我正在使用BLoC从Firestore加载我的预设对象。这是我的Bloc模型：

class StatisticsBloc extends BlocBase {

  List<Preset> _presets;

  StreamController<List<Preset>> _presetsController = new StreamController();

  Stream<List<Preset>> get getPresets => _presetsController.stream.asBroadcastStream();

  StatisticsBloc() {
    print('init Statistics Bloc');
    _presets = [];
    Firestore.instance.collection('Presets').snapshots().asBroadcastStream().listen(_onPresetsLoaded);
  }

  @override
  void dispose() {
    print('Disposed Statistics Bloc');
    _presetsController.close();
  }

  void _onPresetsLoaded(QuerySnapshot data) {
    _presets = [];
    data.documents.forEach((DocumentSnapshot snap) {
      Preset preset = Preset.fromDoc(snap);
      _presets.add(preset);
    });
    _presetsController.sink.add(_presets);
  }
}

然后我显示如下列表：

class StatisticsPage extends StatelessWidget {

  StatisticsPage() {
    print('Created StatisticsPage');
  }

  @override
  Widget build(BuildContext context) {
    final StatisticsBloc statisticsBloc = BlocProvider.of<StatisticsBloc>(context);
    final List<Preset> _ = [];

    print(statisticsBloc.getPresets.isBroadcast);

    return Scaffold(
      appBar: AppBar(
        title: Text('Statistics'),
      ),
      body: StreamBuilder(
        stream: statisticsBloc.getPresets,
        initialData: _,
        builder: (BuildContext context, AsyncSnapshot<List<Preset>> snapshot) {
          if (snapshot.hasData) {
            return ListView(
              children: snapshot.data.map((Preset preset) {
                print(preset.name);
                return new ListTile(
                  title: new Text(preset.name),
                  subtitle: new Text(preset.id),
                );
              }).toList(),
            );
          } else {
            Text('No Data');
            print('No Data');
          }
        }
      )
    );
  }
}

问题是，我StatisticsPage在标签栏中显示了，因此当我切换标签并返回到该标签栏时，这将是多次构建的时间。第一次访问时它可以工作，但是当我切换选项卡并返回到它时，小部件将重新构建，并且出现错误：Bad state: Stream has already been listened to.。getPresets如您所见，我试图将Stream
声明为BroadcastStream，StatisitcsBloc但这不起作用。

还有一个次要问题：是否有更好的方法将Stream<QuerySnapshot>我从Firestore转换为Stream<List<Presets>>？