how to make string as global variable in flutter - flutter

Question

I was create SharedPreferences to save user loading in logon page. Then data of user will be save in SharedPreferences and move to main page. But my problem now in main page I need use this variable in different places in main page. But I cant do that.
I need to make variable of logindata can use in each places in main page I try to use in drawer to make logout. No I get error as:
Undefined name 'logindata'.
this is my code:
void initial() async {
logindata = await SharedPreferences.getInstance();
setState(() {
username = logindata.getString('username');
return username;
});
}
my full code:
import 'package:flutter/material.dart';
import 'dart:convert';
import 'package:http/http.dart' as http;
import 'package:shared_preferences/shared_preferences.dart';
import 'addnewtopics.dart';
import 'DetilesOfMainPage.dart';
import 'loginpage.dart';
class MyApp extends StatelessWidget {
final String email;
MyApp({Key key, #required this.email}) : super(key: key);
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: Text('JSON ListView')
),
drawer: Drawer(
child: ListView(
padding: EdgeInsets.zero,
children: <Widget>[
DrawerHeader(
child: Text('Drawer Header'),
decoration: BoxDecoration(
color: Colors.blue,
),
),
ListTile(
title: Text('Item 1'),
onTap: () {
logindata.setBool('login', true);// here I need to use It ========================
Navigator.pushReplacement(context,
new MaterialPageRoute(builder: (context) => LoginUser()));
Navigator.pop(context);
},
),
ListTile(
title: Text('Item 2'),
onTap: () {
// Navigator.pop(context);
},
),
],
),
),
body: JsonImageList(),
floatingActionButton: FloatingActionButton(
onPressed: () {
Navigator.push(context, MaterialPageRoute(builder: (context) => UploadImageDemo()
),);
},
child: Icon(Icons.add),
),
));
}
}
class Flowerdata {
int id;
String flowerName;
String flowerImageURL;
Flowerdata({
this.id,
this.flowerName,
this.flowerImageURL
});
factory Flowerdata.fromJson(Map<String, dynamic> json) {
return Flowerdata(
id: json['id'],
flowerName: json['nametopics'],
flowerImageURL: json['image']
);
}
}
class JsonImageList extends StatefulWidget {
JsonImageListWidget createState() => JsonImageListWidget();
}
class JsonImageListWidget extends State {
SharedPreferences logindata;
String username;
#override
void initState() {
// TODO: implement initState
super.initState();
initial();
}
void initial() async {
logindata = await SharedPreferences.getInstance();
setState(() {
username = logindata.getString('username');
return username;
});
}
final String apiURL = 'http://xxxxxxxxx/getFlowersList.php';
Future<List<Flowerdata>> fetchFlowers() async {
var response = await http.get(apiURL);
if (response.statusCode == 200) {
final items = json.decode(response.body).cast<Map<String, dynamic>>();
List<Flowerdata> listOfFruits = items.map<Flowerdata>((json) {
return Flowerdata.fromJson(json);
}).toList();
return listOfFruits;
}
else {
throw Exception('Failed to load data from Server.');
}
}
getItemAndNavigate(String item, BuildContext context){
Navigator.push(
context,
MaterialPageRoute(
builder: (context) => SecondScreen(itemHolder : item)
)
);
}
#override
Widget build(BuildContext context) {
return FutureBuilder<List<Flowerdata>>(
future: fetchFlowers(),
builder: (context, snapshot) {
if (!snapshot.hasData) return Center(
child: CircularProgressIndicator()
);
return ListView(
children: snapshot.data
.map((data) => Column(children: <Widget>[
GestureDetector(
onTap: ()=>{
getItemAndNavigate(data.flowerName, context)
},
child: Row(
children: [
Container(
width: 200,
height: 100,
margin: EdgeInsets.fromLTRB(10, 0, 10, 0),
child: ClipRRect(
borderRadius: BorderRadius.circular(8.0),
child:
Image.network(data.flowerImageURL,
width: 200, height: 100, fit: BoxFit.cover,))),
Flexible(child:
Text(data.flowerName,
style: TextStyle(fontSize: 18)))
]),),
Divider(color: Colors.black),
],))
.toList(),
);
},
);
}
}
Anyone know how can make that?

Answer 1

You need var keyword, in your case you can directly use
var logindata = await SharedPreferences.getInstance();
You do not need to make it global, because SharedPreferences.getInstance() is Singleton
Every time you use var logindata = await SharedPreferences.getInstance(); will get the same instance
Also there is no performance issue when you call getInstance(), because it's cached, you can see source code snippet below
class SharedPreferences {
SharedPreferences._(this._preferenceCache);
...
static Future<SharedPreferences> getInstance() async {
if (_completer == null) {
_completer = Completer<SharedPreferences>();
try {
final Map<String, Object> preferencesMap =
await _getSharedPreferencesMap();
_completer.complete(SharedPreferences._(preferencesMap));
} on Exception catch (e) {
// If there's an error, explicitly return the future with an error.
// then set the completer to null so we can retry.
_completer.completeError(e);
final Future<SharedPreferences> sharedPrefsFuture = _completer.future;
_completer = null;
return sharedPrefsFuture;
}
}
return _completer.future;
When you declare a String outside of class and does not contain _ before variable name like _localString it become global
String globalString = ""; //global, import can be seen
String _localString = ""; //local and can only be seen in this file, import can not seen
void main() async{
var logindata = await SharedPreferences.getInstance();
runApp(MyApp());
}

Answer 2

You simply need to put your variable outside of any class or method. An example is to create a file globals.dart then put all your globals in it and import the file when you need.
Example
// globals.dart
String globalString;
int globalInt;
bool globalBool;
// in any other file
import 'globals.dart' as globals;
globals.globalString = "Global String";