Single.fromCallable() - Chaining two calls, one after another

Question

I'm using Single.fromCallable() to make a network call and I want to use the data from the response to make a second call.

How can I chain these two calls instead of nesting them?

private void queryForUser() {
    Single.fromCallable(() -> remoteRepository.queryForUser()))
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(response-> {

                    String username = response.getUsername();

                    //Perform second call
                    performSecondQuery(username);

                }, err -> {
                    Log.e(TAG, "Failed to get user", err);
               
                });

}

private void performSecondQuery(String username){
        Single.fromCallable(() -> remoteRepository.performSecondQuery(username)))
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(secondQueryResult -> {

                    Log.d(TAG, "performSecondQuery: " + secondQueryResult);

                }, err -> {
                    Log.e(TAG, "Failed to perform second query", err);
                });
}

Answer 1

My Java lambda syntax is rusty, but using flatMap() should give you something like this:

private Single<Whatever> performSecondQuery(String username){
        return Single.fromCallable(() -> remoteRepository.performSecondQuery(username)))
}

private void queryForUser() {
    Single.fromCallable(() -> remoteRepository.queryForUser()))
                .flatMap { response -> performSecondQuery(response.getUsername()) }
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(
                   // TODO something with the result
                }, err -> {
                    Log.e(TAG, "Failed to get user", err);
                });
}

(note: I don't know what the second query is doing, so I don't know what Whatever is — it would be the return type of remoteRepository.performSecondQuery())