DropdownButton 下拉按钮

用于下拉选择的按钮，可指定图标、影深、提示等属性，接收选中变化的事件。

相关组件

DropdownButtonHideUnderline

DropdownButton基本用法

   <br />【value】 : 当前值   【T】<br />【items】 : 下拉选框   【List<DropdownMenuItem<T>>】<br />【icon】 : 图标   【Widget】<br />【elevation】 : 影深   【double】<br />【onChanged】 : 选择条目事件   【Function(T)】<br />【backgroundColor】 : 背景色   【Color】<br />![94.gif](https://cdn.nlark.com/yuque/0/2020/gif/326147/1589446312315-8758cdd5-88a3-4beb-8955-116a23f47c41.gif#align=left&display=inline&height=189&margin=%5Bobject%20Object%5D&name=94.gif&originHeight=189&originWidth=404&size=206429&status=done&style=none&width=404)

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

class _CustomDropDownButtonState extends State<CustomDropDownButton> {
  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,
        ),
        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();
}

DropdownButton的样式指定

   <br />【isDense】 : 是否紧排   【bool】<br />【iconSize】 : 图标大小   【double】<br />【hint】 : 提示组件   【Widget】<br />【iconEnabledColor】 : 图标颜色   【Color】<br />![95.gif](https://cdn.nlark.com/yuque/0/2020/gif/326147/1589446397267-ad3a4e4b-0035-4aee-8062-2fc2feec6dd6.gif#align=left&display=inline&height=337&margin=%5Bobject%20Object%5D&name=95.gif&originHeight=337&originWidth=404&size=341959&status=done&style=none&width=404)

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

class _StyleDropDownButtonState extends State<StyleDropDownButton> {
  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(
      crossAxisAlignment: WrapCrossAlignment.center,
      children: <Widget>[
        Container(
          margin: EdgeInsets.symmetric(horizontal: 20),
          width: 50,
          height: 50,
          color: _color??Colors.transparent,
        ),
        DropdownButton<Color>(
            hint: Text('请选择'),
            isDense: true,
            iconSize:20,
            iconEnabledColor:_color??Colors.orange,
            value: _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();
}