rxdart递归变压器

我正在实现一个需要递归调用来获取所有数据的api。我实现了一个具有 recursive transformer . 然而,看起来转换器在递归调用中一直返回一个空累加器。

评论组.dart

class CommentsBloc {
  final _repository = Repository();
  final _commentsFetcher = PublishSubject<int>();
  final _commentsOutput = BehaviorSubject<Map<int, Future<ItemModel>>>();

  // Stream Getters
  Observable<Map<int, Future<ItemModel>>> get itemWithComments =>
      _commentsOutput.stream;

  // Sink getters
  Function(int) get fetchItemWithComments => _commentsFetcher.sink.add;

  CommentsBloc() {
    _commentsFetcher.stream
        .transform(_commentsTransformer())
        .pipe(_commentsOutput);
  }

  _commentsTransformer() {
    return ScanStreamTransformer<int, Map<int, Future<ItemModel>>>(
      (cache, int id, index) {
        cache[id] = _repository.fetchItem(id);
        cache[id].then((ItemModel item) {
          item.kids.forEach((kidId) => fetchItemWithComments(kidId));
        });
      },
      <int, Future<ItemModel>>{},
    );
  }

  dispose() {
    _commentsFetcher.close();
    _commentsOutput.close();
  }
}

小精灵

return MaterialPageRoute(
  builder: (BuildContext context) {
    final itemId = int.parse(settings.name.replaceFirst('/', ''));
    final commentsBloc = CommentsProvider.of(context);

    commentsBloc.fetchItemWithComments(itemId);

    return NewsDetail(itemId: itemId);
  },
  );

误差

E/flutter (17142): [ERROR:flutter/shell/common/shell.cc(178)] Dart Error: Unhandled exception:
E/flutter (17142): NoSuchMethodError: The method '[]=' was called on null.
E/flutter (17142): Receiver: null
E/flutter (17142): Tried calling: []=(19307509, Instance of 'Future<ItemModel>')
E/flutter (17142): #0      _rootHandleUncaughtError.<anonymous closure> (dart:async/zone.dart:1112:29)
E/flutter (17142): #1      _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
E/flutter (17142): #2      _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)

这个 _评论员 只执行引发错误的第一个实例 cache[id] = _repository.fetchItem(id); ;