method reduce

Source

Future<T> reduce(T combine(T previous, T element)) {
  _Future<T> result = new _Future<T>();
  bool seenFirst = false;
  T value;
  StreamSubscription subscription;
  subscription = this.listen(
    (T element) {
      if (seenFirst) {
        _runUserCode(() => combine(value, element),
                     (T newValue) { value = newValue; },
                     _cancelAndErrorClosure(subscription, result));
      } else {
        value = element;
        seenFirst = true;
      }
    },
    onError: result._completeError,
    onDone: () {
      if (!seenFirst) {
        try {
          throw IterableElementError.noElement();
        } catch (e, s) {
          _completeWithErrorCallback(result, e,  s);
        }
      } else {
        result._complete(value);
      }
    },
    cancelOnError: true
  );
  return result;
}