Interface ObservableOperator<Downstream,Upstream>

Type Parameters:
Downstream - the value type of the downstream
Upstream - the value type of the upstream
Functional Interface:
This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

@FunctionalInterface public interface ObservableOperator<@NonNull Downstream,@NonNull Upstream>
Interface to map/wrap a downstream Observer to an upstream Observer.