shared_preferences _CastError

Question

提供的示例可以正常工作

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);
}

，但是假设我想在runApp之前加载数据。只需像这样移动_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);
}

但是现在我得到了一个"_CastError (用于空值的空值检查运算符)“。

如果在runApp之前有一个init()就好了，但是看起来至少shared_preferences在某种程度上依赖于runApp中发生的事情。

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

Answer 1

只需在main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);
}

问题是，SharedPreferences使用本机平台代码来初始化存储。颤动平台通道用于此通信，因此您需要确保已初始化颤动引擎绑定。

Answer 2

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

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(),
      ),
    ));
  }
}

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