Question

在我的flutter应用程序中，我试图创建一个单次/第一次屏幕，该屏幕显示首次下载该应用程序时的教程。我决定使用“共享首选项”来存储数据。但是每当我运行该应用程序时，我都会得到断言失败：布尔表达式不能为空：那是由于代码中的变量 _seen 没有初始化，当我进一步查看时，发现我的代码在用于构建窗口小部件之后正在初始化变量。反正有解决办法吗？

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(),
    );
  } 
}

Answer 1

您可以在下面复制粘贴运行完整代码
步骤1：您可以将逻辑checkFirstTime()放在main()中
步骤2：要在updateFirstTime()中使用build，可以使用initialRoute来避免 seen值更新原因SingupScreen()突然更改为HomeScreen()

代码段

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),
      ),
    );
  }
}

Answer 2

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();
        }
      }),
    );
  }
}

Answer 3

这里的实际问题是由于SharedPreferences API是异步的，因此，按预期在build prefs之前触发了build（）方法。

正在：

1. initState
2. _checkFirstTime (starts)
3. build 
4. _checkFirstTime (completes)

有很多方法可以修复它，可以将其初始化为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4ms或Prefs完成加载所需的时间的空白视图。这在功能上与使用FutureBuilder相同。不要忘记在setState()末尾调用_checkFirstTime来触发刷新。

为什么在构建小部件之前，我的initState（）不调用使用sharedprefs的函数？

3 个答案: