// BAD: Widget rebuilds unnecessarily
class BadExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        ExpensiveWidget(),
        AnotherWidget(),
      ],
    );
  }
}

// GOOD: Use const constructors
class GoodExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return const Column(
      children: [
        ExpensiveWidget(),
        AnotherWidget(),
      ],
    );
  }
}

// GOOD: Extract static widgets
class OptimizedExample extends StatelessWidget {
  static const _staticWidget = ExpensiveWidget();

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        _staticWidget,
        AnotherWidget(),
      ],
    );
  }
}

// BAD: Creates all widgets upfront
ListView(
  children: items.map((item) => ItemWidget(item)).toList(),
)

// GOOD: Lazy loading with builder
ListView.builder(
  itemCount: items.length,
  itemBuilder: (context, index) {
    return ItemWidget(items[index]);
  },
)

// BEST: With separator and const
ListView.separated(
  itemCount: items.length,
  separatorBuilder: (context, index) => const Divider(),
  itemBuilder: (context, index) {
    return ItemWidget(items[index]);
  },
)

// Cached network images
CachedNetworkImage(
  imageUrl: 'https://example.com/image.jpg',
  placeholder: (context, url) => const CircularProgressIndicator(),
  errorWidget: (context, url, error) => const Icon(Icons.error),
  memCacheWidth: 600, // Resize in memory
  memCacheHeight: 600,
)

// Precache images
@override
void didChangeDependencies() {
  super.didChangeDependencies();
  precacheImage(AssetImage('assets/large_image.png'), context);
}

// Use appropriate image formats
// WebP for better compression
// SVG for scalable graphics (flutter_svg package)

// Dispose controllers
class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  late ScrollController _scrollController;
  late StreamSubscription _subscription;

  @override
  void initState() {
    super.initState();
    _scrollController = ScrollController();
    _subscription = someStream.listen((data) {/* ... */});
  }

  @override
  void dispose() {
    _scrollController.dispose();
    _subscription.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return ListView(controller: _scrollController);
  }
}

// Isolate expensive widgets from parent rebuilds
RepaintBoundary(
  child: ExpensiveAnimatedWidget(),
)

// Custom painter optimization
RepaintBoundary(
  child: CustomPaint(
    painter: MyComplexPainter(),
    child: Container(),
  ),
)

// Use compute for heavy calculations
Future<List<Photo>> fetchPhotos() async {
  final response = await http.get(Uri.parse('https://api.example.com/photos'));
  return compute(parsePhotos, response.body);
}

List<Photo> parsePhotos(String responseBody) {
  final parsed = jsonDecode(responseBody).cast<Map<String, dynamic>>();
  return parsed.map<Photo>((json) => Photo.fromJson(json)).toList();
}

// Use Isolates for long-running tasks
Future<void> runInIsolate() async {
  final result = await Isolate.run(() {
    // Heavy computation
    return heavyComputation();
  });
}

# Run with performance profiling
flutter run --profile

# Use DevTools
flutter pub global activate devtools
flutter pub global run devtools

# Performance overlay in app
MaterialApp(
  showPerformanceOverlay: true,
  // ...
)