我正在學習如何在 Flutter 中使用 SharedPreferences 庫。
我創建了這段代碼，我希望在關閉并重新打開應用程式后將 counter 和 counter2 變數保留為最后一次保存。
但是，當我重新打開應用程式時，counter 和 counter2 值回傳 0。誰能向我解釋我哪里出錯了？
謝謝你。
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'data.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({Key? key, required this.title}) : super(key: key);
final String title;
@override
State<MyHomePage> createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int counter = 0;
int counter2 = 0;
increment() {
setState(() {
counter = 1;
counter2 = 2;
});
}
loadData() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
setState(() {
String? json = prefs.getString('UserData');
print('loaded json: $json');
if (json == null) {
print('NO DATA (null)');
} else {
Map<String, dynamic> map = jsonDecode(json);
print('map $map');
final data = Data.fromJson(map);
print('Data ${data.counter}, ${data.counter2}');
}
});
}
saveData() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
final _data = Data(counter: counter, counter2: counter2);
String json = jsonEncode(_data);
print('saved json: $json');
prefs.setString('UserData', json);
}
clearData() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.clear();
print('data cleared');
}
/// dichiarare l' initState()
@override
void initState() {
super.initState();
loadData();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
const Text(
'You have pushed the button this many times:',
),
Text(
'c: $counter, c2: $counter2',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: () {
increment();
saveData();
},
tooltip: 'Increment',
child: const Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
class Data {
int counter = 0;
int counter2 = 0;
Data({required this.counter, required this.counter2});
Map<String, dynamic> toJson() {
return {
'counter': counter,
'counter2': counter2,
};
}
Data.fromJson(Map<String, dynamic> json) {
counter = json['counter'];
counter2 = json['counter2'];
}
}
uj5u.com熱心網友回復：
我同意另一個答案，最好是使用FutureBuilder. 但是您可以通過在末尾添加兩行來使當前代碼正常作業loadData：
loadData() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
setState(() {
String? json = prefs.getString('UserData');
print('loaded json: $json');
if (json == null) {
print('NO DATA (null)');
} else {
Map<String, dynamic> map = jsonDecode(json);
print('map $map');
final data = Data.fromJson(map);
print('Data ${data.counter}, ${data.counter2}');
// add these lines
counter = data.counter;
counter2 = data.counter2;
}
});
}
會發生什么（正如另一個答案所說）是您的小部件是在不知道來自SharedPreferences. 一段時間后，第一次構建完成，loadDatafuture 完成，并且setState重新構建了小部件。
在實際應用程式中，您希望避免不必要的構建，因此您寧愿在加載異步資料時顯示進度指示器，檢查FutureBuilder.
uj5u.com熱心網友回復：
一個簡短的回答是，當您在函式loadData();內部呼叫時initState，相對于小部件的其余部分異步執行，因此您的 Scaffold 是在資料可用之前構建的。這就是為什么您看到的是您的資料，print而不是應用程式中的資料。
解決它的一種方法是給我們一個https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html
轉載請註明出處，本文鏈接：https://www.uj5u.com/net/497264.html