Flutter下拉值如果未选中，则指定初始值

Question

我有一个DropDownButtonFormField，我需要检查一下。如果用户未选择下拉值，则在提交时分配初始值。

自定义下拉菜单

Container myDropDownContainer(String initialVal, List<String> listItems,
String text, Function myFunc, Function validate) {
return Container(
margin: const EdgeInsets.all(8),
child: Row(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.start,
children: [
SizedBox(
width: 120,
child: Text(
text,
style: kTextStyle,
),
),
const SizedBox(
width: 20,
),
Expanded(
child: Container(
height: 50,
decoration: BoxDecoration(
color: Colors.orangeAccent,
borderRadius: BorderRadius.circular(5)),
child: DropdownButtonFormField<String>(
autovalidateMode: AutovalidateMode.always,
//menuMaxHeight: 300,
validator: (value) {
if(value!.isEmpty) {
return "485s4a8sd4as85";
}
} ,
decoration: const InputDecoration(border: InputBorder.none),
isExpanded: true,
onTap: () => myFunc,
//borderRadius: BorderRadius.circular(5),
value: initialVal,
icon: const Icon(
Icons.arrow_downward,
color: Colors.black38,
),
iconSize: 24,
elevation: 16,
dropdownColor: Colors.deepOrange,
style: kTextStyle.copyWith(color: Colors.black),
onChanged: (val) => myFunc(val),
items: listItems.map<DropdownMenuItem<String>>((String? val) {
return DropdownMenuItem(
//TODO: Set default values
value: val,
child: Text(
val,
style: kTextStyle.copyWith(color: Colors.black),
),
);
}).toList(),
),
),
)
],
),
);
}

这是我的onChanged属性，由用户指定选定的值。我添加了一些关于我试图做什么的解释。

String _valueCinsiyet = "Diğer"; // initial value
void onChangedCinsiyet(String? newVal) {
setState(() {
if(newVal==null) {
_formData.setCinsiyet(_valueCinsiyet);
/*
'if newVal is null' means that if the value is not selected by the user
then set the initialValue( _valueCinsiyet)
*/
} else {
/*
if newVal is not null then assign the newVal( which means the selected value)
into my initialValue, then set the data to use it on different pages. What is missing?
*/
_valueCinsiyet = newVal;
_formData.setCinsiyet(_valueCinsiyet);
}
});
}

Answer 1

您可以使用可为null的数据来跟踪DropdownButtonFormField的更改。由于可以为null，您可以检查它是否为null，不需要在onChanged:上做任何额外的操作，只需按常规方式分配新值即可。

建立前状态开启方法：String? value; // value to keep track

child: DropdownButtonFormField<String>(
value: value,
onChanged: (val) {
setState(() {
value = val;
});
},

现在onSaved/submit按钮可以通过检查null来传递值，简单的方法是

value?? "default Value"。在您的情况下，它是value??Diğer