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  static const menuItems = <String>[
    'One',
    'Two',
    'Three',
    'Four',
  ];  

final List<PopupMenuItem<String>> _popUpMenuItems = menuItems
      .map(
        (String value) => PopupMenuItem<String>(
          value: value,
          child: Text(value),
        ),
      )
      .toList();

  String _btn3SelectedVal;

PopupMenuButton<String>(
    onSelected: (String newValue) {
      _btn3SelectedVal = newValue;
      Scaffold.of(context).showSnackBar(
        SnackBar(
          content: Text('$_btn3SelectedVal'),
        ),
      );
    },
    itemBuilder: (BuildContext context) => _popUpMenuItems,
  ),