StatefulBuilder({
  Key? key, 
  required StatefulWidgetBuilder builder
r})

builder: (context, setInnerState) => SomeWidget(/* ...*/)

// main.dart
import 'package:flutter/material.dart';
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void main() {
  runApp(const MyApp());
}
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class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
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  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // Remove the debug banner
      debugShowCheckedModeBanner: false,
      title: '大前端之旅',
      theme: ThemeData(
        primarySwatch: Colors.amber,
      ),
      home: const HomePage(),
    );
  }
}
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// StatelessWidget is used here
class HomePage extends StatelessWidget {
  const HomePage({Key? key}) : super(key: key);
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  @override
  Widget build(BuildContext context) {
    // This is the number shown in the red circle
    // It represents state and stays outside the builder function
    int count = 0;
    return Scaffold(
      appBar: AppBar(title: const Text('大前端之旅')),
      body: Padding(
        padding: const EdgeInsets.all(30),
        child: Center(
          // Implement StatefulBuilder
          child: StatefulBuilder(
            builder: (context, setInnerState) => Column(
              children: [
                Container(
                  width: 300,
                  height: 300,
                  decoration: const BoxDecoration(
                      color: Colors.red, shape: BoxShape.circle),
                  child: Center(
                    // Display the number
                    child: Text(
                      count.toString(),
                      style: const TextStyle(fontSize: 80, color: Colors.white),
                    ),
                  ),
                ),
                const SizedBox(
                  height: 20,
                ),
                // This button is used to crease the number
                ElevatedButton.icon(
                    onPressed: () {
                      // Call setInnerState function
                      setInnerState(() => count++);
                    },
                    icon: const Icon(Icons.add),
                    label: const Text('Increase By One')),
              ],
            ),
          ),
        ),
      ),
    );
  }
}