Completer example

main.dart

import 'dart:async';

final Completer<int> halfwayPointCompleter = Completer();

void main() async {
  print('getting started');
  calculateSomeNumbers(10).then((result) => print(result)); // async
  int halfwayPoint = await halfwayPointCompleter.future;
  print(halfwayPoint);
  // this will print before the above function completes because it's waiting for the //halfwayPointCompleter to complete, which completes halfway through
}

//function is useless, but it's just an example.
Future<int> calculateSomeNumbers(int max) async {
  for (int i = 0; i < max; i++) {
    await Future.delayed(Duration(milliseconds: 500));
    if (i == max ~/ 2) {
      // ~/ is division with rounding. It returns an int.
      halfwayPointCompleter
          .complete(i); // this makes the future return at this point.
    }
  }
  return max;
}