在main.dart中使用ChangeNotifierProvider实现暗黑模式时遇非空断言错误

错误根源 #

你碰到的Failed assertion: Boolean expression must not be null错误，核心问题是**MyApp的isDark参数未被正确传入，导致其值为null**，进而在创建ThemeSettings实例时传递了null的布尔值，触发了非空断言失败。

看你的代码：

• main函数里已经正确获取了isDark值（sharedPreferences.getBool("is_dark") ?? false），但创建MyApp时完全没传这个参数：runApp(MyApp())
• MyApp类的isDark成员没有默认值，默认就是null，当你在create: (context) => ThemeSettings(isDark)时，相当于把null传给了ThemeSettings，最终导致布尔表达式的非空断言错误。

修复步骤 #

1. 给MyApp传入isDark参数
   修改main里的runApp调用：

   return runApp(
     MyApp(isDark: isDark),
   );
   

2. 可选：给MyApp的isDark加默认值
   为避免后续忘记传参再出问题，给构造函数参数加默认值：

   MyApp({Key key, this.isDark = false});
   

3. 确保ThemeSettings正确处理布尔参数
   你的ThemeSettings类需要基于传入的布尔值生成对应主题，示例如下：

   class ThemeSettings extends ChangeNotifier {
     bool _isDark;
     ThemeData get currentTheme => _isDark ? ThemeData.dark() : ThemeData.light();
   
     ThemeSettings(bool isDark) : _isDark = isDark;
   
     // 可选：添加切换主题的方法
     void toggleTheme() {
       _isDark = !_isDark;
       notifyListeners();
     }
   }
   

修复后的完整代码 #

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

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
  final isDark = sharedPreferences.getBool("is_dark") ?? false;

  return runApp(
    MyApp(isDark: isDark),
  );
}

class MyApp extends StatelessWidget {
  final bool isDark;
  MyApp({Key key, this.isDark = false});
  
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (context) => ThemeSettings(isDark),
      builder: (context, snapshot) {
        final settings = Provider.of<ThemeSettings>(context);
        return MaterialApp(
          debugShowCheckedModeBanner: false,
          title: 'Flutter Demo',
          theme: settings.currentTheme,
          home: MyHomePage(),
        );
      },
    );
  }
}

class ThemeSettings extends ChangeNotifier {
  bool _isDark;
  ThemeData get currentTheme => _isDark ? ThemeData.dark() : ThemeData.light();

  ThemeSettings(bool isDark) : _isDark = isDark;

  void toggleTheme() {
    _isDark = !_isDark;
    notifyListeners();
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("暗黑模式演示")),
      body: Center(
        child: ElevatedButton(
          onPressed: () {
            Provider.of<ThemeSettings>(context, listen: false).toggleTheme();
          },
          child: Text("切换主题"),
        ),
      ),
    );
  }
}

内容的提问来源于stack exchange，提问作者Hum Tum