flutter -如何在Flutter中解析带有列表的嵌套JSON结构？

Question

这是我的JSON

{
  "type-1":[
    {
      "name": "type-1-1"
    },
    {
      "name": "type-1-2"
    }
  ],
  "type-2": [
    {
      "name": "type-2-1"
    },
    {
      "name": "type-2-2"
    },
    {
      "name": "type-2-3"
    }
  ]
}

我想解析这个JSON，因此我将我的模型写成这样

class Places {
  final List<PlaceName> names;
  Places({this.names});

  factory Places.fromJson(Map<String, dynamic> parsedJson){
    var list = parsedJson['name'] as List;
    List<PlaceName> placeList = list.map((i) => PlaceName.fromJson(i)).toList();
    return Places(
        names: placeList
    );
  }
}


class PlaceName {
  final String name;
  PlaceName({this.name});
  factory PlaceName.fromJson(Map<String, dynamic> parsedJson){
    return PlaceName(
        name : parsedJson['name'],
    );
  }
}

最后，我在窗口小部件树中编写了如下函数

Future<String> _loadAllPlaces() async {
    return await rootBundle.loadString('assets/location.json');
  }

  Future loadPlaces() async {
    String jsonString = await _loadAllPlaces();
    final jsonResponse = json.decode(jsonString);
    PlaceName place = new PlaceName.fromJson(jsonResponse);
    print(place);
    print(place.name);
    return place;
  }

因此，当我在FutureBuilder中调用loadPlaces()时，它返回null。我试着调试它，但没有成功。请帮帮忙。提前谢谢。

Answer 1

在var list = parsedJson['name']行的Places.fromJson方法中，您试图在引用对象本身之前访问对象内部的列表。相反，您应该通过示例传递其中一个对象的名称，例如type-1，以这种方式传递var list = parsedJson['type-1'] as List;。

但这将只返回第一个对象的列表。要获得所有对象和每个对象的名称列表，需要遍历json对象，在本例中是type-1和type-2，然后遍历其列表。您可以在AllPlaces.fromJson中检查迭代。因此，您需要对代码进行一些更改，以便从json中获取所有对象。您可以尝试下面这样的代码，您可以在DartPad上进行测试

import 'dart:convert';

String jsonString = '{"type-1":[{"name": "type-1-1"},{"name": "type-1-2"}],"type-2": [{"name": "type-2-1"},{"name": "type-2-2"},{"name": "type-2-3"}]}';


class AllPlaces {
  List<PlaceName> placeNames;

  AllPlaces({this.placeNames});

  AllPlaces.fromJson(Map<String, dynamic> json) {
    placeNames = new List<PlaceName>();
    //Iterates over the json objects.
     json.forEach((type, list) {
      //Iterates over the list of each object.
      json[type].forEach((list) {
        //Creates an instance of PlaceName and adds to the placeNames list.
        placeNames.add(new PlaceName.fromJson(list));
      });
    });
  }
}

class PlaceName {
  String name;

  PlaceName({this.name});

  PlaceName.fromJson(Map<String, dynamic> json) {
    name = json['name'];
  }

}

Future loadPlaces() async {
   //String jsonString = await _loadAllPlaces();
   final jsonResponse = json.decode(jsonString);
   AllPlaces allPlaces = new AllPlaces.fromJson(jsonResponse);
  
   //Print the names of each object in the console
   allPlaces.placeNames.forEach((place) => print(place.name));
   
   return allPlaces;
}


void main() {
  loadPlaces();
}