Flutter笔记39：CheckedPopupMenuItem可勾选的弹出菜单

CheckedPopupMenuItem可勾选的弹出菜单，常常与PopupMenuButton，作为PopupMenuButton构造方法的元素：

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'CheckedPopupMenuItem可勾选的弹出菜单',
      home: Scaffold(
        appBar: AppBar(
          title: Text('CheckedPopupMenuItem可勾选的弹出菜单'),
        ),
        body: DemoPage(),
      ),
    );
  }
}

//CheckedPopupMenuItem可勾选的弹出菜单，常常与PopupMenuButton，作为PopupMenuButton构造方法的元素

class DemoPage extends StatefulWidget {
  DemoPage({Key key}) : super(key: key);

  @override
  _DemoPageState createState() => _DemoPageState();
}

class _DemoPageState extends State<DemoPage> {
  List<String> _checkedValues;

  final String _checkedValue1 = '1';
  final String _checkedValue2 = '2';
  final String _checkedValue3 = '3';
  final String _checkedValue4 = '4';

  @override
  void initState() {
    //初始化选中的值
    _checkedValues = <String>[_checkedValue1];
    super.initState();
  }

  bool isChecked(String value) => _checkedValues.contains(value);

  //将选中的值保存在 _checkedValues 中
  void showCheckedMenuSelections(String value) {
    if (_checkedValues.contains(value)) {
      _checkedValues.remove(value);
    } else {
      _checkedValues.add(value);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Theme.of(context).primaryColor,
      child: ListTile(
        title: Text(
          '有选择标题的弹出菜单',
          style: TextStyle(color: Colors.white),
        ),
        trailing: PopupMenuButton<String>(
          padding: EdgeInsets.zero,
          onSelected: showCheckedMenuSelections,
          icon: Icon(
            Icons.menu,
            color: Colors.white,
          ),
          itemBuilder: (BuildContext context) => [
            CheckedPopupMenuItem<String>(
              value: _checkedValue1,
              //是否选中，通过 isChecked 判断 _checkedValues 中是否保存 _checkedValue1
              checked: isChecked(_checkedValue1),
              child: Text(_checkedValue1),
            ),
            CheckedPopupMenuItem<String>(
              value: _checkedValue2,
              //当前是否可选
              enabled: false,
              checked: isChecked(_checkedValue2),
              child: Text(_checkedValue2),
            ),
            CheckedPopupMenuItem<String>(
              value: _checkedValue3,
              checked: isChecked(_checkedValue3),
              child: Text(_checkedValue3),
            ),
            CheckedPopupMenuItem<String>(
              value: _checkedValue4,
              checked: isChecked(_checkedValue4),
              child: Text(_checkedValue4),
            ),
          ],
        ),
      ),
    );
  }
}

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