如何在注册表单的下拉列表中检查值

Question

我的应用程序中有一个注册表单，我想检查DropDown值是否为空。所以我会给屏幕一个警告。但我不能在checkFieldStatus函数中使用DropDown值。我怎么能拿到这个？

这些是我在应用程序中使用的代码：

class Register extends StatefulWidget {
@override
_RegisterState createState() => _RegisterState();
}
class _RegisterState extends State<Register> {
List listGender = ["Erkek", "Kız"];
List listTeacher = ["Oğulcan Baybars", "Kübra Yeşilkazak"];
final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
String genderHolder;
String teacherHolder;
var _imageFile = null;
@override
Widget build(BuildContext context) {
return SingleChildScrollView(
child: Form(
key: _formKey,
child: Column(
children: [
CustomDropDownField(
list: listGender,
hintText: "Cinsiyet",
value: genderHolder,
),
CustomDropDownField(
list: listTeacher,
hintText: "Öğretmeniniz",
value: teacherHolder,
),
ElevatedButton(
onPressed: () {
checkFieldStatus();
},
child: Text("Kayıt Ol")),
],
),
),
);
}
Future<void> checkFieldStatus() async {
if (_imageFile != null) {
showDialog(
context: context,
builder: (context) {
return ErrorAlertDialog(
message: "Resim yüklendi",
);
});
} else { 
**Where I want to do the checks**
? registerUser()
: displayDialog("Lütfen formdaki bütün alanları doldurun.";
}
}
}

我的自定义下拉字段如下：

import 'package:flutter/material.dart';
class CustomDropDownField extends StatefulWidget {
final List list;
final String hintText;
String value;
CustomDropDownField({
Key key,
this.list,
this.hintText,
this.value,
}) : super(key: key);
@override
_CustomDropDownFieldState createState() => _CustomDropDownFieldState();
}
class _CustomDropDownFieldState extends State<CustomDropDownField> {
@override
Widget build(BuildContext context) {
return Container(
child: DropdownButton(
isExpanded: true,
hint: Text(widget.hintText),
items: widget.list.map((valueItem) {
return DropdownMenuItem(value: valueItem, child: Text(valueItem));
}).toList(),
value: widget.value,
onChanged: (newValue) {
setState(() {
widget.value = newValue;
});}),);}

Answer 1

CustomDropDownField只更改其自身状态下的String值，它不会反映到_RegisterState屏幕上。您可以做一些不同的事情：

1. 传递一个回调函数，更新_RegisterState屏幕中的值

或更好的

2. 使用类似Provider或Bloc的状态管理来更新值