I'm having issues using Shared Preferences on another page

Question

I have been having issues retrieving the "username" String saved in Shared Preferences (from the Settings page) for the home page text field (~Line 60 main.dart). I have tried a few methods to retrieve it, but so far I haven't had any luck with trying to grab it. The last attempt I tried was using '$user' (~Line 29), but I still haven't had any luck. I'm still very new to Flutter programming, but I had assumed you could access Shared Preferences data globally as long as you had the Key. So far when I tried using the methods I saw online and in documentation I had no luck transferring the data. Thank you for your help!

main.dart

import 'package:bit/shared_preferences.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:bit/saved_data.dart';

void main() {
  runApp(MaterialApp(
    title: 'App',
    themeMode: ThemeMode.system,
    theme: MyThemes.lightTheme,
    darkTheme: MyThemes.darkTheme,
    home: MyApp(),
  ));
}

class MyApp extends StatelessWidget {
  MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final theme = MediaQuery.of(context).platformBrightness == Brightness.dark
        ? 'Dark Theme'
        : 'Light Theme';
    final user = ''; // Empty String Line 29

    var scaffold = Scaffold(
      appBar: AppBar(
        title: const Text('Home'),
      ),
// Body Home Page Beginning
      body: SingleChildScrollView(
          child: Center(
        child: Text('Hello $theme!'),
      )),
// Body Home Page End
      drawer: Drawer(
        // Drawer Beginning
        child: ListView(
          children: [
// Drawer Header
            DrawerHeader(
              decoration: const BoxDecoration(
                color: Colors.blue,
              ),
              child: Stack(
                children: const [
                  Align(
                      alignment: Alignment.centerLeft,
                      child: CircleAvatar(
                        backgroundColor: Colors.white,
                        radius: 50.0,
                      )),
                  Align(
                      alignment: Alignment.centerRight,
                      child: Text('$user',             // Area To Input Text Line 60
                        style: TextStyle(
                          color: Colors.white,
                          fontSize: 20.0,
                        ),
                      )),
                  Align(
                      alignment: Alignment(1, 0.3),
                      child: Text(
                        'Supporter',
                        style: TextStyle(
                          color: Colors.white,
                          fontSize: 15.0,
                        ),
                      ))
                ],
              ),
            ),
// Drawer List
            ListTile(
              title: const Text('Settings'),
              subtitle: const Text('Account Info & Settings'),
              onTap: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(builder: (context) => Settings()),
                );
              },
              trailing: const Icon(Icons.arrow_forward_ios_rounded),
            ),
          ],
        ),
      ),
// Drawer End
    );
    return MaterialApp(
      title: 'App',
      themeMode: ThemeMode.system,
      theme: MyThemes.lightTheme,
      darkTheme: MyThemes.darkTheme,
      home: scaffold,
    );
  }
}

// Settings Page & Account Information
class Settings extends StatefulWidget {
  Settings({Key? key}) : super(key: key);

  @override
  State<Settings> createState() => _SettingsState();
}

class _SettingsState extends State<Settings> {
  final _preferencesService = PreferencesService();
  final _usernameController = TextEditingController();
  void initState() {
    super.initState();
    _populateFields();
  }

  void _populateFields() async {
    final settings = await _preferencesService.getSettings();
    setState(() {
      _usernameController.text = settings.username;
    });
  }

  @override
  Widget build(BuildContext context) {
    final theme = MediaQuery.of(context).platformBrightness == Brightness.dark
        ? 'Dark Theme'
        : 'Light Theme';

    return Scaffold(
        appBar: AppBar(
          title: const Text(
              'Settings'), /* actions: <Widget>[
          IconButton(
              onPressed: () async {
                _saveSettings;
              },
              icon: const Icon(Icons.save),
              tooltip: 'Save Settings')
        ] */
        ),
        body: SingleChildScrollView(
            child: Padding(
          padding: const EdgeInsets.fromLTRB(8, 8, 8, 8),
          child: Column(
            children: [
              Column(
                // Account
                children: [
                  const Padding(
                      padding: EdgeInsets.fromLTRB(0, 12, 0, 0),
                      child: Text('Account Information',
                          style: TextStyle(
                            fontSize: 17.0,
                          ))),
                  Padding(
                    padding: const EdgeInsets.fromLTRB(12, 0, 12, 12),
                    child: TextField(
                      controller: _usernameController,
                      inputFormatters: [LengthLimitingTextInputFormatter(25)],
                      decoration: InputDecoration(
                        hintText: 'Username',
                        labelText: 'Username',
                        border: OutlineInputBorder(
                            borderRadius: BorderRadius.circular(10.0)),
                      ),
                    ),
                  ),
                ],
              ),
              Container(
                child: Column(
                  // App Settings
                  children: [
                    // SwitchListTile(value: DarkMode, onChanged: Light => Dark => Light)
                    // ChangeThemeButtonWidget(),
                    TextButton(
                      onPressed: _saveSettings,
                      child: const Text('Save Settings'),
                    )
                  ],
                ),
              ),
            ],
          ),
        )));
  }

  void _saveSettings() {
    final newSettings = SettingsModal(
      username: _usernameController.text,
    );

    print(newSettings);
    print(_usernameController.text);
    _preferencesService.saveSettings(newSettings);
  }
}

shared_preferences.dart

import 'package:bit/main.dart';
import 'package:bit/saved_data.dart';
import 'package:shared_preferences/shared_preferences.dart';

class PreferencesService {
  Future saveSettings(SettingsModal settings) async {
    final preferences = await SharedPreferences.getInstance();

    await preferences.setString('username', settings.username);

    print('Saved Settings');
  }

  Future<SettingsModal> getSettings() async {
    final preferences = await SharedPreferences.getInstance();

    final username = preferences.getString('username')!;

    return SettingsModal(
      username: username,
    );
  }
}

saved_data.dart

import 'package:shared_preferences/shared_preferences.dart';
import 'package:bit/main.dart';

class SettingsModal {
  final String username;

  SettingsModal({
    required this.username,
  });
}

Answer 1

The issue is coming because, you like to use user which is not a constant. While adding const on Stack's children as Constance, which can be happened in this case, remove const and it won't show any errors.

child: Stack(
  children: [
    Align(
        alignment: Alignment.centerLeft,
        child: CircleAvatar(
          backgroundColor: Colors.white,
          radius: 50.0,
        )),
    Align(
        alignment: Alignment.centerRight,
        child: Text(
          user, 
          style: TextStyle(
            color: Colors.white,
            fontSize: 20.0,
          ),
        )),
  ],
),

To receive data from future(SharedPreference) we need to wait. You can use FutureBuilder in this case.

We can provide default value instead of using ! and make it static .

 static Future<SettingsModal> getSettings() async {
    final preferences = await SharedPreferences.getInstance();

    final username = preferences.getString('username') ?? "Not found";

    return SettingsModal(
      username: username,
    );
  }

Use PreferencesService.getSettings(), to get data.

Align(
    alignment: Alignment.centerRight,
    child: FutureBuilder<SettingsModal>(
      future: PreferencesService.getSettings(),
      builder: (context, snapshot) {
        if (snapshot.hasData &&
            snapshot.connectionState ==
                ConnectionState.done) {
          return Text(
            snapshot.data!.username,
            style: TextStyle(
              color: Colors.white,
              fontSize: 20.0,
            ),
          );
        }

        /// better to handle other cases, included on answer
        return CircularProgressIndicator();
      },
    )),

More about FutureBuilder