src/main/java/io/reactivex/plugins/RxJavaPlugins.java
@SuppressWarnings({ "rawtypes", "unchecked" })
    @NonNull
    public static <T> ParallelFlowable<T> onAssembly(@NonNull ParallelFlowable<T> source) {
        Function<? super ParallelFlowable, ? extends ParallelFlowable> f = onParallelAssembly;
        if (f != null) {
            return apply(f, source);
        }
        return source;
    }
@SuppressWarnings({ "rawtypes", "unchecked" })
    @NonNull
    public static <T> ParallelFlowable<T> onAssembly(@NonNull ParallelFlowable<T> source) {
        Function<? super ParallelFlowable, ? extends ParallelFlowable> f = onParallelAssembly;
        if (f != null) {
            return apply(f, source);
        }
        return source;
    }
Calls the associated hook function.
<p>History: 2.0.6 - experimental; 2.1 - beta
@param <T> the value type of the source
@param source the hook's input value
@return the value returned by the hook
@since 2.2