为什么以下flutter代码不能正常运行？

Question

希望点击按钮以后可以看到数字加一

代码：

import 'package:flutter/material.dart';

class Choose extends StatefulWidget {
  @override
  _ChooseState createState() => _ChooseState();
}

class _ChooseState extends State<Choose> {
  @override
  void initState() {
    print('init');
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    int count = 0;
    print('build');
    return Scaffold(
        appBar: AppBar(
            backgroundColor: Colors.grey[200],
            title: Text('choose'),
            centerTitle: true,
            elevation: 0),
        body: Column(children: <Widget>[
          RaisedButton(
              onPressed: () {
                setState(() {
                  count++;
                });
              },
              child: Text('count add')),
          Text('$count')
        ]));
  }

  @override
  void dispose() {
    print('dispose');
    super.dispose();
  }
}

Answer 1

原来count要写在外面
不然每次build都赋值为0了