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