completers and await

main.dart

import 'dart:async';
void main() {

  print('hi');
  var res = doCalc();
  print('value = ${res}');
  
  res.then((result) => print('result = $result'));
}

Future<int> doCalc() async
{
  var done = Completer<int>();
  
  await Future.delayed(Duration(seconds: 4), 
                 () { 
                   done.complete(10);
                   
                     });
  
  return done.future;
}