How to combine RxJava Single & Completable Retrofit calls in Android Application

Question

My current Android Application employs Retrofit and RxJava to choreograph my network calls.

I have modelled my HTTP GET(s) as Single<Response<String>> and POST(s) as Completable.

The sequence of calls that I require are as follows:-

Sequentially call GET(1), GET(2), GET(3)

Parallel call POST(1), POST(2)

When both POST(1) & POST(2) have completed OK, call GET(4).

I have a partial solution. I have coded calls for the the First three GET(s) followed by the POST calls

My code resembles this:-

Single.concat(getRequests())
                .subscribeOn(Schedulers.single())
                .doOnError(throwable -> Log.e(TAG, "Manage Totals Failed", throwable))
                .doFinally(new Action() {
                    @Override
                    public void run() throws Exception {
                        manageExecutions(combineExecutions());
                    }
                })
                .subscribe();

    /**
     * @return
     */
    private static Iterable<Single<Response<String>>> getRequests() {
        final API_CALL_GET[] apiCalls = API_CALL_GET.values();
        final List<Single<Response<String>>> requests = new ArrayList<>(apiCalls.length);

        for (final API_CALL_GET apiCall : apiCalls) {
            requests.add(apiCall.request());
        }

        return requests;
    }

public enum API_CALL_GET {

    GET_ONE {
        @Override
        public Single<Response<String>> request() {
            return RETRO_SERVICE
                .getOne(authToken, new HashMap<>())
                .doAfterSuccess(this::persistDataOne)
                .doOnError(error -> ever(error));
        }
    }, GET_TWO {
        @Override
        public Single<Response<String>> request() {
            return RETRO_SERVICE
                .getTwo(authToken, new HashMap<>())
                .doAfterSuccess(this::persistDataTwo)
                .doOnError(error -> ever(error));
        }
    },
    GET_THREE {
        @Override
        public Single<Response<String>> request() {
            return RETRO_SERVICE
                .getThree(authToken, new HashMap<>())
                .doAfterSuccess(this::persistDataThree)
                .doOnError(error -> ever(error));
        }
    };

    public abstract Single<Response<String>> request();

}


    private static Action manageExecutions(final List<Completable> completables) {

        return new Action() {
            @Override
            public void run() throws Exception {
                Completable
                .concat(completables)
                .subscribeOn(Schedulers.io())
                .doOnError(throwable -> Log.e(TAG, "Manage Totals Failed", throwable))
                .doOnComplete(new Action() {
                    @Override
                    public void run() throws Exception {
                        accumulateAmounts();
                    }
                })
                .subscribe();
            }
        };
    }


    /**
     * @return
     */
    private static List<Completable> combineExecutions() {
        final API_CALL_POST[] apiCalls = API_CALL_POST.values();
        final List<Completable> requests = new ArrayList<>(apiCalls.length);

        for (final API_CALL_POST apiCall : apiCalls) {
            requests.addAll(apiCall.requests());
        }

        return Lists.newArrayList(Iterables.unmodifiableIterable(requests));
    }

public enum API_CALL_POST {

    POST_ONE {
        @Override
        public List<Completable> requests() {
            return NetworkController.postRecommenderExecutions();
        }
    },
    POST_TWO {
        @Override
        public List<Completable> requests() {
            return NetworkController.postSavedSearcheExecutions();
        }
    };

    public abstract List<Completable> requests();

}


    public static List<Completable> postONE() {
        final List<Completable> completables = new ArrayList<>();

        final List<OneDO> oneDOS = fetchOnes();

        for (final OneDO oneDO : oneDOS) {
            completables.add(RETRO_SERVICE.runCompletableOnes(authToken, oneDO.getId())
                    .doOnError(new Consumer<Throwable>() {
                        @Override
                        public void accept(final Throwable throwable) throws Exception {
                            Log.e(TAG, "accept: ", throwable);
                        }
                    }));
        }

        return completables;
    }




    public static List<Completable> postTWO() {
        final List<Completable> completables = new ArrayList<>();

        final List<TwoDO> twoDOS = fetchTwos();

        for (final TwoDO twoDO : twoDOS) {
            completables.add(RETRO_SERVICE.runCompletableTwos(authToken, twoDO.getId())
                    .doOnError(new Consumer<Throwable>() {
                        @Override
                        public void accept(final Throwable throwable) throws Exception {
                            Log.e(TAG, "accept: ", throwable);
                        }
                    }));
        }

        return completables;
    }

What I am having difficulty with is correctly chaining my calls

e.g. I thought I would be able to develop a solution that resembled this pseudo code

Single.concat(GET_1... GET_N).onComplete(POST_1... POST_N).onComplete(GET_LAST)

however my current partial solution only calls the First group of GET(s) followed by the POST(s) and the GET and POST calls are not "chained"

I cannot see how to create a chain of calls that support my use case.

Is it possible to combine Single -> Completable -> Single in a chained call?

UPDATE

Based on Daniil answer I ended up with this solution:-

 Single.concat(getRequests())
                .subscribeOn(Schedulers.io())
                .doOnError(throwable -> Log.e(TAG, "accept[0000]: ", throwable))
                .ignoreElements()
                .andThen(Completable.merge(combineExecutions()))
                .doOnError(throwable -> Log.e(TAG, "accept: ", throwable))
                .doOnComplete(() -> Controller.accumulateTotals())
                .subscribe();

Answer 1

In kotlin it would look something like this:

fun generateGetRequests(): List<Single<Response<String>>> {
    return listOf(retrofit.firstGet(), retrofit.secondGet(), ... ,retrofit.lastGet())
}

fun generatePostRequests(): List<Completable> {
    return listOf(retrofit.firstPost(), ..., retrofit.lastPost())
}

fun doSomethingWithResponses(responses: Array<Any>) {
    // Do Something, like save to db
}

fun runRequests() {
    Single.zip(generateGetRequests(), { responses ->
        doSomethingWithResponses(responses)
    }).ignoreElements()
        .andThen(Completable.merge(generatePostRequests()))
        .subscribeOn(Schedulers.io())
        .subscribe()
}

Answer 2

Chaining up different types can happen either by converting them onto a shared reactive type (such as Observable) and concatenating, or using one of the continuation approaches via flatMapX and andThen:

 someSingle
 .flatMapCompletable(result1 -> {
     sideEffect(result1);
     return someCompletable;
 })
 .andThen(Single.defer(() -> {
     sideEffectAfterCompletable();
     return someOtherSingle;
 }))
 ...