src/main/java/io/reactivex/plugins/RxJavaPlugins.java
@SuppressWarnings({ "rawtypes", "unchecked" })
    @NonNull
    public static <T> MaybeObserver<? super T> onSubscribe(@NonNull Maybe<T> source, @NonNull MaybeObserver<? super T> observer) {
        BiFunction<? super Maybe, ? super MaybeObserver, ? extends MaybeObserver> f = onMaybeSubscribe;
        if (f != null) {
            return apply(f, source, observer);
        }
        return observer;
    }
@SuppressWarnings({ "rawtypes", "unchecked" })
    @NonNull
    public static <T> MaybeObserver<? super T> onSubscribe(@NonNull Maybe<T> source, @NonNull MaybeObserver<? super T> observer) {
        BiFunction<? super Maybe, ? super MaybeObserver, ? extends MaybeObserver> f = onMaybeSubscribe;
        if (f != null) {
            return apply(f, source, observer);
        }
        return observer;
    }
Calls the associated hook function.
@param <T> the value type
@param source the hook's input value
@param observer the subscriber
@return the value returned by the hook