rxDart BehaviorSubject: add condition to adding value on listen

Question

I am looking for a recipe that would enable me to fulfill the following requirements.

I want to have a BehaviorSubject like stream controller of a nullable type that:

• allows for multiple listeners to listen to the controller's stream
• will provide the current value of the controller's stream to any new listener if and only if there is a current non-null value
• will not add any new value to other listeners when a new listener registers

To illustrate:

final controller = MyCustomController<int?>();

final sub1 = controller.stream.listen((val) => print("sub1: {value}");

controller.add(1);

final sub2 = controller.stream.listen((val) => print("sub2: {value}");

controller.add(null);

final sub3 = controller.stream.listen((val) => print("sub3: {value}");

The output would be:

sub1: 1
sub2: 1  // received current value on listening subscription2
sub1: null
sub2: null
// no value streamed at sub3 listen event because current value was null

Thanks a lot for your insights.