, dynamic>? toMap(T object);

  List<T> fromMapList(List<dynamic> dynamicDataList);

  List<Map<String, dynamic>?> toMapList(List<T> objectList);
}

核心问题 #

Firebase更新后，event.snapshot.value返回的类型从Map<String, dynamic>变为_Map<Object?, Object?>，直接将其强制转换为Chat类型本身逻辑错误——value是原始Map数据，必须通过Chat类的fromMap方法转换为对象，而非直接强转。

修复步骤 #

1. 修正出错的代码行
   替换错误的类型转换逻辑，先处理Map类型转换，再通过fromMap生成Chat对象：

   // 原错误代码
   // itemId = (event.snapshot.value as Chat).itemId;
   
   // 修正后代码
   final rawData = event.snapshot.value as Map<Object?, Object?>;
   final chat = Chat().fromMap(rawData.cast<String, dynamic>());
   itemId = chat?.itemId;
   

2. 优化Chat类的fromMap方法
   给方法参数添加明确的类型处理，避免后续类型警告：

   @override
   Chat? fromMap(dynamic dynamicData) {
     if (dynamicData != null) {
       // 将Map<Object?, Object?>安全转换为Map<String, dynamic>
       final castedData = (dynamicData as Map<Object?, Object?>).cast<String, dynamic>();
       return Chat(
         itemId: castedData['itemId'],
         receiverId: castedData['receiver_id'],
         senderId: castedData['sender_id'],
       );
     } else {
       return null;
     }
   }
   

3. 可选：简化抽象类泛型逻辑
   如果业务允许，可调整MyObject的泛型参数为非空类型，减少可空处理的复杂度：

   // 修改抽象类
   abstract class MyObject<T> {
     String? key = '0';
   
     String? getPrimaryKey();
   
     T fromMap(Map<String, dynamic> dynamicData);
   
     Map<String, dynamic> toMap(T object);
   
     List<T> fromMapList(List<Map<String, dynamic>> dynamicDataList);
   
     List<Map<String, dynamic>> toMapList(List<T> objectList);
   }
   
   // 修改Chat类继承及方法
   class Chat extends MyObject<Chat> {
     // ... 其他属性、==、hashCode等代码不变
     
     @override
     Chat fromMap(Map<String, dynamic> dynamicData) {
       return Chat(
         itemId: dynamicData['itemId'],
         receiverId: dynamicData['receiver_id'],
         senderId: dynamicData['sender_id'],
       );
     }
   
     @override
     Map<String, dynamic> toMap(Chat object) {
       final Map<String, dynamic> data = <String, dynamic>{};
       data['itemId'] = object.itemId;
       data['receiver_id'] = object.receiverId;
       data['sender_id'] = object.senderId;
       return data;
     }
   
     // 同步修改fromMapList和toMapList的参数与返回值类型
   }
   

内容的提问来源于stack exchange，提问作者Vahit Behrouz