Poor Share Replay implementation

share_replay.dart

import 'dart:async';

import 'package:rxdart/rxdart.dart';

class ShareReplayTransformer<T> extends StreamTransformerBase<T, T> {
  final StreamTransformer<T, T> transformer;

  ShareReplayTransformer({int maxSize = 1, bool sync = false}) : transformer = _buildTransformer(maxSize, sync);

  @override
  Stream<T> bind(Stream<T> stream) {
    return transformer.bind(stream);
  }

  static StreamTransformer<T, T> _buildTransformer<T>(int maxSize, bool sync) {

    StreamSubscription<T> subscription;
    ReplaySubject<T> subject;

    return new StreamTransformer<T, T>((Stream<T> input, bool cancelOnError) {

      if(subject == null) {
        subject = ReplaySubject<T>(
          maxSize: maxSize,
          sync: sync,
          onListen: () {
            subscription = input.listen(
              subject.add,
              onError: subject.addError,
              onDone: subject.close
            );
          },
          onCancel: () {
            subscription.cancel();
            subscription = null;
          },
        );
      }

      return subject.stream.listen(null);
    });
  }
}