Blocking method execution until asynchronous RxJava call returns

Question

I am pretty sure someone must have asked this question before but I cannot find it. I am trying to do something quite simple even for a RxJava beginner like me. I have the following method:

 public ILocation findLocationDetails() {
    requestLocationDetails();
    return buildLocationFromDetails();
}

The method requestLocationDetails contains a REST call which is executed inside another thread:

private void requestLocationDetails() {
    compositeDisposable.add(
        Observable
        .fromCallable((() -> JsonRestCaller.readJsonFromUrl(buildUrl())))
        .subscribeOn(Schedulers.io())
        .subscribeWith(new DisposableObserver<JsonObject>() {
            @Override
            public void onNext(JsonObject jsonObject) {
                try {
                    parseJson(jsonObject);
                } catch (IOException e) {
                    Log.e(TAG, e.getMessage());
                }
            }
            @Override
            public void onError(Throwable e) {
                Log.e(TAG, e.getMessage());
            }
            @Override
            public void onComplete() {
            }
    }));
}

Now all I want to do is wait for the parseJson() to complete so the method buildLocationFromDetails can work with the details retrieved in parseJson.

I read something about blocking operators but I am not sure how to make them work in my case. Also I found examples where someone just waited for a while to make sure a result was available but I would not know how long to wait for and that does not strike me as the right approach.

Answer 1

You can try something like below

//Create a method which will parse json and return the result in String 
//(I am assuming string as parse response for simplicity)
public Observable<String> parseJson(JsonObject jsonObject) {
    return Observable
        .create(
            e -> {
                //parse json code goes here.
                // Once parsing done pass the details
                e.onNext("Details from parsing json");
                e.onComplete();
            }
        );
}

Now modify the above code which you have written,

Observable
    .fromCallable((() -> JsonRestCaller.readJsonFromUrl(buildUrl())))
    //Pass the jsonObject from above to parseJson() method
    .flatMap(jsonObject -> parseJson(jsonObject))
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    //details is the string from parsing json
    .subscribe(
        details -> buildLocationFromDetails(details),
        error -> new Throwable(error)
    );

Hope this helps.