Flutter: not being able to show all incoming data

Question

I was using shared_preferences plugin in my Flutter application, I set my data when I click the button, but I try to get my data is not coming.

I set my data here and when I print it, I can see the output on my console.

shows.forEach((element) {
    showCode=element.showName;
    show.setString('showName', showCode);
});

I want to get my data here, above the override method. But when I print here, I just see 1 item, but I need to print 2 items.

  String showCode;

  Future<String> getCode() async{
    final codeValue = await SharedPreferences.getInstance();
    setState((){
      showyCode =codeValue.getString('showName');
    });
    print('here $showCode');
    return showCode;
  }

But I cannot call this future in my function, even if I have 2 data to come but I just see 1 item. I tried to include it in Listview then it fills an entire list with 1 item. Anyone have an idea where I'm doing wrong?

_showProgramData
              ? Container(
            height: MediaQuery.of(context).size.height/2,
            child: Padding(
              padding: const EdgeInsets.all(16.0),
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: [
                  Column(
                    children: [
                      IconButton(icon:Icon(Icons.close) , onPressed:() {Navigator.pushNamed(context, SecondScreen.routeName);}),
                      Text('Please Select Show',style: TextStyle(fontWeight:FontWeight.bold,fontSize: 24),),
                    ],
                  ),
                  Text(showCode),
                ],
              ),
            ),
          ) 

Answer 1

You can copy paste run full code below
Step 1: You need to use setStringList and getStringList
Step 2: getCode() return Future<List<String>>
Step 3: In forEach use showCodeList.add(element.showName);

code snippet

  void _setData() async {
    shows.forEach((element) {
      showCodeList.add(element.showName);
    });

    final codeValue = await SharedPreferences.getInstance();
    await codeValue.setStringList('showName', showCodeList);

    setState(() {});
  }

  Future<List<String>> getCode() async {
    final codeValue = await SharedPreferences.getInstance();
    showCodeList = await codeValue.getStringList('showName');
    if (showCodeList == null) {
      showCodeList = [];
    }
    print('here ${showCodeList.toString()}');
    return showCodeList;
  }
 ... 
 FutureBuilder(
          future: _future,
          builder: (context, AsyncSnapshot<List<String>> snapshot) {
           ...
                } else {
                  return ListView.builder(
                      itemCount: snapshot.data.length,
                      itemBuilder: (context, index) {
                        return Card(
                            elevation: 6.0,
                            child: Padding(
                              padding: const EdgeInsets.only(
                                  top: 6.0, bottom: 6.0, left: 8.0, right: 8.0),
                              child: Row(
                                crossAxisAlignment: CrossAxisAlignment.start,
                                children: <Widget>[
                                  Text(snapshot.data[index]),  

working demo

full code

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() async{
  WidgetsFlutterBinding.ensureInitialized();
  final codeValue = await SharedPreferences.getInstance();
  await codeValue.setStringList('showName', []);
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class Show {
  String showName;
  Show({this.showName});
}

class _MyHomePageState extends State<MyHomePage> {
  Future<List<String>> _future;

  List<Show> shows = [Show(showName: "a"), Show(showName: "b")];
  String showCode;
  List<String> showCodeList = [];

  void _setData() async {
    shows.forEach((element) {
      showCodeList.add(element.showName);
    });

    final codeValue = await SharedPreferences.getInstance();
    await codeValue.setStringList('showName', showCodeList);

    setState(() {});
  }

  Future<List<String>> getCode() async {
    final codeValue = await SharedPreferences.getInstance();
    showCodeList = await codeValue.getStringList('showName');
    if (showCodeList == null) {
      showCodeList = [];
    }
    print('here ${showCodeList.toString()}');
    return showCodeList;
  }

  @override
  void initState() {
    _future = getCode();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: FutureBuilder(
          future: _future,
          builder: (context, AsyncSnapshot<List<String>> snapshot) {
            switch (snapshot.connectionState) {
              case ConnectionState.none:
                return Text('none');
              case ConnectionState.waiting:
                return Center(child: CircularProgressIndicator());
              case ConnectionState.active:
                return Text('');
              case ConnectionState.done:
                if (snapshot.hasError) {
                  return Text(
                    '${snapshot.error}',
                    style: TextStyle(color: Colors.red),
                  );
                } else {
                  return ListView.builder(
                      itemCount: snapshot.data.length,
                      itemBuilder: (context, index) {
                        return Card(
                            elevation: 6.0,
                            child: Padding(
                              padding: const EdgeInsets.only(
                                  top: 6.0, bottom: 6.0, left: 8.0, right: 8.0),
                              child: Row(
                                crossAxisAlignment: CrossAxisAlignment.start,
                                children: <Widget>[
                                  Text(snapshot.data[index]),
                                ],
                              ),
                            ));
                      });
                }
            }
          }),
      floatingActionButton: FloatingActionButton(
        onPressed: _setData,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}