下拉选择框

 DropdownButtonHideUnderline(
        child: DropdownButton<String>(
        value: 'One',
        onChanged: null,
        items: <String>['One', 'Two', 'Free', 'Four']
            .map<DropdownMenuItem<String>>((String value) {
          return DropdownMenuItem<String>(
            value: value,
            child: Text(value),
          );
        }).toList(),
      ),
    )