有状态组件

StatefulWidget 是有状态组件类。

我们需要改变页面的状态就需要使用这个类：

import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
        return MaterialApp(
            title: 'Retrieve Text Input',
            home: Scaffold(
                appBar: AppBar(
                    title: Text('Hello Flutter'),
                ),
                body: MyCustomForm(),
            ),
        );
    }
}
// StatefulWidget 有状态组件
class MyCustomForm extends StatefulWidget {
    @override
    _MyCustomFormState createState() => _MyCustomFormState();
}
class _MyCustomFormState extends State<MyCustomForm> {
    int countNum = 0;
    @override
    Widget build(BuildContext context) {
        return Center(
            child: Column(
                children: [
                    Text('$countNum'),
                    ElevatedButton(
                        child: Text('按钮'),
                        onPressed: () {
                            // 注意 ， 需要调用 setState
                            setState(() {
                                this.countNum++;
                            });
                        },
                    ),
                ],
            ),
        );
    }
}