flutter :sharedpreference retrieving null value [duplicate] - firebase

Question

This question already has answers here:
What is a NoSuchMethod error and how do I fix it?
(2 answers)
Closed 2 years ago.
The method 'getStringList' was called on null.
Receiver: null
Tried calling: getStringList("userCart")
The relevant error-causing widget was:
Consumer .the details are stored in firebase,the error occurs when displaying the no of items the user have added to the cart.
the CartItemCounter dart file is that thrown error when calling consumer.initially useCart is given as garbage value when registering in firebase.
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.

Answer 1

The error:
getStringList("...") was called on null
implies that the method was called on a null object which means that you are trying to call sharedPrefs.getStringList()
but, sharedPrefs is null at the moment as it has not been loaded yet. Hence, you encounter the problem. A simple hack to solve this problem:
#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..