flutter bloc how stop execution loop inside event? - flutter

Question

I use a Provider with ChangeNotifier, but inside it I need to make a StreamController that will process the data. For example, let there be a list of sites
class ExampleProvider extends ChangeNotifier {
//list of sites for data processing
final List<String> _weblinks = [];
ExampleProvider() {
//Fill in demo data
_weblinks.add("https://stackoverflow.com/");
_weblinks.add("https://github.com");
_weblinks.add("http://microsoft.com");
//subscribe to the stream in the constructor
eventStream.listen((event) async {
if (event is StartProcessing) {
for (var i = 0; i < _weblinks.length; i++) {
//Doing some fake calculations
await Future.delayed(Duration(seconds: 3));
//add the result to the stream
dataSink.add(_weblinks[i]);
}
}
if (event is FinishProcessing) {}
});
}
final _streamController = StreamController<String>.broadcast();
Stream<String> get dataStream => _streamController.stream;
StreamSink<String> get dataSink => _streamController.sink;
final _eventController = StreamController<StreamControllerAction>.broadcast();
Stream<StreamControllerAction> get eventStream => _eventController.stream;
StreamSink<StreamControllerAction> get eventSink => _eventController.sink;
}
The problem is that the list of sites can be very large and if a data processing thread has started, then I can’t interrupt or cancel it, I cannot stop the execution of the loop, and until the list of sites ends, the thread will be executed. I also cannot use _streamController.close() because the stream is inside the provider, which is and the stream will be disabled forever, and I would like to reuse it. is there a way to interrupt the execution of a stream if the data is evaluated in a loop?
Full code
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
ChangeNotifierProvider.value(
value: ExampleProvider(),
),
],
child: MaterialApp(
home: MyHomePage(),
));
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key}) : super(key: key);
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
#override
Widget build(BuildContext context) {
return Scaffold(
body: SafeArea(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
StreamBuilder<String>(
initialData: null,
stream: Provider.of<ExampleProvider>(context, listen: false)
.dataStream,
builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
List<Widget> children;
if (snapshot.hasError) {
children = <Widget>[Text("error")];
} else {
switch (snapshot.connectionState) {
case ConnectionState.waiting:
children = <Widget>[
ElevatedButton(
child: Text("processing data"),
onPressed: () {
Provider.of<ExampleProvider>(context, listen: false)
.eventSink
.add(StartProcessing());
},
),
];
break;
default:
{
children = <Widget>[
Column(
children: [
ElevatedButton(
child: Text("finish"),
onPressed: () {},
),
Text(snapshot.data),
],
)
];
}
break;
}
}
return Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: children,
);
},
)
],
),
),
);
}
}
class ExampleProvider extends ChangeNotifier {
final List<String> _weblinks = [];
ExampleProvider() {
_weblinks.add("https://stackoverflow.com/");
_weblinks.add("https://github.com");
_weblinks.add("http://microsoft.com");
eventStream.listen((event) async {
if (event is StartProcessing) {
for (var i = 0; i < _weblinks.length; i++) {
await Future.delayed(Duration(seconds: 3));
dataSink.add(_weblinks[i]);
}
}
if (event is FinishProcessing) {}
});
}
final _streamController = StreamController<String>.broadcast();
Stream<String> get dataStream => _streamController.stream;
StreamSink<String> get dataSink => _streamController.sink;
final _eventController = StreamController<StreamControllerAction>.broadcast();
Stream<StreamControllerAction> get eventStream => _eventController.stream;
StreamSink<StreamControllerAction> get eventSink => _eventController.sink;
}
abstract class StreamControllerAction {}
class StartProcessing extends StreamControllerAction {}
class FinishProcessing extends StreamControllerAction {}

Answer 1

If you want to stop listening to eventStream you can save its subscription in a StreamSubscription object and then you will have access to .pause() and .resume().
When you use stream.listen() it returns a StreamSubscription you can save in a variable and use later.
You can read more about it in the documentation https://api.dart.dev/stable/2.12.4/dart-async/StreamSubscription-class.html