为什么我的 initState() 在构建小部件之前不调用使用 sharedprefs 的函数?
Why does my initState() not call a function that uses sharedprefs before the widget is built?
在我的 flutter 应用程序中,我尝试创建一个一次性/第一次屏幕,在首次下载该应用程序时显示教程。我决定使用共享首选项来存储数据。但是每当我 运行 我得到 Failed assertion: boolean expression must not be null 的应用程序时:那是由于我的变量 _seen代码未被初始化,当我进一步查看时,我发现我的代码在用于构建小部件后正在初始化变量。有办法解决这个问题吗?
我的代码
void main() => runApp(Start());
class Start extends StatefulWidget
{
@override
App createState() => new App();
}
class App extends State<Start>
{
bool _seen;
@override
void initState()
{
print(1);
_checkFirstTime();
super.initState();
}
_checkFirstTime() async
{
print(1.1);
SharedPreferences prefs = await SharedPreferences.getInstance();
_seen = (prefs.getBool('seen') ?? false);
print(1.2);
}
_updateFirstTime() async
{
SharedPreferences prefs = await SharedPreferences.getInstance();
_seen = true;
prefs.setBool('seen', true);
}
Widget build(BuildContext context)
{
print(2);
bool seen = _seen;
if (_seen == false) {_updateFirstTime();}
print(2.1);
return MaterialApp(
debugShowCheckedModeBanner: false,
home: seen ? HomeScreen() : SignUpScreen(),
);
}
}
您可以复制粘贴运行下面的完整代码
第 1 步:您可以将逻辑 checkFirstTime()
放入 main()
第二步:要在build
中使用updateFirstTime()
,可以使用initialRoute
来避免
seen
值更新导致SingupScreen()
突然变为HomeScreen()
代码片段
Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
SharedPreferences prefs = await SharedPreferences.getInstance();
seen = (prefs.getBool('seen') ?? false);
runApp(MyApp());
}
...
initialRoute: seen == false || seen == null ? "/sign" : "/",
routes: {
'/': (context) => HomeScreen(
title: "demo",
),
"/sign": (context) => SignUpScreen(),
工作演示
完整代码
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:flutter/services.dart';
bool seen;
Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
SharedPreferences prefs = await SharedPreferences.getInstance();
seen = (prefs.getBool('seen') ?? false);
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
initialRoute: seen == false || seen == null ? "/sign" : "/",
routes: {
'/': (context) => HomeScreen(
title: "demo",
),
"/sign": (context) => SignUpScreen(),
},
);
}
}
class SignUpScreen extends StatelessWidget {
_updateFirstTime() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
seen = true;
prefs.setBool('seen', true);
}
@override
Widget build(BuildContext context) {
if (seen == false) {
_updateFirstTime();
}
return Text('SignUpScreen');
}
}
class HomeScreen extends StatefulWidget {
HomeScreen({Key key, this.title}) : super(key: key);
final String title;
@override
_HomeScreenState createState() => _HomeScreenState();
}
class _HomeScreenState extends State<HomeScreen> {
int _counter = 0;
void _incrementCounter() {
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),
),
);
}
}
在这种情况下,您可以使用 Future 构建器。这是一个例子
import 'package:flutter/material.dart';
void main() => runApp(Start());
class Start extends StatefulWidget {
@override
App createState() => new App();
}
class App extends State<Start> {
bool _seen;
@override
void initState() {
print(1);
_checkFirstTime();
super.initState();
}
Future<bool> _checkFirstTime() async {
print(1.1);
SharedPreferences prefs = await SharedPreferences.getInstance();
_seen = (prefs.getBool('seen') ?? false);
print(1.2);
return _seen;
}
_updateFirstTime() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
_seen = true;
prefs.setBool('seen', true);
}
Widget build(BuildContext context) {
print(2.1);
return MaterialApp(
debugShowCheckedModeBanner: false,
home: FutureBuilder<bool>(
future: _checkFirstTime(),
builder: (BuildContext context, AsyncSnapshot<bool> snapshot) {
if (snapshot.hasData) {
if (_seen == false) {
_updateFirstTime();
}
return snapshot.data ? HomeScreen() : SignUpScreen();
} else {
//you can put anything while retrieving data
return Container();
}
}),
);
}
}
这里的实际问题是由于 SharedPreferences API 是异步的,所以 build() 方法在加载 prefs 之前触发,正如预期的那样。
进行中:
1. initState
2. _checkFirstTime (starts)
3. build
4. _checkFirstTime (completes)
有很多方法可以修复它,可以初始化为 false,可以使用 FutureBuilder,可以使用 _isInitComplete 标志等。
一个简单的解决方法是将其添加到构建的顶部:
if(_seen == null) return Container();
现在,将显示空视图 4 毫秒或 Prefs 完成加载所需的任何时间。这在功能上与使用 FutureBuilder 相同。不要忘记在 _checkFirstTime
末尾调用 setState()
来触发刷新。
在我的 flutter 应用程序中,我尝试创建一个一次性/第一次屏幕,在首次下载该应用程序时显示教程。我决定使用共享首选项来存储数据。但是每当我 运行 我得到 Failed assertion: boolean expression must not be null 的应用程序时:那是由于我的变量 _seen代码未被初始化,当我进一步查看时,我发现我的代码在用于构建小部件后正在初始化变量。有办法解决这个问题吗?
