CODEWITHSUNDEEP
flutterdartfuture
Brian Oh
Brian Oh

Reputation: 10780

Using Flutter and Dart, how can I get this Future to work?

I know that there are other ways of achieving this, but I want to use a Future via initState() to obtain a List using SharedPreferences. The following illustrates what I want to achieve, but it does not work as required, because the Future returns immediately before completing the task.

How should this be structured?


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

class _MyHomePageState extends State<MyHomePage> {
  SharedPreferences _prefs;
  String _sMessage = "No response from getting params";
  List<String> _lsCategories;

  @override
  void initState() {
    super.initState();
    _initPreferences().then((_) {
      try {
        _lsCategories = _prefs.getStringList("categories") ?? [];
        debugPrint("Categories = $_lsCategories");
        _sMessage = "Categories loaded OK";
      } catch (vError) {
        _sMessage = ("Error loading categories = $vError");
      }
      setState(() {});
    });
  }

  Future<void> _initPreferences() async {
    SharedPreferences.getInstance().then((prefs) {
      _prefs = prefs;
      debugPrint("Preferences initialized OK");
    }).catchError((vError) {
      debugPrint("Error initializing preferences = ${vError.toString()}");
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(_sMessage,
                style: TextStyle(
                    color: Colors.red[500],
                    fontWeight: FontWeight.bold,
                    fontSize: 20)),
          ],
        ),
      ),
    );
  }
}

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Future Test',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: "Flutter Future Test"),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

Upvotes: 0

Views: 121

Answers (2)

Jordan Davies
Jordan Davies

Reputation: 10881

Change your _initPreferences method to the following:

Future<void> _initPreferences() async {
  try {
    final prefs = await SharedPreferences.getInstance();
    _prefs = prefs;
    debugPrint("Preferences initialized OK");
  } catch (e) {
    debugPrint("Error initializing preferences = ${e.toString()}");
  }
}

The issue is that when using .then() on a Future you are not waiting for that future to finish, using await does wait.

Upvotes: 1

Augusto
Augusto

Reputation: 4253

Try it. 

Future<void> _initPreferences() async {
    _prefs = await SharedPreferences.getInstance().catchError((vError) { 
          debugPrint("Error initializing preferences = ${vError.toString()}");
    };
    debugPrint("Preferences initialized OK");
}

Upvotes: 0

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