如何使用 onListen 回调开始产生事件？

Question

我正在尝试制作一个广播流以仅在有侦听器后才开始发出事件，因为如果当时没有任何侦听器，则事件刚刚被丢弃。

如果我使用单订阅流，它可以正常工作，但前提是我第一次进入屏幕。第二次显示错误Bad state: Stream has already been listened to.，所以我决定切换到广播。

我已经按照我在 github 上找到的 Dart 开发人员之一进行了 suggested 的尝试。

StreamController.broadcast(onListen: () => _cacheLength.sink.add(0));

错误信息是Only static members can be accessed in initializers.

关于如何在 onListen 向接收器添加事件有什么建议吗？

---

CarsListScreen.dart

import './../../bloc/provider.dart';

int _itemCount = 200;

class CarsList extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final bloc = Provider.provideBloc(context);
    return Scaffold(
      body: Column(
        children: <Widget>[
          Expanded(
              child: StreamBuilder(
                  stream: bloc.cacheLength,
                  builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
                    if (snapshot.hasData && snapshot.data != null) {
                      if (snapshot.data != 0) {
                        _itemCount = snapshot.data;
                      }
                      return ListView.builder(
                        itemCount: _itemCount,
                        itemBuilder: (BuildContext context, int index) {
                          return StreamBuilder(
                              stream: bloc.cars,
                              builder: (BuildContext context, AsyncSnapshot snapshot) {
                                if (snapshot.hasData &&
                                    snapshot.data != null &&
                                    snapshot.data.length > index) {
                                  return Text('''
                                          ${snapshot.data[index].brand}
                                          ${snapshot.data[index].model}
                                          ${snapshot.data[index].year}''');
                                } else {
                                  bloc.addId(index);
                                  return CircularProgressIndicator();
                                }
                              });
                        },
                      );
                    } else {
                      return Padding(
                        padding: EdgeInsets.all(8),
                        child: Center(
                          child: CircularProgressIndicator(),
                        ),
                      );
                    }
                  })),
        ],
      ),
    );
  }
}

---

bloc.dart

import './../data-sources/carsCache.dart';
import './../models/carModel.dart';

class Bloc {
  Bloc() {
    init();
  }

  CarsCache cache = CarsCache();

  //stream cars from cache to cars-list screen//
  StreamController<List<CarModel>> _cars = StreamController.broadcast();
  Stream<List<CarModel>> get cars => _cars.stream;

  //stream IDs to load from cars-list screen to cache//
  StreamController<int> _carIds = StreamController();
  Function(int) get addId => _carIds.sink.add;

  //stream cache length to cars-list//
  StreamController<int> _cacheLength =
      StreamController.broadcast(onListen: () => _cacheLength.sink.add(0));
  Stream<int> get cacheLength => _cacheLength.stream;

  void init() {
    _carIds.stream.listen((id) {
      if (cache.repo.length <= id) {
        cache.fetchCarsFromDB().then((endOfList) {
          if (endOfList) {
            _cacheLength.sink.add(cache.repo.length);
          } else {
            _cacheLength.sink.add(0);
          }
        });
      }
      _cars.sink.add(cache.repo);
    });
  }
}

Answer 1

尝试将声明与赋值分开：

  StreamController<int> _fin;
  _fin = StreamController<int>.broadcast(onListen: () => _fin.sink.add(0));

如果不是这样，请发布更多代码，以便我们查看上下文。

像这样更改您的更新代码....

  //stream cache length to cars-list//
  StreamController<int> _cacheLength;

  Stream<int> get cacheLength => _cacheLength.stream;

  void init() {
    _cacheLength =
        StreamController.broadcast(onListen: () => _cacheLength.sink.add(0));