我想像Flutter 中的DropdownButtonSpinner 一样在下面打开 DropDown 对话框。现在它在 Button 小部件上打开，当我选择最后一个项目并重新打开时。
代码：
import 'package:flutter/material.dart';
class DropDown extends StatefulWidget {
DropDown() : super();
final String title = "DropDown Demo";
@override
DropDownState createState() => DropDownState();
}
class Company {
int id;
String name;
Company(this.id, this.name);
static List<Company> getCompanies() {
return <Company>[
Company(1, 'Apple'),
Company(2, 'Google'),
Company(3, 'Samsung'),
Company(4, 'Sony'),
Company(5, 'LG'),
];
}
}
class DropDownState extends State<DropDown> {
//
List<Company> _companies = Company.getCompanies();
List<DropdownMenuItem<Company>> _dropdownMenuItems;
Company _selectedCompany;
@override
void initState() {
_dropdownMenuItems …