grammar_level.dart

  Widget createLevelMenu() {
    return PopupMenuButton<GrammarLevel>(
      color: kBaseThemeData.primaryColor,
      onSelected: _selectLevel,
      itemBuilder: (BuildContext context) {
        return GrammarLevel.values.map((level) {
          String title = level.toString().split('.').last;
          return PopupMenuItem<GrammarLevel>(
            value: level,
            child: Text(
              title,
              style: TextStyle(color: Colors.white),
            ),
          );
        }).toList();
      },
    );
  }

  void _selectLevel(GrammarLevel level) {
    _level = level;
    _grammarManager.getGrammarItemList(grammarLevel: _level).then((list) {
      setState(() {
        _grammarItemList.clear();
        _grammarItemList.addAll(list);
      });
    });
  }