我的代码
void main() => runApp(Start());
class Start extends StatefulWidget
{
@override
App createState() => new App();
}
class App extends State<Start>
{
bool _seen;
@override
void initState()
{
print(1);
_checkFirstTime();
super.initState();
}
_checkFirstTime() async
{
print(1.1);
SharedPreferences prefs = await SharedPreferences.getInstance();
_seen = (prefs.getBool('seen') ?? false);
print(1.2);
}
_updateFirstTime() async
{
SharedPreferences prefs = await SharedPreferences.getInstance();
_seen = true;
prefs.setBool('seen', true);
}
Widget build(BuildContext context)
{
print(2);
bool seen = _seen;
if (_seen == false) {_updateFirstTime();}
print(2.1);
return MaterialApp(
debugShowCheckedModeBanner: false,
home: seen ? HomeScreen() : SignUpScreen(),
);
}
}
您可以复制粘贴运行下面的完整代码
第 1 步:您可以将逻辑 checkFirstTime()
放入 main()
第二步:要在build
中使用updateFirstTime()
,可以使用initialRoute
来避免
seen
值更新导致SingupScreen()
突然变为HomeScreen()
代码片段
Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
SharedPreferences prefs = await SharedPreferences.getInstance();
seen = (prefs.getBool('seen') ?? false);
runApp(MyApp());
}
...
initialRoute: seen == false || seen == null ? "/sign" : "/",
routes: {
'/': (context) => HomeScreen(
title: "demo",
),
"/sign": (context) => SignUpScreen(),
工作演示
完整代码
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:flutter/services.dart';
bool seen;
Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
SharedPreferences prefs = await SharedPreferences.getInstance();
seen = (prefs.getBool('seen') ?? false);
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
initialRoute: seen == false || seen == null ? "/sign" : "/",
routes: {
'/': (context) => HomeScreen(
title: "demo",
),
"/sign": (context) => SignUpScreen(),
},
);
}
}
class SignUpScreen extends StatelessWidget {
_updateFirstTime() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
seen = true;
prefs.setBool('seen', true);
}
@override
Widget build(BuildContext context) {
if (seen == false) {
_updateFirstTime();
}
return Text('SignUpScreen');
}
}
class HomeScreen extends StatefulWidget {
HomeScreen({Key key, this.title}) : super(key: key);
final String title;
@override
_HomeScreenState createState() => _HomeScreenState();
}
class _HomeScreenState extends State<HomeScreen> {
int _counter = 0;
void _incrementCounter() {
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),
),
);
}
}
在这种情况下,您可以使用 Future 构建器。这是一个例子
import 'package:flutter/material.dart';
void main() => runApp(Start());
class Start extends StatefulWidget {
@override
App createState() => new App();
}
class App extends State<Start> {
bool _seen;
@override
void initState() {
print(1);
_checkFirstTime();
super.initState();
}
Future<bool> _checkFirstTime() async {
print(1.1);
SharedPreferences prefs = await SharedPreferences.getInstance();
_seen = (prefs.getBool('seen') ?? false);
print(1.2);
return _seen;
}
_updateFirstTime() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
_seen = true;
prefs.setBool('seen', true);
}
Widget build(BuildContext context) {
print(2.1);
return MaterialApp(
debugShowCheckedModeBanner: false,
home: FutureBuilder<bool>(
future: _checkFirstTime(),
builder: (BuildContext context, AsyncSnapshot<bool> snapshot) {
if (snapshot.hasData) {
if (_seen == false) {
_updateFirstTime();
}
return snapshot.data ? HomeScreen() : SignUpScreen();
} else {
//you can put anything while retrieving data
return Container();
}
}),
);
}
}
这里的实际问题是由于 SharedPreferences API 是异步的,所以 build() 方法在加载 prefs 之前触发,正如预期的那样。
进行中:
1. initState
2. _checkFirstTime (starts)
3. build
4. _checkFirstTime (completes)
有很多方法可以修复它,可以初始化为 false,可以使用 FutureBuilder,可以使用 _isInitComplete 标志等。
一个简单的解决方法是将其添加到构建的顶部:
if(_seen == null) return Container();
现在,将显示空视图 4 毫秒或 Prefs 完成加载所需的任何时间。这在功能上与使用 FutureBuilder 相同。不要忘记在 _checkFirstTime
末尾调用 setState()
来触发刷新。