与 Flutter 共享首选项，我做错了什么?

Question

我正在尝试使用共享首选项来简化数据持久性 跨不同的页面/片段，(忽略这里和那里写的"json"( 在您询问之前，这只是一次性数据，该应用程序将基于 SQL 构建。

所以我创建了一个类来处理共享首选项

import 'dart:async';
import 'package:shared_preferences/shared_preferences.dart';
class JsonHelper {
Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
getJson(String _key) async {
final SharedPreferences prefs = await _prefs;
final String _json = prefs.getString('$_key');
return _json;
}
setJson(String _key, String _json) async {
final SharedPreferences prefs = await _prefs;
prefs.setString('$_key', _json);
}
delJson(String _key) async {
final prefs = await SharedPreferences.getInstance();
prefs.remove('$_key');
}
}

和一个"简单"的主页，简单到不起作用

import 'package:flutter/material.dart';
import 'package:flutter_3/DATABASE/JsonHelper.dart';
class LandingPage extends StatefulWidget {
@override
_LandingPageState createState() => _LandingPageState();
}
class _LandingPageState extends State<LandingPage> {
JsonHelper _jHelp;
String _localKey = 'test';
TextEditingController _textCTRL;
@override
void initState() {
_jHelp.setJson(_localKey, "start");
super.initState();
}
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: Text(_jHelp.getJson(_localKey)),
),
body: Center(
child: Column(
children: [
TextField(
controller: _textCTRL,
decoration: InputDecoration(labelText: 'write here'),
),
RaisedButton(onPressed:_pressButton)
],
),
),
),
);
}
void _pressButton () {
_jHelp.setJson(_localKey, _textCTRL.toString());
Navigator.of(context).pop();
}
}

我做错了什么？ 提前感谢您的帮助

Answer 1

您也可以在没有FutureBuilder的情况下实现。

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'SharedPreferences Demo',
home: LandingPage(),
);
}
}
class LandingPage extends StatefulWidget {
@override
_LandingPageState createState() => _LandingPageState();
}
class _LandingPageState extends State<LandingPage> {
JsonHelper _jHelp;
String _localKey = "test";
TextEditingController _textCTRL;
Future<String> data;
String text = "start";
final myController = TextEditingController();
@override
void initState() {
_jHelp = JsonHelper();
_jHelp.setJson(_localKey, "start");
super.initState();
}
@override
void dispose() {
myController.dispose();
super.dispose();
}

@override
Widget build(BuildContext context) {
return MaterialApp(
home:  Scaffold(
appBar: AppBar(
title: Text(text),
),
body: Center(
child: Column(
children: [
TextField(
controller: myController,
decoration: InputDecoration(labelText: 'write here'),
),
RaisedButton(
onPressed:_pressButton,
child: new Text("check"),
)
],
),
),
)
);
}
void _pressButton () async{
await _jHelp.setJson(_localKey,myController.text.toString());
String getdata = await _jHelp.getJson(_localKey) as String;
setState(() {
text = getdata;
});
}
}
class JsonHelper {

Future<String> getJson(String _key) async {
SharedPreferences prefs = await SharedPreferences.getInstance();
String _json = prefs.getString('$_key') ?? "start";
return _json;
}
setJson(String _key, String _json) async {
SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setString('$_key',_json);
}
delJson(String _key) async {
final prefs = await SharedPreferences.getInstance();
prefs.remove('$_key');
}
}

Answer 2

那是因为你的方法getJson是异步的，你可以使用FutureBuilder来获取响应，看看这里：https://docs.flutter.io/flutter/widgets/FutureBuilder-class.html

@override
Widget build(BuildContext context) {
return MaterialApp(
home: FutureBuilder(
initialData: "",
future: _jHelp.getJson(_localKey),
builder: (context, snapshot) {
return Scaffold(
appBar: AppBar(
title: Text(snapshot.data),
),
body: Center(
child: Column(
children: [
TextField(
controller: _textCTRL,
decoration: InputDecoration(labelText: 'write here'),
),
RaisedButton(onPressed:_pressButton)
],
),
),
);
},
),
);
}