Future<S> fold<S>(S initialValue, S combine(S previous, T element))

Reduces a sequence of values by repeatedly applying combine.

Source 

Future<S> fold<S>(S initialValue,
    S combine(S previous, T element)) {

  _Future<S> result = new _Future<S>();
  S value = initialValue;
  StreamSubscription subscription;
  subscription = this.listen(
    (T element) {
      _runUserCode(
        () => combine(value, element),
        (S newValue) { value = newValue; },
        _cancelAndErrorClosure(subscription, result)
      );
    },
    onError: (e, st) {
      result._completeError(e, st);
    },
    onDone: () {
      result._complete(value);
    },
    cancelOnError: true);
  return result;
}