firebase - 颤振：sharedpreference 检索空值
问题描述
在 null 上调用了方法“getStringList”。Receiver: null 尝试调用：getStringList("userCart") 导致错误的相关小部件是：Consumer。详细信息存储在firebase中，显示用户已添加到购物车的商品数量时发生错误。
CartItemCounter dart 文件是调用消费者时抛出的错误。最初在 firebase 中注册时，useCart 被作为垃圾值给出。
import 'package:flutter_staggered_grid_view/flutter_staggered_grid_view.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:fluttertoast/fluttertoast.dart';
import 'package:provider/provider.dart';
import 'package:shared_preferences/shared_preferences.dart';
class StoreHome extends StatefulWidget {
@override
_StoreHomeState createState() => _StoreHomeState();
}
class _StoreHomeState extends State<StoreHome> {
SharedPreferences sharedPreferences;
@override
void initState() {
super.initState();
SharedPreferences.getInstance().then((prefs){
setState(() {
sharedPreferences=prefs;
});
});
}
@override
Widget build(BuildContext context) {
final _width=MediaQuery.of(context).size.width;
return SafeArea(
child: Scaffold(
appBar: AppBar(
title: Text("E_shop",style: TextStyle(color:Colors.red,),
),
centerTitle: true,
actions: [
Stack(
children: [
IconButton(icon:Icon(Icons.add_shopping_cart,color: Colors.grey,),
onPressed: (){
Navigator.pushReplacement(context, MaterialPageRoute(builder: (context)=>StoreHome()));
},
),
Positioned(child: Stack(
children: [
Icon(Icons.brightness_1,size: 20.0,
color: Colors.green,),
Positioned(
top: 3.0,
bottom: 4.0,
left:4.0,
child: Consumer<CartItemCounter>(
builder: (context, counter,_){
return Text(
counter.count.toString(),
style:TextStyle(color:Colors.white,fontSize:12.0,fontWeight:FontWeight.w500),
);
},),
),
],
),
),
],
),
],
),
drawer: MyDrawer(),
body: CustomScrollView(
slivers: [
SliverPersistentHeader(
pinned: true,
delegate: SearchBoxDelegate(),
),
StreamBuilder<QuerySnapshot>(
stream: FirebaseFirestore.instance.collection("items").limit(15).orderBy("publishedDate",descending: true).snapshots(),
builder: (context,dataSnapshot){
return !dataSnapshot.hasData
?SliverToBoxAdapter(child: Center(child: circularProgress(),),)
:SliverStaggeredGrid.countBuilder(
crossAxisCount: 1,
itemCount: 5,
staggeredTileBuilder: (c)=>StaggeredTile.fit(1),
itemBuilder: (context,index){
ItemModel model=ItemModel.fromJson(dataSnapshot.data.docs[index].data());
return sourceInfo(model,context);
},
);
},
),
],
),
),
);
}
circularProgress(){
return Container(
alignment: Alignment.center,
padding: EdgeInsets.only(top: 12.0),
child: CircularProgressIndicator(valueColor: AlwaysStoppedAnimation(Colors.lightGreenAccent),),
);
}
Widget sourceInfo(ItemModel model, BuildContext context , {Color background, removeCartFunction}) {
return InkWell(
onTap: (){
Navigator.pushReplacement(context, MaterialPageRoute(builder: (context)=>ProductPage(itemModel:model)));
},
splashColor: Colors.pink,
child: Padding(
padding: EdgeInsets.all(6.0),
child: Container(
height: 300.0,
width: MediaQuery.of(context).size.width,
child: Row(
children: [
Image.network(model.thumbnailUrl ,width:140.0,height: 140.0,),
SizedBox(width: 4.0,),
Expanded(
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
SizedBox(height: 16.0,),
Container(
child: Row(
mainAxisSize: MainAxisSize.max,
children: [
Expanded(child: Text(
model?.title ?? '',style: TextStyle(color: Colors.black,fontSize: 14.0),
),
)
],
),
),
SizedBox(height: 5.0,),
Container(
child: Row(
mainAxisSize: MainAxisSize.max,
children: [
Expanded(child: Text(
model?.shortInfo ?? '',style: TextStyle(color: Colors.black54,fontSize: 12.0),
),
)
],
),
),
SizedBox(height: 20.0,),
Row(
children: [
Container(
decoration: BoxDecoration(
shape: BoxShape.rectangle,
color: Colors.pink,
),
