RxJava subscribe onNext is not called when adding element asynchronously - retrofit2

Question

I have a Observable like this
Observable<String> gitHubRepoModelObservable;
I have this code
repoNames = new ArrayList<String>();
gitHubRepoModelObservable = Observable.fromIterable(repoNames);
repoNames.add("Hello");
gitHubRepoModelObservable
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Observer<String>() {
#Override
public void onSubscribe(Disposable d) {
}
#Override
public void onNext(String s) {
System.out.println(s);
}
#Override
public void onError(Throwable e) {
}
#Override
public void onComplete() {
}
});
repoNames is just a list of string. When I am adding a string "hello" manually the onNext is getting called but when I am adding string from a API call like bellow
call.enqueue(new Callback<List<GitHubRepoModel>>() {
#Override
public void onResponse(Call<List<GitHubRepoModel>> call, Response<List<GitHubRepoModel>> response) {
for (GitHubRepoModel repo : response.body()) {
repoNames.add(repo.getName());
}
}
#Override
public void onFailure(Call<List<GitHubRepoModel>> call, Throwable t) {
}
});
I am adding strings from the API into the repoNames the "onNext" is not getting called.
I have seen
.addCallAdapterFactory(RxJava2CallAdapterFactory.create())
can be added while initializing retrofit but I want to better understand the rxjava so in this experiment it is not working.
Please help!

Answer 1

It can't not be work.
When you create you api request and try subscribe you list is emty, so Observable does not work.
You need to create Observable such, that your subcribe will run your request!
Observable<String> gitHubRepoModelObservable = Observable.create(
new Observable.OnSubscribe<String>() {
#Override
public void call(final Subscriber<? super String> sub) {
call.enqueue(new Callback<List<GitHubRepoModel>>() {
#Override
public void onResponse(Call<List<GitHubRepoModel>> call, Response<List<GitHubRepoModel>> response) {
for (GitHubRepoModel repo : response.body()) {
sub.onNext(repo.getName()); //send result to rx
}
sub.onCompleted();
}
#Override
public void onFailure(Call<List<GitHubRepoModel>> call, Throwable t) {
}
});
}
}
);
gitHubRepoModelObservable
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Observer<String>() {
#Override
public void onNext(String s) {
System.out.println(s);
}
#Override
public void onCompleted() {
}
#Override
public void onError(Throwable e) {
}
});

Answer 2

Why would onNext get called if you are just adding element to plain List?
In the first example you are seeing onNext being called because modified list is passed through the stream during subscribe.
Create Subject ex. PublishSubject and pass list to Subject.onNext in onResponse, subscribe to it and you will get what you want.
Second option is adding RxJava2CallAdapterFactory and return Observable<Response<List<GithubRepoModel>>>. This way you don't need to create stream yourself.