Adaptive UI Talk: Example of handling light mode and dark mode in Cupertino.

main.dart

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart' show Icons;

void main() {
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  Brightness _brightness = Brightness.light;

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
    _brightness = WidgetsBinding.instance.window.platformBrightness;
  }

  @override
  void didChangePlatformBrightness() {
    if (mounted) {
      setState(() {
        _brightness = WidgetsBinding.instance.window.platformBrightness;
      });
    }

    super.didChangePlatformBrightness();
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return CupertinoApp(
      debugShowCheckedModeBanner: false,
      theme: CupertinoThemeData(
        brightness: _brightness,
      ),
      home: const CupertinoPageScaffold(
        navigationBar: CupertinoNavigationBar(
          middle: Text('Flutter Forward 2023'),
        ),
        child: _MyPage(),
      ),
    );
  }
}

class _MyPage extends StatelessWidget {
  const _MyPage();

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Icon(
        CupertinoTheme.of(context).brightness == Brightness.light
            ? Icons.light_mode
            : Icons.dark_mode,
        color: const CupertinoDynamicColor.withBrightness(
          color: CupertinoColors.black,
          darkColor: CupertinoColors.white,
        ).resolveFrom(context),
        size: 100.0,
      ),
    );
  }
}