src/main/java/io/reactivex/plugins/RxJavaPlugins.java
@SuppressWarnings("rawtypes")
    public static void setOnSingleSubscribe(@Nullable BiFunction<? super Single, ? super SingleObserver, ? extends SingleObserver> onSingleSubscribe) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onSingleSubscribe = onSingleSubscribe;
    }
@SuppressWarnings("rawtypes")
    public static void setOnSingleSubscribe(@Nullable BiFunction<? super Single, ? super SingleObserver, ? extends SingleObserver> onSingleSubscribe) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onSingleSubscribe = onSingleSubscribe;
    }
Sets the specific hook function.
@param onSingleSubscribe the hook function to set, null allowed