How to wait/observe incoming data from server correctly with rxjava/retrofit

Question

I have a problem with downloading data from server, to be more specific I cannot implement a "waiting/observing incoming data" behavior. I want to fill up my RecyclerView and Spinner with lists come from server, but right know, the methods return null/empty list. I understand, the problem is that the method finish before the data arrives.

I've checked these two questions which are similar, but none of the answers worked for me:

Waiting for API Calls to complete with RxJava and Retrofit

RXJava retrofit waiting having the UI wait for data to be fetched from API

My code:

API:

@GET("/api/categories")
Observable<Response<JsonObject>> getCategories();

netWorker:

public List<String> getCat() {
    List<String> incomingDataSet = new ArrayList<>();
    
    compositeDisposable.add(
        apiConnector.getCategories()
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(response -> {
                if (response.code() >= 200 && response.code() < 300) {
                    JsonArray incomingArray = response.body().getAsJsonArray("category");
                    
                    for (int i = 0; i < incomingArray.size(); i++) {
                        incomingDataSet.add(incomingArray.get(i).getAsJsonObject().get("categoryname").toString().replaceAll("\"", ""));
                        Log.i("LOG", "downloadCategories response: " + incomingDataSet.toString());
                    }

                } else {
                    Log.i("LOG", "Response error: " + response.code());
                }
            })
    );

    Log.i("LOG", "getCat: " + incomingDataSet.toString());
    
    return incomingDataSet;
}

MainActivity:

List<String> categorylist = new ArrayList<>();
categorylist = netWorker.getCat();
Log.i("LOG", "list:" + categorylist );

So, what I see in log:

First:

getCat: []

list: []

and then:

downloadCategories response: [cat1]

downloadCategories response: [cat1, cat2]

downloadCategories response: [cat1, cat2, cat3]

Answer 1

Show spinner in doOnSubscribe() and hide in doFinally() . Put code that fills RecyclerView inside subscribe() to fill it when data arrives. Otherwise you just get empty new list on Main thread and do not update RecyclerView further.