DropdownButtonHideUnderline 下拉按钮隐藏线

用于去除DropdownButton的下划线，本身没有什么应用价值。

相关组件

DropdownButton

DropDownButtonHideUnderline使用

   <br />【child】 : 子组件   【Widget】,<br />![217.gif](https://cdn.nlark.com/yuque/0/2020/gif/326147/1589512378305-0e59db17-a04c-4a63-a838-a089018e4cbe.gif#align=left&display=inline&height=369&margin=%5Bobject%20Object%5D&name=217.gif&originHeight=369&originWidth=402&size=184418&status=done&style=none&width=402)

import 'package:flutter/material.dart';
class CustomDropDownButtonHideUnderline extends StatefulWidget {
  @override
  _CustomDropDownButtonHideUnderlineState createState() =>
      _CustomDropDownButtonHideUnderlineState();
}

class _CustomDropDownButtonHideUnderlineState
    extends State<CustomDropDownButtonHideUnderline> {
  Color _color = Colors.red;
  final _colors = [Colors.red, Colors.yellow, Colors.blue, Colors.green];
  final _info = ["红色", "黄色", "蓝色", "绿色"];

  @override
  Widget build(BuildContext context) {
    return Wrap(
      children: <Widget>[
        Container(
          margin: EdgeInsets.symmetric(horizontal: 20),
          width: 50,
          height: 50,
          color: _color,
        ),
        DropdownButtonHideUnderline(
          child: DropdownButton<Color>(
              value: _color,
              elevation: 1,
              icon: Icon(
                Icons.expand_more,
                size: 20,
                color: _color,
              ),
              items: _buildItems(),
              onChanged: (v) => setState(() => _color = v)),
        ),
      ],
    );
  }

  List<DropdownMenuItem<Color>> _buildItems() => _colors
      .map((e) => DropdownMenuItem<Color>(
          value: e,
          child: Text(
            _info[_colors.indexOf(e)],
            style: TextStyle(color: e),
          )))
      .toList();
}