Flutter- compute method

Question

I'm trying to execute heavy method by compute().
I tried like so. After loop executed, Text widget update but result returns 0 almost right after button was pressed.
Does anyone know what am I missing?

 int _counter;

  static int loop(int val) {
    int count = 0;
    for (int i = 1; i <= val; i++) {
      count += i;
    }
    return count;
  }

  Future<void> _onPressed() async {
    int result = await compute(loop, 1000000000000000000);
    setState(() {
      _counter = result;
    });
  }

Answer 1

According to https://api.flutter.dev/flutter/foundation/compute-constant.html,

The callback argument must be a top-level function, not a closure or an instance or static method of a class.

Please remove the static keyword, it should work fine

Answer 2

Here is an example of compute:

final sum = await compute(computationallyExpensiveTask, 1000000000);

And this is the function that is run in the compute isolate:

int computationallyExpensiveTask(int value) {
  var sum = 0;
  for (var i = 0; i <= value; i++) {
    sum += i;
  }
  print('finished');
  return sum;
}

Notes:

• As mentioned in the accepted answer, the function that you give to compute must be a top level function (not inside a class).

Code

Here is the full code in context. The circular progress indicator stops spinning because the UI is blocked when you do the computationallyExpensiveTask directly.

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: BodyWidget(),
      ),
    );
  }
}

class BodyWidget extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          CircularProgressIndicator(),
          ElevatedButton(
            child: Text('start'),
            onPressed: () async {
              final sum = computationallyExpensiveTask(1000000000);
              //final sum = await compute(computationallyExpensiveTask, 1000000000);
              print(sum);
            },
          )
        ],
      ),
    );
  }
}


int computationallyExpensiveTask(int value) {
  var sum = 0;
  for (var i = 0; i <= value; i++) {
    sum += i;
  }
  print('finished');
  return sum;
}