singleWhere method Null safety

Future<T> singleWhere(
  1. bool test(
    1. T element
    ),
  2. {T orElse(
      )?}
    )

    Finds the single element in this stream matching test.

    Like lastWhere, except that it is an error if more than one matching element occurs in this stream.

    Example:

    var result = await Stream.fromIterable([1, 2, 3, 6, 9, 12])
    .singleWhere((element) => element % 4 == 0, orElse: () => -1);
print(result); // 12

result = await Stream.fromIterable([2, 6, 8, 12, 24, 32])
    .singleWhere((element) => element % 9 == 0, orElse: () => -1);
print(result); // -1

result = await Stream.fromIterable([2, 6, 8, 12, 24, 32])
    .singleWhere((element) => element % 6 == 0, orElse: () => -1);
// Throws.

    Implementation

    Future<T> singleWhere(bool test(T element), {T orElse()?}) {
  _Future<T> future = new _Future<T>();
  late T result;
  bool foundResult = false;
  StreamSubscription<T> subscription =
      this.listen(null, onError: future._completeError, onDone: () {
    if (foundResult) {
      future._complete(result);
      return;
    }
    if (orElse != null) {
      _runUserCode(orElse, future._complete, future._completeError);
      return;
    }
    try {
      throw IterableElementError.noElement();
    } catch (e, s) {
      _completeWithErrorCallback(future, e, s);
    }
  }, cancelOnError: true);

  subscription.onData((T value) {
    _runUserCode(() => test(value), (bool isMatch) {
      if (isMatch) {
        if (foundResult) {
          try {
            throw IterableElementError.tooMany();
          } catch (e, s) {
            _cancelAndErrorWithReplacement(subscription, future, e, s);
          }
          return;
        }
        foundResult = true;
        result = value;
      }
    }, _cancelAndErrorClosure(subscription, future));
  });
  return future;
}