alignment: Alignment.topLeft,
width: 40.0,
height: 43.0,
child: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(
"50%",style: TextStyle(
fontSize: 15.0,color: Colors.white,
fontWeight: FontWeight.bold,
),
),
Text(
"OFF",style: TextStyle(
fontSize: 12.0,color: Colors.white,
fontWeight: FontWeight.bold,
),
),
],
),
),
),
SizedBox(width: 10.0,),
Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Padding(
padding: EdgeInsets.only(top: 0.0),
child: Row(
children: [
Text(
"Original Price :%",
style: TextStyle(
fontSize: 14.0,
color: Colors.grey
),
),
Text(
(model?.price??'').toString(),
style: TextStyle(
fontSize: 15.0,
color: Colors.grey,
decoration: TextDecoration.lineThrough,
),
)
],
),
),
Padding(
padding: EdgeInsets.only(top: 5.0),
child: Row(
children: [
Text(
"New Price :%",
style: TextStyle(
fontSize: 16.0,
color: Colors.red,
),
),
Text(
"%",
style: TextStyle(
fontSize: 15.0,
color: Colors.grey,
),
),
Text(
(model.price+model.price).toString(),
style: TextStyle(
fontSize: 15.0,
color: Colors.grey
),
),
],
),
),
],
),
],
),
Flexible(child: Container(
),
),
//to implement cart add/remove remove
Align(
alignment: Alignment.centerRight,
child: removeCartFunction==null
?IconButton(
icon: Icon(Icons.add_shopping_cart,color: Colors.pinkAccent,),
onPressed: (){
checkItemInCart(model.shortInfo,context);
},)
:IconButton(
icon: Icon(Icons.delete),
onPressed: null)
)
],
))
],
),
),
),
);
}
void checkItemInCart(String shortInfoAsId, BuildContext context) {
sharedPreferences.getStringList(EcommerceApp.userCartList).contains(shortInfoAsId)
?Fluttertoast.showToast(msg: "item already in cart")
:addItemToCart(shortInfoAsId,context);
}
addItemToCart(String shortInfoAsId, BuildContext context) {
List tempCartList=sharedPreferences.getStringList(EcommerceApp.userCartList);
tempCartList.add(shortInfoAsId);
FirebaseFirestore.instance.collection("users").doc(sharedPreferences.getString(EcommerceApp.userUID))
.update({
EcommerceApp.userCartList: tempCartList,
}).then((value){
Fluttertoast.showToast(msg: "Item added to cart successfully");
sharedPreferences.setStringList(EcommerceApp.userCartList, tempCartList);
Provider.of<CartItemCounter>(context,listen: false).displayResult();
});
}
}```
the CartItemCounter dart file is that thrown error when calling consumer<cartitemcounter>.initially useCart is given as garbage value when registering in firebase.
```class CartItemCounter extends ChangeNotifier{
static SharedPreferences sharedPreferences;
int _counter=sharedPreferences.getStringList("userCart").length-1;
int get count=>_counter;
Future<void> displayResult() async{
int _counter=sharedPreferences.getStringList("userCart").length-1;
await Future.delayed(const Duration(milliseconds: 100),(){
notifyListeners();
});
}
}```
The method 'getStringList' was called on null.
Receiver: null
Tried calling: getStringList("userCart")
The relevant error-causing widget was:
Consumer<CartItemCounter> .the details are stored in firebase,the error occurs when displaying the no of items the user have added to the cart.
解决方案
错误：
getStringList("...") was called on null
意味着该方法是在一个空对象上调用的，这意味着您正在尝试调用 sharedPrefs.getStringList() 但是，由于它尚未加载，因此 sharedPrefs 目前为空。因此，您会遇到问题。解决此问题的简单技巧：
@override
Widget build(BuildContext context) {
final _width=MediaQuery.of(context).size.width;
return SafeArea(
child:
sharedPrefs == null? // If sharedPrefs is not retreived yet
Scaffold(body: Center(child: Text("Hold on :)"))): // Show this widget
Scaffold( // Else do your normal job
appBar: AppBar(
title: Text("E_shop",style: TextStyle(color:Colors.red,),
),
......... and so on..
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