Flutter

Question

我即将在 Flutter 中使用数据库 "Sembast"。具有 string 和 int 等数据类型的简单对象可以正常工作。但是，使用列表时会出现问题。

我已经创建了一个示例并以以下教程为导向：https://resocoder.com/2019/04/06/flutter-nosql-database-sembast-tutorial-w-bloc/ 在我的示例中，有水果和树叶作为对象。水果包含叶子列表。

class Fruit {
  final String id;
  final String name;
  final bool isSweet;
  final List<Leaves> leaves;
...
}

class Leaves {
  final String id;
  final String name;
...
}

//Create a sample object
var leaveOne = Leaves(id: "1", name: "leaveOne");
var leaveTwo = Leaves(id: "2", name: "leaveTwo");
var leaveThree = Leaves(id: "3", name: "leaveThree");

var leavesList = List<Leaves>();
leavesList.add(leaveOne);
leavesList.add(leaveTwo);
leavesList.add(leaveThree);

var fruit = Fruit(id: "1", name: "Apple", isSweet: true, leaves: leavesList);
_fruitDao.insert(fruit);


// The fruitDao.insert makes following
Future insert(Fruit fruit) async {
  await _fruitStore.add(await _db, fruit.toJson());
}

JSON 看起来像这样：{id: 1, name: Apple, isSweet: true, leaves: ['Leaves' 的实例, 'Leaves' 的实例, [ 的实例=21=]]}

错误如下： [错误：flutter/lib/ui/ui_dart_state.cc(148)] 未处理的异常：无效参数：值 'Leaves' 不支持类型 Leaves

的实例

Answer 1

如前所述，Instance of 'Leaves' 不是有效类型，因此每个 Leave 也必须进行转换。很难在没有看到 toJson() 的情况下猜出你在做什么，但像这样的东西应该可以工作（可以在很大程度上进行优化）：

class Fruit {
  final String id;
  final String name;
  final bool isSweet;
  final List<Leaves> leaves;

  Fruit({this.id, this.name, this.isSweet, this.leaves});

  Map<String, dynamic> toJson() => <String, dynamic>{
        'id': id,
        'name': name,
        'isSweet': isSweet,
        'leaves': leaves?.map((leave) => leave.toJson())?.toList(growable: false)
      };
}

class Leaves {
  final String id;
  final String name;

  Leaves({this.id, this.name});

  Map<String, dynamic> toJson() => <String, dynamic>{'id': id, 'name': name};
}

你的 json 应该是这样的：

{
  "id": "1",
  "name": "Apple",
  "isSweet": true,
  "leaves": [
    {
      "id": "1",
      "name": "leaveOne"
    },
    {
      "id": "2",
      "name": "leaveTwo"
    },
    {
      "id": "3",
      "name": "leaveThree"
    }
  ]
}

Answer 2

除了@alextk 的回答之外，这里还有一个示例，在没有任何代码生成或库的情况下来回转换。

class Fruit {
  final String id;
  final String name;
  final bool isSweet;
  final List<Leaves> leaves;

  Fruit({this.id, this.name, this.isSweet, this.leaves});

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
      'isSweet': isSweet,
      'leaves': leaves.map((leave) => leave.toMap()).toList(growable: false)
    };
  }

  static Fruit fromMap(Map<String, dynamic> map) {
    return Fruit(
      id: map['id'],
      name: map['name'],
      isSweet: map['isSweet'],
      leaves: map['leaves'].map((mapping) => Leaves.fromMap(mapping)).toList().cast<Leaves>(),
    );
  }
}

class Leaves {
  final String id;
  final String name;

  Leaves({this.id, this.name});

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
    };
  }

  static Leaves fromMap(Map<String, dynamic> map) {
    return Leaves(
      id: map['id'],
      name: map['name'],
    );
  }
}

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