How to chain multiple observables with RxJava?

Question

I am trying to perform some operatations when a activity onDestroy is called. I want start a Observable with a ID, then retrieve some data from Realm and execute a HTTP request to a backend based on the retrieved data and afterwards store the retrieved data to the row given with the starting id.

Summary:

Retrieve data from database with id
Use data to perform a request to backend
Store retrieved data to row with the id from step 1

Graphical:

Code: what I ended up with and got stuck

Observable.just(id)
        .observeOn(Schedulers.io())
        .map(new Function<String, Person>() {
            @Override
            public Person apply(@NonNull String id) throws Exception {
                Realm realm = Realm.getDefaultInstance();

                Person person = realm.copyFromRealm(realm.where(Person.class).equalTo("id", id).findFirst());

                realm.close();

                return person;
            }
        })
        .switchMap(new Function<Person, Observable<Directions>>() {
            @Override
            public Observable<Directions> apply(@NonNull Person person) throws Exception {
                return Utils.getRemoteService().getDirections(person.getAddress()); // retrofit
            }
        })
        .map(new Function<Directions, Object>() {
            @Override
            public Object apply(@NonNull Directions directions) throws Exception {

                // how do I get the id here to store the data to the correct person

                return null;
            }
        })
        .subscribe();

Note:

POJO's are fictive
It's my first time using RxJava

score 0 · Accepted Answer · answered Jun 17 '17 at 19:42

The information has to be passed down the stream, it can be done like below. It would be much more readable when you wrap it in a class rather than Pair.

 Observable.just(id)
            .observeOn(Schedulers.io())
            .map(new Function<String, Person>() {
                @Override
                public Person apply(@NonNull String id) throws Exception {
                    Realm realm = Realm.getDefaultInstance();

                    Person person = realm.copyFromRealm(realm.where(Person.class).equalTo("id", id).findFirst());

                    realm.close();

                    return person;
                }
            })
            .switchMap(new Function<Person, Observable<Directions>>() {
                @Override
                public Observable<Directions> apply(@NonNull Pair<String, Person> pair) throws Exception {
                    // assuming that id is available by getId
                    return Pair(person.getId(), Utils.getRemoteService().getDirections(person.getAddress())); // retrofit
                }
            })
            .map(new Function<Pair<String, Directions>, Object>() {
                @Override
                public Object apply(@NonNull Pair<String, Directions> pair) throws Exception {

                    // how do I get the id here to store the data to the correct person
                    // pair.first contains the id
                    // pair.second contains the Directions
                    return null;
                }
            })
            .subscribe();

How to chain multiple observables with RxJava?

1 Answers1