为什么futureBuilder返回null值

Question

当我尝试在主页中获取对象时，我没有收到错误，但即使我在下一个屏幕中使用相同的方法来显示数据库中的对象，FutureBuilder也不能获取任何值。

@override
Widget build(BuildContext context) {
return WillPopScope(
child: SafeArea(
child: Scaffold(
backgroundColor: kColorTheme1,
appBar: AppBar(
centerTitle: true,
automaticallyImplyLeading: false,
elevation: 20,
backgroundColor: Color(0xFFF2C3D4).withOpacity(1),
title:TitleBorderedText(title:"Sevimli Yemekler", textColor: Color(0xFFFFFB00)),
actions: [
CircleAvatar(
radius: 27,
backgroundColor: Colors.transparent,
backgroundImage: AssetImage(kCuttedLogoPath),
),
],
),
body: FutureBuilder<Map>(
future: HiveHelper().getCategoryModels(),
builder: (BuildContext context,AsyncSnapshot<Map> snapshot){
if(snapshot.hasData) {
_list = snapshot.data.values.toList();
}
Future.delayed(Duration(milliseconds: 200));
return Container(
decoration: BoxDecoration(
image: DecorationImage(
image: AssetImage(kBGWithLogoOpacity),
fit: BoxFit.cover,
),
),
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Expanded(
child:GridView.builder(
scrollDirection: Axis.vertical,
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
itemCount: _list.length+1,
itemBuilder: (context,index){
if(_list.length==0){
return EmptyCard(where: "homeScreen",);
}
if(_flag==1){
return EmptyCard(where: "homeScreen",);
}
if(index==_list.length-1){
_flag=1;
CategoryModel categoryModel = _list[index];
return CategoryCard(category: categoryModel);
}
else{
CategoryModel categoryModel =_list[index];
return CategoryCard(category: categoryModel);
}
}
),
),
Column(
mainAxisAlignment: MainAxisAlignment.end,
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Padding(
padding: EdgeInsets.all(10),
child: Container(
decoration: BoxDecoration(
border: Border.all(style: BorderStyle.solid),
color: kColorTheme7,
borderRadius: BorderRadius.circular(40),
),
child: TextButton(
onPressed: (){
showModalBottomSheet(
isDismissible: false,
enableDrag: false,
context: context,
builder: (BuildContext context)=> AddMenuScreen(buttonText: "Menü Ekle",route: "homeScreen",),
);
},
child: TitleBorderedText(title: "LEZZET GRUBU EKLE",textColor: Colors.white,)
),
),
),
],
)
],
),
);
},
),
),
),
onWillPop: ()async{
var response = await showAlertDialog(context);
print(response);
return response;
},
);
}

我可以在这里显示我的对象，不会有任何错误。我使用下面相同的数据库和构建器，但我可以取一个值。

@override
Widget build(BuildContext context) {
return SafeArea(
child: Scaffold(
body: FutureBuilder<Map>(
future: HiveHelper().getCategoryModel(widget.categoryId),
builder: (BuildContext context,AsyncSnapshot<Map> snapshot) {
if(snapshot.hasData) {
_list=snapshot.data.values.toList();
}
Future.delayed(Duration(milliseconds: 200));
return  Scaffold(
appBar: AppBar(
automaticallyImplyLeading: false,
centerTitle: true,
title: BorderedText(
child:Text(
_list[4],
style: TextStyle(
color: Color(0XFFFFFB00),
fontSize: 30,
fontFamily: "OpenSans"
),
),
strokeWidth: 5,
strokeColor: Colors.black,
),
elevation: 5,
backgroundColor: Color(0xFFF2C3D4).withOpacity(1),
leading: IconButton(
icon: Icon(Icons.arrow_back),
onPressed: (){
Navigator.pop(context);
},
iconSize: 40,
color: Color(0xFFA2000B),
),
actions: [
CircleAvatar(
radius: 27,
backgroundColor: Colors.transparent,
backgroundImage: AssetImage("images/cuttedlogo.PNG"),
)
],
),
body:Container(
decoration: BoxDecoration(
image: DecorationImage(
image: AssetImage("images/logoBGopacity.png"),
fit: BoxFit.cover,
),
),
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Expanded(
child: GridView.builder(
scrollDirection: Axis.vertical,
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
itemCount:_list[5].length+1,
itemBuilder: (context,index){
if(_list[5].length==0){
return EmptyCard(where: "subCategoryScreen",categoryId: widget.categoryId,);
}
if(_flag==1){
return EmptyCard(where: "homeScreen",);
}
if(index==_list[5].length-1){
_flag=1;
SubCategoryModel subCategoryModel =SubCategoryModel();
return SubCategoryCard(subCategoryCardId:index,subCategoryId:subCategoryModel.subCategoryId,subcategoryName: subCategoryModel.subCategoryName,
subCategoryImagePath:subCategoryModel.subCategoryImagePath,
subCategoryCardColor: subCategoryModel.categoryColor,);
}
else{
SubCategoryModel subCategoryModel =SubCategoryModel();
return SubCategoryCard(subCategoryCardId:index,subCategoryId:subCategoryModel.subCategoryId,subcategoryName: subCategoryModel.subCategoryName,
subCategoryImagePath:subCategoryModel.subCategoryImagePath,
subCategoryCardColor: subCategoryModel.categoryColor,);
}
}
),
),
Column(
mainAxisAlignment: MainAxisAlignment.end,
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Padding(
padding: EdgeInsets.all(10),
child: Container(
decoration: BoxDecoration(
border: Border.all(style: BorderStyle.solid),
color: kColorTheme7,
borderRadius: BorderRadius.circular(40),
),
child: TextButton(
onPressed: (){
showModalBottomSheet(
isDismissible: false,
enableDrag: false,
context: context,
builder: (BuildContext context)=> AddMenuScreen(categoryId:widget.categoryId,buttonText: "Tarif Ekle", route:"subCategoryScreen"),
);
},
child: BorderedText(
strokeWidth: 5,
strokeColor: Colors.black,
child:Text("Tarif Ekle",style: TextStyle(
color: Colors.white,
fontFamily:'OpenSans',
fontSize:30,
),
),
),
),
),
),
],
)
],
),
),
);
}
),
),
);
}

class HiveHelper{
void addCategoryModel(CategoryModel categoryModel)async{
var box = await Hive.openBox('categoryModelsInBox');
await box.add(categoryModel);
await Hive.close();
}
Future <Map> getCategoryModels()async{
var box = await Hive.openBox('categoryModelsInBox');
var boxToMap=box.toMap();
return boxToMap;
}
Future <Map> getCategoryModel(int index) async {
var box = await Hive.openBox('categoryModelsInBox');
var boxToMap=box.toMap();
return boxToMap[index];
}
void addSubCategory(SubCategoryModel subCategoryModel, key)async{
var box = await Hive.openBox('categoryModelsInBox');
box.put(subCategoryModel,key);
}
}

这是我的蜂房课。如何解决此问题？

Answer 1

不要在"未来：；FutureBuilder的参数。正如《精细手册》的第一大段所说：

future必须提前获得，例如在State.initState、State.didUpdateWidget或State.didChangeDependencies期间。在构造FutureBuilder时，不能在State.build或StatelessWidget.build方法调用期间创建。如果future与FutureBuilder同时创建，那么每次重建FutureBuilders的父级时，异步任务都将重新启动。

一般的指导原则是假设每个构建方法都可以在每帧调用，并将省略的调用视为优化。

https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html

Answer 2

我不确定，但你可以试试：

Future <Map> getCategoryModel(int index) async {
var box = await Hive.openBox('categoryModelsInBox');
var boxToMap=box.toMap();
return boxToMap["$index"];
}