src/main/java/io/reactivex/plugins/RxJavaPlugins.java
@NonNull
    public static Runnable onSchedule(@NonNull Runnable run) {
        ObjectHelper.requireNonNull(run, "run is null");

        Function<? super Runnable, ? extends Runnable> f = onScheduleHandler;
        if (f == null) {
            return run;
        }
        return apply(f, run);
    }
@NonNull
    public static Runnable onSchedule(@NonNull Runnable run) {
        ObjectHelper.requireNonNull(run, "run is null");

        Function<? super Runnable, ? extends Runnable> f = onScheduleHandler;
        if (f == null) {
            return run;
        }
        return apply(f, run);
    }
Called when a task is scheduled.
@param run the runnable instance
@return the replacement runnable