@SuppressWarnings({ "rawtypes", "unchecked" })
    @NonNull
    public static <T> Observer<? super T> onSubscribe(@NonNull Observable<T> source, @NonNull Observer<? super T> observer) {
        BiFunction<? super Observable, ? super Observer, ? extends Observer> f = onObservableSubscribe;
        if (f != null) {
            return apply(f, source, observer);
        }
        return observer;
    }