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