can I know when a StreamSubscription is cancelled? - dart

Question

is there anything like onCancel for a StreamSubscription?
example:
var subscription = someStream.listen((item) => null);
subscription.cancel(); // does this trigger any event?
I ended up creating a _StreamSubscriptionDelegate that delegates all methods and so I can put some logic when the subscription is cancelled, however, maybe there is an easier solution to it.

Answer 1

If the stream comes from a StreamController, then the controller is notified of the cancel. The listener is expected to keep track of their own subscription, so if one part of the client code needs to know that another part has cancelled the stream, then wrapping the subscription in something which records that you cancelled it, is a perfectly good approach.
Another approach could be to wrap the stream before listening to it:
Stream<T> onCancel<T>(Stream<T> source, void onCancel()) async* {
bool isCancelled = true;
try {
await for (var event in source) {
yield event; // exits if cancelled.
}
isCancelled = false;
} finally {
if (isCancelled) onCancel();
}
}
or
Stream<T> onCancel<T>(Stream<T> source, void onCancel()) {
var sink = StreamController<T>();
sink.onListen = () {
var subscription = source.listen(sink.add, onError: sink.onError, onDone: sink.close);
sink
..onPause = subscription.pause
..onResume = subscription.resume
..onCancel = () {
subscription.cancel();
onCancel();
};
};
return sink.stream;
}