Chaining requests in Retrofit + RxJava

Question

I have 2 APIs that I want to make request to in sequence and store their data in SQLite.

First I want to make request to API A and store its data in SQL table a. Then make request to API B and store its data in table b and some data in table a_b. The data stored in a_b is from request B alone.

How can I do this using RxJava. I read somewhere about using flatMap for this, something like this

apiService.A()
    // store in DB here? How? maybe use map()?
    .flatMap(modelA -> {
        // or maybe store modelA in DB here?
        return apiService.B().map(modelB -> {
            storeInDB()l // store B here ?
            return modelB;
        });
    });

If I wasn't using lambda functions, this would look as ugly as normal nested calls. Is this a better way to do it?

Answer 1

I don't think using map operator is the best way to go with things like storing the result of the api call.

What I like to do is to separate those things inside doOnNext operators. So your example would be something like this:

apiService.A()         .doOnNext(modelA -> db.store(modelA))         .flatMap(modelA -> apiService.B())         .doOnNext(modelB -> db.store(modelB)); 

(add necessary observeOn and subscribeOn yourself, exactly like you need them)

Answer 2

Yes, you can use flatmap for this exact purpose. See the below example (Assuming your service A returns Observable<FooA> and service B returns Observable<FooB>)

api.serviceA()         .flatMap(new Func1<FooA, Observable<FooB>>() {             @Override             public Observable<FooB> call(FooA fooA) {                 // code to save data from service A to db                  // call service B                 return api.serviceB();             }          })          .subscribeOn(Schedulers.io())          .observeOn(AndroidSchedulers.mainThread())          .subscribe(new Subscriber<FooB>() {              @Override              public void onCompleted() {              }               @Override              public void onError(Throwable e) {               }               @Override              public void onNext(FooB fooB) {                  // code to save data from service B to db               }         });