main.dart

class ChainProcessor {
  var arg = '';
  var didUpdateArg = false;
  bool isComputing = false;

  void _computation() {
    // do stuff
  }

  void chain() async {
    var argcopy = arg;
    print('Computation('+argcopy+') start');
    // do computation
    await Future.delayed(const Duration(seconds: 3), _computation).then((x) {
      print('Computation(' + argcopy + ') done');
      // when done check if need to compute again
      if (didUpdateArg) {
        didUpdateArg = false;
        chain();
      }
      isComputing = false;
    });
  }

  void compute(nextarg) {
    arg = nextarg;
    if (!isComputing) {
      isComputing = true;
      chain();
    } else {
      didUpdateArg = true;
    }
  } 
}

Future<void> main() async {
  final processor = ChainProcessor();

  // initial computation request
  processor.compute('1');

  // new computation requests with different arguments
  Future.delayed(const Duration(seconds: 1), () => processor.compute('2'));
  Future.delayed(const Duration(milliseconds: 1500), () => processor.compute('3'));

  // but only the argument in the most recent request is computed
  Future.delayed(const Duration(milliseconds: 1600), () => processor.compute('4'));

  // when no computation is being done, more chains can be started
  Future.delayed(const Duration(seconds: 11), () => processor.compute('5'));
  Future.delayed(const Duration(seconds: 12), () => processor.compute('6'));
}