Future<T> single

Returns the single element.

If an error event occurs before or after the first data event, the resulting future is completed with that error.

If this is empty or has more than one element throws a StateError.

Source 

Future<T> get single {
  _Future<T> future = new _Future<T>();
  T result = null;
  bool foundResult = false;
  StreamSubscription subscription;
  subscription = this.listen(
    (T value) {
      if (foundResult) {
        // This is the second element we get.
        try {
          throw IterableElementError.tooMany();
        } catch (e, s) {
          _cancelAndErrorWithReplacement(subscription, future, e, s);
        }
        return;
      }
      foundResult = true;
      result = value;
    },
    onError: future._completeError,
    onDone: () {
      if (foundResult) {
        future._complete(result);
        return;
      }
      try {
        throw IterableElementError.noElement();
      } catch (e, s) {
        _completeWithErrorCallback(future, e, s);
      }
    },
    cancelOnError: true);
  return future;
}