我有一个ViewTotalItemProvider类，它扩展了ChangeNotifier。在课程内部，有一个像这样的列表。
class ViewTotalItemProvider extends ChangeNotifier{
List<CartPlantLists> cartPlantList3 = [];
}
此外，还有3个屏幕，包括PlantFeatureScreen1，ParticularPlant2，CartDetais3之类的类。所有这些都是有状态的小部件，我将在第二个屏幕中添加一些项目，即ParticularPlant2类。
当我尝试在第二个屏幕和第三个屏幕中显示列表中的项目时，它起作用了。
但是，该值不会在firstScreen（即PlantFeatureScreen1）中更新。但是，当我重新加载应用程序时，它会显示更新后的值。
为什么会这样呢？我该怎么解决？
代码
ViewTotalItemProvider
List<CartPlantLists> cartPlantList3 = [];
class ViewTotalItemProvider extends ChangeNotifier{
addQuantity(index){
cartPlantList3[index].qu++;
notifyListeners();
}
subtrachQuantity(index){
cartPlantList3[index].qu--;
notifyListeners();
}
}
firstScreen PlantFeatureScreen1（在这里我要更新最后一个小部件中的值）
class PlantFeatureScreen1 extends StatefulWidget {
@override
_PlantFeatureScreen1State createState() => _PlantFeatureScreen1State();
}
class _PlantFeatureScreen1State extends State<PlantFeatureScreen1> {
@override
Widget build(BuildContext context) {
return ChangeNotifierProvider<ViewTotalItemProvider>(
create: (context) => ViewTotalItemProvider(),
child:Builder(builder: (context) {
return Column(
children: <Widget>[
TopAppBar(),
Expanded(
flex: 1,
child: Align(
alignment: Alignment(-1, 0),
child: Container(
decoration: BoxDecoration(
color: Colors.white,
),
child: Text(
"Plants",
style: TextStyle(fontSize: 30, fontWeight: FontWeight.w700),
),
),
),
),
Expanded(
flex: 5,
child: Container(
width: double.infinity,
decoration: BoxDecoration(
color: Colors.blue,
),
child: DefaultTabController(
length: 5,
child: Column(
children: [
Container(
height: 50,
width: double.infinity,
child: TabBar(
isScrollable: true,
tabs: ourAllLists.tabMaker(),
),
),
Container(
height: 317,
width: double.infinity,
decoration: BoxDecoration(color: Colors.white),
child: TabBarView(
children: ourAllLists.tabViewerMaker(context),),),
],
),
),
),
),
Padding(
padding: const EdgeInsets.fromLTRB(20, 0, 20, 20),
child: Container(
alignment: Alignment.bottomRight,
height: 120,
width: double.infinity,
child: Stack(
overflow: Overflow.visible,
children: [
Container(
height: 70,
width: 105,
decoration: BoxDecoration(
color: Color(0xFF96CA2D),
borderRadius: BorderRadiusDirectional.horizontal(
end: Radius.circular(32),
start: Radius.circular(32))),
child: Icon(FontAwesomeIcons.shoppingBag,color:Colors.white,size:30),
),
Positioned(
// top: 0,
bottom: 50,
right: 0,
child: Consumer<ViewTotalItemProvider>(
builder: (context, value, child){
return Container(
height: 35,
width: 35,
decoration: BoxDecoration(
color: Colors.white,
borderRadius: BorderRadius.circular(50),
border: Border.all(color: Color(0xFF96CA2D),width: 4)
),
child: Center(child: Text(ourAllLists.totalquantity().toString(),style:TextStyle(fontSize: 20,color: Color(0xFF96CA2D)))),
);
}),
),
],
),
),
)
],
);
})
);
}
}
secondScreen ParticularPlant2
class ParticularPlant2 extends StatefulWidget {
final indexNumber;
ParticularPlant2({@required this.indexNumber});
@override
_ParticularPlant2State createState() => _ParticularPlant2State();
}
class _ParticularPlant2State extends State<ParticularPlant2> {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: SafeArea(
child: Scaffold(
body: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
TopAppBar(),
Container(
decoration: BoxDecoration(
color: Colors.red,
borderRadius: BorderRadiusDirectional.only(
bottomStart: Radius.circular(50),
),
),
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Text(
ourAllLists
.mainListAllPlantDetailsList1[widget.indexNumber].pN,
style: kPlantNameStyle,
),
Text(
ourAllLists
.mainListAllPlantDetailsList1[widget.indexNumber].ca
.toUpperCase(),
style: TextStyle(
fontSize: 15,
),
),
Text(
"\$" +
ourAllLists
.mainListAllPlantDetailsList1[widget.indexNumber]
.pr
.toString(),
style: kItemPrice,
),
SizedBox(height: 100),
Row(
mainAxisAlignment: MainAxisAlignment.spaceAround,
children: [
Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
Container(
height: 80,
width: 80,
decoration: BoxDecoration(
color: Colors.white,
borderRadius: BorderRadius.circular(50)),
child: Icon(
FontAwesomeIcons.flag,
color: Color(0xFF9DCD3C),
),
),
SizedBox(
height: 50,
),
FlatButton(
onPressed: () {
final tile = cartPlantList3.firstWhere(
(item) =>
item.pN ==
ourAllLists
.mainListAllPlantDetailsList1[
widget.indexNumber]
.pN,
orElse: () => null);
if (tile != null) {
} else {
cartPlantList3.add(
CartPlantLists(
quantity: 1,
plantName: ourAllLists
.mainListAllPlantDetailsList1[
widget.indexNumber]
.pN,
category: ourAllLists
.mainListAllPlantDetailsList1[
widget.indexNumber]
.ca,
price: ourAllLists
.mainListAllPlantDetailsList1[
widget.indexNumber]
.pr,
),
);
}
print(cartPlantList3.length);
},
child: Container(
height: 80,
width: 80,
decoration: BoxDecoration(
color: Color(0xFF9DCD3C),
borderRadius: BorderRadius.circular(50)),
child: Icon(FontAwesomeIcons.shoppingBag,
color: Colors.white),
),
)
],
),
Container(
height: 250,
child: Image(image: AssetImage("assets/tulip.png")),
)
],
)
],
),
)
],
),
),
),
);
}
}
答案 0 :(得分：2)
似乎您使用提供者的方式错误。在您的方案中，执行此操作的最佳方法是使用MultiProvider将MaterialApp包裹在main.dart文件中的MyApp（）中。尝试如下操作：https://pub.dev/packages/provider#multiprovider您可以在其中放置一个ChangeNotifierProvider。
return MultiProvider(
providers: [
ChangeNotifierProvider<ViewTotalItemProvider>(
create: (context) => ViewTotalItemProvider()),
],
child: MaterialApp(...)
);
此外，您还必须在模型中放置吸气剂和吸气剂。这是一个示例：
class ImageModel extends ChangeNotifier {
String _base64Image;
get base64Image => _base64Image;
set base64Image(String base64Image) {
_base64Image = base64Image;
notifyListeners();
}
}
我还建议您使用Selector代替Consumer（理想情况下，您应该使用Selector代替Consumer，这样才使小部件仅在其侦听的值发生更改时才重建）这是基于上述模型的示例：
@override
Widget build(BuildContext context) {
//other widgets
Selector<ImageModel, String>(
selector: (_, model) => model.base64Image,
builder: (_, image, __) {
return Text(image);
},
);
}
)
}
以下是使用RaisedButton进行设置的方法：
class _PlantFeatureScreen1State extends State<PlantFeatureScreen1> {
final itemModel;
List<CartPlantLists> myList=[];
@override
Widget build(BuildContext context) {
itemModel = Provider.of<ViewTotalItemProvider>(context,listen:false);
print(itemModel.yourVariable); //getting the value
return Container(
child: RaisedButton(
child:Text("Set Item");
onPressed:(){
itemModel.yourVariable=myList; //setting the value
},
),
);
}
}
希望这会有所帮助！祝你好运！
答案 1 :(得分：2)
步骤1： 在 pubspec.yaml 文件
中添加提供者模式的依赖项dependencies:
flutter:
sdk: flutter
provider: ^4.1.2
步骤2： 在单独的文件中创建提供程序：
class ViewTotalItemProvider with ChangeNotifier{
List<CartPlantLists> _cartPlantList1 = [];
get cartPlantList1 => _cartPlantList1 ;
set cartPlantList1 (List<CartPlantLists> selected){
_cartPlantList1 = selected;
notifyListeners();
}
}
步骤3： 使用MultiProvider将MaterialApp小部件包装在 main.dart 中。
void main() => runApp(
MultiProvider (providers: [
ChangeNotifierProvider<ViewTotalItemProvider>.value(value:
ViewTotalItemProvider()),
],
child: MyApp()
)
);
class MyApp extends StatelessWidget {
Widget build(BuildContext context) {
return MaterialApp(
home: HomePage(),
);
}
}
第4步： 在屏幕PlantFeatureScreen1中使用提供程序：
class PlantFeatureScreen1 extends StatefulWidget {
@override
_PlantFeatureScreen1State createState() => _PlantFeatureScreen1State();
}
class _PlantFeatureScreen1State extends State<PlantFeatureScreen1> {
var viewTotalItemProvider;
@override
Widget build(BuildContext context) {
viewTotalItemProvider = Provider.of<ViewTotalItemProvider>(context);
return Scaffold(
.......
);
}
}
步骤5： 获取cartPlantList1。
List<CartPlantLists> list = viewTotalItemProvider.cartPlantList1;
第6步：设置cartPlantList1。
List<CartPlantLists> list = [];
...
viewTotalItemProvider.cartPlantList1 = list;
类似地，您可以将其用于其他两个类。