description

# fold<S> method Null safety

Future<S> fold <S>(

1. S initialValue,
2. S combine(
   1. S previous,
   2. T element
   )

)

Combines a sequence of values by repeatedly applying `combine`.

Similar to Iterable.fold, this function maintains a value, starting with `initialValue` and updated for each element of this stream. For each element, the value is updated to the result of calling `combine` with the previous value and the element.

When this stream is done, the returned future is completed with the value at that time. For an empty stream, the future is completed with `initialValue`.

If this stream emits an error, or the call to `combine` throws, the returned future is completed with that error, and processing is stopped.

## Implementation

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