將 Json 數組轉換為列表<object>在 Flutter<div id="text_translate"><p> 我是 Flutter 和 Dart 的新手，我一直在努力保存共享首選項中的值以在我的應用程序重新啟動時使用。 我已經成功存儲了變量，但我還需要存儲 object 列表。 我知道由於共享首選項只接受字符串列表，我需要將我的 object 列表轉換為 JSON 數組，當應用程序重新啟動時，檢索此列表並將其再次轉換為 object 列表。</p><p> 我能夠將 object 列表編碼為 JSON 並且得到如下信息：</p><pre> [{name: Rent, amount: 250}, {name: Insurance, amount: 105}]</pre><p> 我通過以下 function 實現了這一點：</p><pre> void SaveLists(key, value) async { //where value is a List&lt;Object&gt; final prefs = await SharedPreferences.getInstance(); List&lt;dynamic&gt; json_list = (value.map((i) =&gt; i.toJson())).toList(); prefs.setStringList(key, json_list); //this line causes the error print('$json_list'); }</pre><p> List json_list 包含如上所示的 JSON 數組。 但是，當我嘗試使用prefs.setStringList(key, json_list); 我收到以下錯誤：</p><pre> Unhandled Exception: type 'List&lt;dynamic&gt;' is not a subtype of type 'List&lt;String&gt;'</pre><p> 現在，假設我以某種方式成功地將其存儲在共享首選項中，如何在使用prefs.getString('key')調用時將 JSON 數組轉換回 List ？</p><p> 如果您需要查看 class，這里是：</p><pre> import 'dart:convert'; class Random_expenses { String name; double amount; Random_expenses({this.name, this.amount}); Random_expenses.fromJson(Map&lt;String, dynamic&gt; json): this.name = json['name'], this.amount = json['amount']; Map&lt;String, dynamic&gt; toJson() =&gt; {'name': this.name, 'amount': this.amount}; }</pre></div></object>

Question

我是 Flutter 和 Dart 的新手，我一直在努力保存共享首選項中的值以在我的應用程序重新啟動時使用。 我已經成功存儲了變量，但我還需要存儲 object 列表。 我知道由於共享首選項只接受字符串列表，我需要將我的 object 列表轉換為 JSON 數組，當應用程序重新啟動時，檢索此列表並將其再次轉換為 object 列表。

我能夠將 object 列表編碼為 JSON 並且得到如下信息：

[{name: Rent, amount: 250}, {name: Insurance, amount: 105}]

我通過以下 function 實現了這一點：

  void SaveLists(key, value) async { //where value is a List<Object>
    final prefs = await SharedPreferences.getInstance();
    List<dynamic> json_list = (value.map((i) => i.toJson())).toList();
    prefs.setStringList(key, json_list); //this line causes the error
    print('$json_list');
  }

List json_list 包含如上所示的 JSON 數組。 但是，當我嘗試使用prefs.setStringList(key, json_list); 我收到以下錯誤：

Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'List<String>'

現在，假設我以某種方式成功地將其存儲在共享首選項中，如何在使用prefs.getString('key')調用時將 JSON 數組轉換回 List ？

如果您需要查看 class，這里是：

import 'dart:convert';
class Random_expenses {

 String name;
 double amount;

 Random_expenses({this.name, this.amount});

 Random_expenses.fromJson(Map<String, dynamic> json)
     : this.name = json['name'],
       this.amount = json['amount'];

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

Answer 1

您可以在下面復制粘貼運行完整代碼
您可以使用randomExpensesToJson(value)將List<RandomExpenses>保存為 json 字符串
您可以使用randomExpensesFromJson(keyString)獲取List<RandomExpenses>

代碼片段

List<RandomExpenses> randomExpensesFromJson(String str) =>
    List<RandomExpenses>.from(
        json.decode(str).map((x) => RandomExpenses.fromJson(x)));

String randomExpensesToJson(List<RandomExpenses> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));
...
void saveData(String key, List<RandomExpenses> value) async {
    final prefs = await SharedPreferences.getInstance();
    prefs.setString(key, randomExpensesToJson(value));
  }

Future<List<RandomExpenses>> getData(String key) async {
  final prefs = await SharedPreferences.getInstance();
  String keyString = prefs.getString(key);
  return Future.value(randomExpensesFromJson(keyString));
}
...
await saveData("key", randomExpensesList);
List<RandomExpenses> list = await getData("key");

print('${list[0].name} ${list[0].amount}');
print('${list[1].name} ${list[1].amount}');

output

I/flutter (24879): Rent 250
I/flutter (24879): Insurance 105

完整代碼

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'dart:convert';

List<RandomExpenses> randomExpensesFromJson(String str) =>
    List<RandomExpenses>.from(
        json.decode(str).map((x) => RandomExpenses.fromJson(x)));

String randomExpensesToJson(List<RandomExpenses> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class RandomExpenses {
  String name;
  int amount;

  RandomExpenses({
    this.name,
    this.amount,
  });

  factory RandomExpenses.fromJson(Map<String, dynamic> json) => RandomExpenses(
        name: json["name"],
        amount: json["amount"],
      );

  Map<String, dynamic> toJson() => {
        "name": name,
        "amount": amount,
      };
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  List<RandomExpenses> randomExpensesList = [
    RandomExpenses(name: "Rent", amount: 250),
    RandomExpenses(name: "Insurance", amount: 105)
  ];

  void saveData(String key, List<RandomExpenses> value) async {
    final prefs = await SharedPreferences.getInstance();
    prefs.setString(key, randomExpensesToJson(value));
  }

  Future<List<RandomExpenses>> getData(String key) async {
    final prefs = await SharedPreferences.getInstance();
    String keyString = prefs.getString(key);
    return Future.value(randomExpensesFromJson(keyString));
  }

  void _incrementCounter() async {
    await saveData("key", randomExpensesList);
    List<RandomExpenses> list = await getData("key");

    print('${list[0].name} ${list[0].amount}');
    print('${list[1].name} ${list[1].amount}');

    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}