subscribeOnUpdates method
Opens the server-side update stream for channel.
One stream per channel; the connection is the streams, so subscribing is
connecting and the last cancellation disconnects. Called by
DwSocketService, never by an app directly.
Implementation
@override
Stream<SerializableModel> subscribeOnUpdates({required String channel}) =>
answerSubscribe?.call(channel) ??
Stream<SerializableModel>.fromFuture(
Completer<SerializableModel>().future,
);