Flutter 例程：经典计数器——理解 StatefulWidget

目标

通过官方经典的计数器例子，理解 Flutter 核心概念：Widget 树、StatefulWidget、setState、MaterialApp。

完整代码

// lib/main.dart
import 'package:flutter/material.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorSchemeSeed: Colors.blue,
        useMaterial3: true,
      ),
      home: const MyHomePage(title: '计数器 Demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({super.key, required this.title});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  void _decrementCounter() {
    setState(() {
      if (_counter > 0) _counter--;
    });
  }

  void _resetCounter() {
    setState(() {
      _counter = 0;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
        centerTitle: true,
        actions: [
          IconButton(
            icon: const Icon(Icons.refresh),
            onPressed: _resetCounter,
            tooltip: '重置',
          ),
        ],
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Text(
              '你已经点了这么多次：',
              style: TextStyle(fontSize: 18),
            ),
            const SizedBox(height: 12),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.displayLarge?.copyWith(
                    fontWeight: FontWeight.bold,
                    color: _counter > 10 ? Colors.red : null,
                  ),
            ),
            const SizedBox(height: 24),
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                FloatingActionButton(
                  onPressed: _decrementCounter,
                  child: const Icon(Icons.remove),
                ),
                const SizedBox(width: 16),
                FloatingActionButton(
                  onPressed: _incrementCounter,
                  child: const Icon(Icons.add),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

运行步骤

flutter create flutter_counter
# 将上面代码覆盖 lib/main.dart
flutter run

预期表现

• 点击 + 按钮计数器加一
• 点击 - 按钮计数器减一（最小为 0）
• 点击 AppBar 刷新图标重置计数器
• 计数器超过 10 时数字变为红色

核心概念

概念	说明
StatelessWidget	无状态组件，属性不变
StatefulWidget	有状态组件，setState() 触发重建
setState()	通知框架状态已变，需要重新 build
MaterialApp	Material Design 根组件
Scaffold	页面脚手架（含 AppBar、body、FAB）