stateful_mixin.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp(name: 'Dash'));
}

class MyApp extends StatelessWidget {
  const MyApp({super.key, required this.name});

  final String name;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Counter(
        name: name,
        startValue: 10,
      ),
    );
  }
}

// The stateful parts of the counter class
mixin CounterState on State<Counter> {
  int count = 0;
}

class Counter extends GeneratedStatefulWidget<_GeneratedCounterState> {
  const Counter({super.key, required this.name, required this.startValue});

  final String name;
  final int startValue;

  @override
  Widget build(CounterState state) {
    final ThemeData theme = Theme.of(state.context);
    return Scaffold(
      appBar: AppBar(title: Text('Hello $name', style: theme.textTheme.bodyLarge)),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          state.count += 1;
        },
      ),
      body: Center(
        child: Text('Count: ${startValue + state.count}'),
      ),
    );
  }

  //////////////// GENERATED ///////////////////////////

  @override
  State<StatefulWidget> createState() => _GeneratedCounterState();
}

abstract class GeneratedStatefulWidget<S extends State> extends StatefulWidget {
  const GeneratedStatefulWidget({super.key});

  Widget build(S state);
}

class _GeneratedCounterState extends State<Counter> with CounterState {
  @override
  set count(int value) {
    // Since this is generated code, it can be optimized to not incur the cost
    // of an extra closure. Instead, it could just directly call markNeedsBuild(),
    // which is way cheaper.
    setState(() {
      super.count = value;
    });
  }

  @override
  Widget build(BuildContext context) {
    return widget.build(this);
  }
}