rrousselGit/functions_vs_class_performance.dart

import 'package:flutter/material.dart';

// This example showcases how extracting widgets into StatelessWidgets
// instead of functions can improve performances, by rebuilding
// only what needs to update when the state changes

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(context) {
    return Counter(
      count: Count(),
      child: MaterialApp(
        home: Home(),
      ),
    );
  }
}

class Count extends ValueNotifier<int> {
  Count() : super(0);
}

class Counter extends InheritedNotifier {
  const Counter({
    Key? key,
    required this.count,
    required Widget child,
  }) : super(key: key, child: child, notifier: count);

  final Count count;

  static Count of(BuildContext context, {bool listen = true}) {
    if (listen) {
      return context.dependOnInheritedWidgetOfExactType<Counter>()!.count;
    } else {
      final Counter counter =
          context.getElementForInheritedWidgetOfExactType<Counter>()!.widget as Counter;
      return counter.count;
    }
  }
}

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('Home build');
    return Scaffold(
      body: Center(
        // By extracting Title in a StatelessWidget, Home doesn't rebuild when the counter changes
        child: Title(),
        // Uncommenting this code causes Home to rebuild when the counter changes
        // child: title(context)
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () => Counter.of(context, listen: false).value++,
        child: const Icon(Icons.add),
      ),
    );
  }
}

class Title extends StatelessWidget {
  @override
  Widget build(context) {
    print('build Title');
    return Text('Count ${Counter.of(context).value}');
  }
}

Widget title(context) {
  print('build Title');
  return Text('Count ${Counter.of(context).value}');
}