reactfx/src/main/java/org/reactfx/EventStreams.java
public static EventStream<Void> invalidationsOf(Observable observable) {
        return new EventStreamBase<Void>() {
            @Override
            protected Subscription observeInputs() {
                InvalidationListener listener = obs -> emit(null);
                observable.addListener(listener);
                return () -> observable.removeListener(listener);
            }
        };
    }
public static EventStream<Void> invalidationsOf(Observable observable) {
        return new EventStreamBase<Void>() {
            @Override
            protected Subscription observeInputs() {
                InvalidationListener listener = obs -> emit(null);
                observable.addListener(listener);
                return () -> observable.removeListener(listener);
            }
        };
    }
Creates an event stream that emits an impulse on every invalidation
of the given observable.