Dropdownbutton

import 'package:flutter/material.dart';
import 'dart:ui';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: LearnDropdownButton(),
    );
  }
}

class LearnDropdownButton extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _LearnDropdownButton();
  }
}

class _LearnDropdownButton extends State<LearnDropdownButton> {
  List<DropdownMenuItem> getListData() {
    List<DropdownMenuItem> items = List();

    DropdownMenuItem dropdownMenuItem1 = DropdownMenuItem(
      child: Text('1'),
      value: '1',
    );
    items.add(dropdownMenuItem1);

    DropdownMenuItem dropdownMenuItem2 = DropdownMenuItem(
      child: Text('2'),
      value: '2',
    );
    items.add(dropdownMenuItem2);

    DropdownMenuItem dropdownMenuItem3 = DropdownMenuItem(
      child: Text('3'),
      value: '3',
    );
    items.add(dropdownMenuItem3);

    return items;
  }

  var value;

/*  _LearnDropdownButton(){
    value=getListData()[0].value;
  }*/

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Column(
        children: <Widget>[
          Center(
            child: DropdownButton(
              items: getListData(),
              hint: Text('请选择：'), //当没有默认值的时候可以设置的提示
              value: value, //下拉菜单选择完之后显示给用户的值
              onChanged: (T) {
                //下拉菜单item点击之后的回调
                setState(() {
                  value = T;
                });
              },
              elevation: 10, //设置阴影的高度
              style: TextStyle(
                  //设置文本框里面文字的样式
                  fontSize: 18,
                  fontWeight: FontWeight.bold,
                  color: Colors.red),
//              isDense: false,//减少按钮的高度。默认情况下，此按钮的高度与其菜单项的高度相同。如果isDense为true，则按钮的高度减少约一半。 这个当按钮嵌入添加的容器中时，非常有用
              iconSize: 70.0, //设置三角标icon的大小
            ),
          ),
        ],
      ),
    );
  }
}