CODEWITHSUNDEEP
user-interfaceasynchronousflutter
Teddy
Teddy

Reputation: 1095

Flutter UI hangs on async operation

How to structure the following code correctly? I launch an expensive async operation. The "runExpensiveOperation" method stops the spinning CircularProgressIndicator so it seems it is executed on the main thread. Is there anything like "dispatch_async" like on native iOS?

class _MyHomePageState extends State<MyHomePage> {

  void _pressed() async {
    print("I am pressed");
    print("I will make a cheap operation now");
    await runExpensiveOperation();
  }

  Future<void> runExpensiveOperation() async  {
    print('starting expensive operation');
    for (var i = 0; i<1000000; i++) {
      List<int> bytes = utf8.encode('somethingtohash');
      String hash = sha256.convert(bytes).toString();
    }
    print('finished expensive operation');
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            CircularProgressIndicator(),
            FlatButton(
              child: Text("Press me for tough operation"),
              onPressed: _pressed,
            )
          ],
        ),
      ),
    );
  }
}

Upvotes: 6

Views: 5248

Answers (1)

CopsOnRoad
CopsOnRoad

Reputation: 268134

class _MyHomePageState extends State<MyHomePage> {

  void _pressed() async {
    print("I am pressed");
    print("I will make a cheap operation now");

    // string returned is "Good Morning"
    String string = await compute(runExpensiveOperation, null); // this is what you need
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            CircularProgressIndicator(),
            FlatButton(
              child: Text("Press me for tough operation"),
              onPressed: _pressed,
            )
          ],
        ),
      ),
    );
  }
}

// make sure this is outside your class definition 
Future<String> runExpensiveOperation(void _) async  {
  print('starting expensive operation');
  for (var i = 0; i<1000000; i++) {
      List<int> bytes = utf8.encode('somethingtohash');
      String hash = sha256.convert(bytes).toString();
  }
  print('finished expensive operation');
  return "Good Morning";
}

Upvotes: 6

Related Questions