如何仅打印 json 映射中值的特定部分？

Question

我是 Flutter 的新手，我关注了 StackOverflow 上关于从 json 映射中检索数据的另一篇文章。它有效，但现在我只想从地图中访问我需要的数据。我也无法从地图中删除多余的数据，因为我需要相同的地图用于应用程序的另一部分。

这就是代码的样子。

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CupertinoApp(
      localizationsDelegates: <LocalizationsDelegate<dynamic>>[
        DefaultMaterialLocalizations.delegate,
        DefaultWidgetsLocalizations.delegate,
        DefaultCupertinoLocalizations.delegate,
      ],
      theme: CupertinoThemeData(
        primaryColor: Color(0xffffffff),
      ),
      home: TransactionHistoryScreen(),
    );
  }
}

class TransactionHistoryScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CupertinoPageScaffold(
      child: Scaffold(body: Container(child: MakeList(),),),
    );
  }
}

class MakeList extends StatelessWidget {
final List<Map<String, String>> json = [
  {"branch": "B1", "xyz": "0", "ABC": "2", "MN": "2"},
  {"branch": "B2", "xyz": "0", "ABC": "0", "MN": "0"},
  {"branch": "B3", "xyz": "1", "ABC": "1", "MN": "42"},
  {"branch": "B4", "xyz": "0", "ABC": "5", "MN": "69"},
];

@override
Widget build(BuildContext context) {
  return ListView.builder(
    itemCount: json.length,
    itemBuilder: (BuildContext context, int index) {
      return ListTileClass(jsonObject: json[index]);
    },
   );
  }
}

class ListTileClass extends StatefulWidget {
final Map<String, String> jsonObject;

ListTileClass({this.jsonObject});

@override
_ListTileClassState createState() => _ListTileClassState();
}

class _ListTileClassState extends State<ListTileClass> {
  @override
  Widget build(BuildContext context) {
    return ListTile(
      title: Container(
        child: Row(
          children: makeWidgetChildren(widget.jsonObject),
        ),
      ),
    );
  }

  List<Widget> makeWidgetChildren(jsonObject) {
    List<Widget> children = [];
    jsonObject.keys.forEach(
      (key) => {
        children.add(
          Padding(
              child: Text('${jsonObject[key]}'), padding: EdgeInsets.all(8.0)),
        ),
      },
    );
    return children;
  }
}

使用此代码，我可以打印地图每个键的值。如何访问特定密钥？例如，如何仅打印“分支”和“xyz”值？

Answer 1

在许多情况下，您可以在 dart 中使用 forEach：

void main() {
  final List<Map<String, String>> json = [
  {"branch": "B1", "xyz": "0", "ABC": "2", "MN": "2"},
  {"branch": "B2", "xyz": "0", "ABC": "0", "MN": "0"},
  {"branch": "B3", "xyz": "1", "ABC": "1", "MN": "42"},
  {"branch": "B4", "xyz": "0", "ABC": "5", "MN": "69"},
];
  json.forEach((item) => print("Branch : ${item['branch']}, xyz : ${item['xyz']}"));
}

输出：

Branch : B1, xyz : 0
Branch : B2, xyz : 0
Branch : B3, xyz : 1
Branch : B4, xyz : 0