When does onDone event work for StreamSubscription in Dart? - asynchronous

Question

void sample_stream_1() {
final myGenerator = NumberCreator();
final myStream = myGenerator.stream;
myStream
.listen(
(data) => print('Data: $data'),
onError: (err) => print('Error!'),
cancelOnError: false,
onDone: () {
myGenerator.finish();
print('Done!');
},
);
}
class NumberCreator {
final _controller = StreamController<int>();
var _count = 1;
NumberCreator() {
Timer.periodic(Duration(seconds: 1), (t) {
this._controller.sink.add(this._count);
this._count++;
if (_count > 10) t.cancel();
});
}
Stream<int> get stream => this._controller.stream;
void finish() => this._controller.close();
}
main(List<String> args) {
sample_stream_1();
print('waiting...');
}
The output is:
[Running] dart "./future_stream_demo.dart"
waiting...
Data: 1
Data: 2
Data: 3
Data: 4
Data: 5
Data: 6
Data: 7
Data: 8
Data: 9
Data: 10
[Done] exited with code=0 in 11.164 seconds
onDone event is not triggered, if I change sample_stream_1() procedure to this:
void sample_stream_1() {
final myGenerator = NumberCreator();
final myStream = myGenerator.stream;
myStream
.take(5)
.listen(
(data) => print('Data: $data'),
onError: (err) => print('Error!'),
cancelOnError: false,
onDone: () {
myGenerator.finish();
print('Done!');
},
);
}
The output is:
[Running] dart "./future_stream_demo.dart"
waiting...
Data: 1
Data: 2
Data: 3
Data: 4
Data: 5
Done!
Unhandled exception:
Bad state: Cannot add event after closing
#0 _StreamController.add (dart:async/stream_controller.dart:623:24)
#1 _StreamSinkWrapper.add (dart:async/stream_controller.dart:900:13)
#2 new NumberCreator.<anonymous closure> (file:./future_stream_demo.dart:275:29)
#3 _Timer._runTimers (dart:isolate-patch/timer_impl.dart:397:19)
#4 _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:428:5)
#5 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)
[Done] exited with code=255 in 6.298 seconds
onDone event is triggered but it raises an exception also.
So:
Why does onDone event work for the prior one and not for the latter one?
Why do I get an exception in the latter one?
What's the correct way to implement onDone event?

Answer 1

The onDone callback is called when the Stream is closed. You're never closing the Stream in the first example you provided, so the callback is never called. If you call the closing function at the same time as the timer cancel, it works the way you expected.
Example:
Timer.periodic(Duration(seconds: 1), (t) {
this._controller.sink.add(this._count);
this._count++;
if (_count > 10) {
t.cancel();
finish();
}
});
You get an exception with the second example because you're trying
to add an event to a Stream that you closed already. After the 5
taken elements finishes, your onDone method is called, closing
the Stream from the NumberCreator, but the Timer is still ticking, and tries to add an event to the closed Stream. The confusion may have come from the fact that the take method returns a Stream that closes after it emits the provided number of events, in this case 5.
It's already being done correctly for the most part, you just never close the Stream so it's never expected to be called when you think it should.