src/main/java/io/reactivex/plugins/RxJavaPlugins.java
@NonNull
    public static Scheduler onSingleScheduler(@NonNull Scheduler defaultScheduler) {
        Function<? super Scheduler, ? extends Scheduler> f = onSingleHandler;
        if (f == null) {
            return defaultScheduler;
        }
        return apply(f, defaultScheduler);
    }
@NonNull
    public static Scheduler onSingleScheduler(@NonNull Scheduler defaultScheduler) {
        Function<? super Scheduler, ? extends Scheduler> f = onSingleHandler;
        if (f == null) {
            return defaultScheduler;
        }
        return apply(f, defaultScheduler);
    }
Calls the associated hook function.
@param defaultScheduler the hook's input value
@return the value returned by the hook