shared_preferences_CastError

Question

提供的示例有效

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

void main() {
  runApp(MaterialApp(
    home: Scaffold(
      body: Center(
      child: RaisedButton(
        onPressed: _incrementCounter,
        child: Text('Increment Counter'),
        ),
      ),
    ),
  ));
}

_incrementCounter() async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  int counter = (prefs.getInt('counter') ?? 0) + 1;
  print('Pressed $counter times.');
  await prefs.setInt('counter', counter);
}

，但是说我想在runApp之前加载数据。像这样移动 _incrementCounter()

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

void main() {
 _incrementCounter(); 
  runApp(MaterialApp(
    home: Scaffold(
      body: Center(
      child: RaisedButton(
        onPressed: _incrementCounter,
        child: Text('Increment Counter'),
        ),
      ),
    ),
  ));
}

_incrementCounter() async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  int counter = (prefs.getInt('counter') ?? 0) + 1;
  print('Pressed $counter times.');
  await prefs.setInt('counter', counter);
}

但现在我收到“_CastError（用于空值的空检查运算符）”。

在 runApp 之前有一个 init() 真的很好，但似乎至少 shared_preferences 以某种方式依赖于 runApp 中发生的事情。

如果不是在runApp之前，那么使用shared_preferences的初始化应该放在哪里？

Answer 1

只需在主函数中添加WidgetsFlutterBinding.ensureInitialized();：

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

Future<void> main() async {
 WidgetsFlutterBinding.ensureInitialized(); // Add this

 await _incrementCounter(); // Also, add await here since the function is async

  runApp(MaterialApp(
    home: Scaffold(
      body: Center(
      child: RaisedButton(
        onPressed: _incrementCounter,
        child: Text('Increment Counter'),
        ),
      ),
    ),
  ));
}

Future<void> _incrementCounter() async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  int counter = (prefs.getInt('counter') ?? 0) + 1;
  print('Pressed $counter times.');
  await prefs.setInt('counter', counter);
}

事实是，SharedPreferences 使用本机平台代码来初始化存储。本次通信使用了Flutter平台通道，所以需要保证Flutter引擎绑定已经初始化。

Answer 2

您可以在 StatefulWidget 中使用 initState()。因为它是启动 StatefullWidget 时的第一个函数运行。

import 'package:flutter/material.dart';
class _MyAppState extends State<MyApp> {
  @override
  void initState() {
    // Get shared_preferences in here
    print('Async done');
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    print('object');
    print('object2');
    print('object3');
    return MaterialApp(
        home: Scaffold(
      body: Center(
        child: RaisedButton(),
      ),
    ));
  }
}

您可以访问网站了解StatefullWidget的生命周期https://api.flutter.dev/flutter/widgets/StatefulWidget-class.html

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