flutter - 如何使用 onListen 回调开始产生事件?
问题描述
我正在尝试制作一个广播流以仅在有监听器之后才开始发出事件,因为如果当时没有任何监听器,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 开发人员之一建议的方式进行了尝试。
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 向接收器添加事件的任何建议?
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(),
),
);
}
})),
],
),
);
}
}
块飞镖
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);
});
}
}
解决方案
尝试将声明与分配分开:
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));
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