12-flutter-单选和多选框

多看官方文档吧 我看的是2019年的视频，跟着写代码发现了不少错误，又去看官方文档修改，太难了，学不动了！！！

import 'package:flutter/material.dart';
// import 'package:english_words/english_words.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Welcome to Flutter',
      home: new Scaffold(
        appBar: new AppBar(
          title: new Text('Welcome to Flutter12'),
        ),
        body: Column(
          children: <Widget>[
            Image.asset(
            'static/pic/mg3414.jpg',
              width: 200,
              height: 300,
            ),
            Image.asset('static/pic/162.jpg',
            width: 200,),
            Image.network('http://8.129.221.250:3000/uploads/fc3eace4355f22498f5d1008a942da30-NrJn4jqrY5Dd492d82806fe4d6b2a5707d806722350c.png'),
            Icon(Icons.zoom_out_sharp),
            new MySelect(),
            new MySwitchAndCheckBox()
          ]
        ),
      ),
    );
  }
}
// 下拉框
class MySelect extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _MySelect();
  }
}
class _MySelect extends State<MySelect> {
  // List getList() {
  //   List<DropdownMenuItem> lists = [];
  //   lists.add(DropdownMenuItem(child: new Text("上海"), value: 'sh'));
  //   lists.add(DropdownMenuItem(child: new Text("北京"), value: 'bj'));
  //   lists.add(DropdownMenuItem(child: new Text("广州"), value: 'gz'));
  //   lists.add(DropdownMenuItem(child: new Text("深圳"), value: 'sz'));
  //   return lists;
  // }
  var selectValue;
  @override
  Widget build(BuildContext context) {
    return new Column(
      children: <Widget>[
        new  DropdownButton(
          items: [
            (DropdownMenuItem(child: new Text("上海"), value: 'sh')),
            (DropdownMenuItem(child: new Text("北京"), value: 'bj')),
            (DropdownMenuItem(child: new Text("广州"), value: 'gz')),
            (DropdownMenuItem(child: new Text("深圳"), value: 'sz')),
          ],
          hint: new Text('请选择城市'),
          value: selectValue,
          onChanged: (val) {
            setState(() {
              this.selectValue = val;
            });
          }
        )
      ],
    );
  }
}
// 单选和多选
class MySwitchAndCheckBox extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _MySwitchAndCheckBox();
  }
}
class _MySwitchAndCheckBox extends State<MySwitchAndCheckBox> {
  @override
  bool _switchValue = true;
  bool _checkboxValue = false;
  Widget build(BuildContext context) {
    return new Column(
      children: [
        new Switch(value: _switchValue, onChanged: (val) {
          setState(() {
            this._switchValue = val;
          });
        }),
        new Checkbox(value: _checkboxValue, onChanged: (bool? val) {
          setState(() {
            this._checkboxValue = val!;
          });
        })
      ],
    );
  }
}