Read/Write value from SharedPreferences

Issue

I'm tring to set/get sharedpreferences with no luck , my code throw exception in setState(Writing) and InitState (Reading) .

My code :

class _MyHomePageState extends State<MyHomePage> {
  final Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
  int _counter = 0;

  Future<void> _incrementCounter() async {
    final SharedPreferences prefs = await _prefs;
    final int counter = (prefs.getInt('counter') ?? 0) + 1;

    setState(() {
      _counter = prefs.setInt('counter', counter).then((bool success) { //error here 
        return counter;
      });
    });
  }

  @override
  void initState() {
    super.initState();

    _counter = _prefs.then((SharedPreferences prefs) { // error here 
   return prefs.getInt('counter') ?? 0;
     });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () async {
          _incrementCounter();
        },
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

exception tells me that can't cast Future to int

Error: A value of type 'Future' can't be assigned to a variable of type 'int'

Solution

The flutter compiler is right to point out that you are trying to assign a Future to an int value.

A Future in Dart is basically a value, which will be available later (in the future), hence directly assigning the value to it does not make sense. Try waiting for the future using await or register a callback using .then().

Your snippet modified the below-mentioned way should work.

class _MyHomePageState extends State<MyHomePage> {
final Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
int _counter = 0;

Future<void> _incrementCounter() async {
  final SharedPreferences prefs = await _prefs;
  final int counter = (prefs.getInt('counter') ?? 0) + 1;

  prefs.setInt('counter', counter).then((bool success) {
    setState(() {
      _counter = counter;
    });
  });
}

@override
void initState() {
  super.initState();
  _prefs.then((SharedPreferences prefs) => prefs.getInt('counter') ?? 0).then((value) => _counter = value);
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text(widget.title),
    ),
    body: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          const Text(
            'You have pushed the button this many times:',
          ),
          Text(
            '$_counter',
            style: Theme.of(context).textTheme.headline4,
          ),
        ],
      ),
    ),
    floatingActionButton: FloatingActionButton(
      onPressed: () async {
        _incrementCounter();
      },
      tooltip: 'Increment',
      child: const Icon(Icons.add),
    ), // This trailing comma makes auto-formatting nicer for build methods.
  );
}

}

Read more about Futures here. The Dart documentation is extremely detailed about this.

Answered By - Swanav