如何在颤振中使字符串成为全局变量

如何解决如何在颤振中使字符串成为全局变量

我创建了SharedPreferences来保存登录页面中的用户负载。然后，用户数据将保存在SharedPreferences中并移至主页。但是我现在的问题在主页中，我需要在主页的不同位置使用此变量。但是我做不到。

我需要使logindata的变量可以在我尝试在抽屉中用于注销的主页的每个位置使用。不，我收到以下错误消息：

Undefined name 'logindata'.

这是我的代码：

void initial() async {
    logindata = await SharedPreferences.getInstance();
    setState(() {
      username = logindata.getString('username');
      return username;
    });
  }

我的完整代码：

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(10,10,0),child: ClipRRect(
                            borderRadius: BorderRadius.circular(8.0),child:
                            Image.network(data.flowerImageURL,width: 200,fit: BoxFit.cover,))),Flexible(child:
                    Text(data.flowerName,style: TextStyle(fontSize: 18)))

                  ]),Divider(color: Colors.black),))
              .toList(),);

      },);

  }

}

有人知道怎么做吗？

解决方法

您只需要将变量放在任何类或方法之外。一个示例是创建文件globals.dart，然后将所有全局变量放入其中，并在需要时导入文件。

示例

// globals.dart
String globalString;
int globalInt;
bool globalBool;

// in any other file
import 'globals.dart' as globals;

globals.globalString = "Global String";

,

您需要var关键字，在这种情况下，您可以直接使用

var logindata = await SharedPreferences.getInstance();

您不需要将其设置为全局，因为SharedPreferences.getInstance()是Singleton
每次使用var logindata = await SharedPreferences.getInstance();都将获得相同的实例
调用getInstance()时也没有性能问题，因为它已被缓存，您可以在下面看到源代码片段

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;

当您在类外声明String且在变量名称（如_之类的变量名称之前不包含_localString时，它将成为全局变量

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());
}