getLayersObservable(id)
                .flatMap(new Function<List<Layer>, ObservableSource<Layer>>() {
                    @Override
                    public ObservableSource<Layer> apply(@NonNull List<Layer> layers) throws Exception {
                        return Observable.fromIterable(layers);
                    }
                })
                .flatMap(new Function<Layer, ObservableSource<ResponseBody>>() {
                    @Override
                    public ObservableSource<ResponseBody> apply(@NonNull Layer layer) throws Exception {
                        String serverUrl = //buildurl
                        return contentApiManager.downloadFile(serverUrl);
                    }
                })
                .flatMap(new Function<ResponseBody, ObservableSource<File>>() {
                    @Override
                    public ObservableSource<File> apply(@NonNull ResponseBody body) throws Exception {
                        return processResponseBody(body);
                    }
                })
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .toList()
                .subscribe(new Consumer<List<File>>() {
                    @Override
                    public void accept(@NonNull List<File> files) throws Exception {
                        //todo process result.
                    }
                }, new Consumer<Throwable>() {
                    @Override
                    public void accept(@NonNull Throwable throwable) throws Exception {
                        Log.w("TAWWW", throwable);
                    }
                });