RxJava + retrofit, get a List and add extra info for each item

Question

I'm playing around with RXJava, retrofit in Android. I'm trying to accomplish the following:

I need to poll periodically a call that give me a Observable> (From here I could did it)

Once I get this list I want to iterate in each Delivery and call another methods that will give me the ETA (so just more info) I want to attach this new info into the delivery and give back the full list with the extra information attached to each item.

I know how to do that without rxjava once I get the list, but I would like to practice.

This is my code so far:

pollDeliveries = Observable.interval(POLLING_INTERVAL, TimeUnit.SECONDS, Schedulers.from(AsyncTask.THREAD_POOL_EXECUTOR))
            .map(tick -> RestClient.getInstance().getApiService().getDeliveries())
                    .doOnError(err -> Log.e("MPB", "Error retrieving messages" + err))
                    .retry()
                    .subscribe(deliveries -> {
                        MainApp.getEventBus().postSticky(deliveries);
                    });

This is giving me a list of deliveries. Now I would like to accomplish the second part.

Hope I been enough clear. Thanks

Not sure, but you might be able to use flatmap to transform the deliveries into a new observable containing ETA's http://reactivex.io/documentation/operators/flatmap.html — cYrixmorten, Feb 05 '15 at 10:58
Yes I saw some examples, but I could not really understand how to do it... it's the first time I'm trying to use rxjava — Marcel, Feb 05 '15 at 10:59
Here is an example subscribing on GPS location, then transforming into address lookup Observable https://github.com/mcharmas/Android-ReactiveLocation/blob/master/sample/src/main/java/pl/charmas/android/reactivelocation/sample/MainActivity.java line 90 — cYrixmorten, Feb 05 '15 at 12:01
@cYrixmorten thank to share this resource, it's useful but not exactly my situation. He gets the list
and then he just take an address make it as string then send it to the view. I need to get for each item modify and return the full list — Marcel, Feb 05 '15 at 13:10
Not sure that I agree.. `.flatMap` receives a Location object (this could be a `List`), then inside `.flatMap` the Location is transformed into `List
`, finally in `.map` the list of addresses is transformed into a string. If `.map` was excluded from the example, then `.subscribe` would have received `List
`. So to sum up the events, the observer initial receives a Location, which is then transformed: `Location -> flatMap(Location) -> List
-> map(List
) -> String -> subscribe(String)`. — cYrixmorten, Feb 05 '15 at 14:23
You are right if we don't use the map() then we receive a list
in the subscribe but it's not the situation I have. I need to iterate over the list get more info and then return the list with the extra info inside. In this example I get a location I make it a list and that was it. Do you see what I mean? — Marcel, Feb 05 '15 at 14:44
Just dropping in here but why not `.flatMap()` the Delivery into ETA (so you iterate over the output in reactive style), and then `toList()` at the end so that the final output is an observable publishing a list of ETAs? — Adam S, Feb 05 '15 at 14:54
@AdamS could you provide an example? I'm a bit nob on this... — Marcel, Feb 05 '15 at 15:15

score 4 · Accepted Answer · answered Feb 10 '15 at 10:19

Finally I found a nice way to do it.

private void startPolling() {
    pollDeliveries = Observable.interval(POLLING_INTERVAL, TimeUnit.SECONDS, Schedulers.from(AsyncTask.THREAD_POOL_EXECUTOR))
            .flatMap(tick -> getDeliveriesObs())
            .doOnError(err -> Log.e("MPB", "Error retrieving messages" + err))
            .retry()
            .subscribe(this::parseDeliveries, Throwable::printStackTrace);
}

private Observable<List<Delivery>> getDeliveriesObs() {
    return RestClient.getInstance().getApiService().getDeliveries()
            .flatMap(Observable::from)
            .flatMap(this::getETAForDelivery)
            .toSortedList((d1, d2) -> {
                if (d1.getEta() == null) {
                    return -1;
                }
                if (d2.getEta() == null) {
                    return 1;
                }
                return d1.getEta().getDuration().getValue() > d2.getEta().getDuration().getValue() ? 1 : -1;
            });
}

Let's go step by step.

First we create an Observable that triggers every POLLING_INTERVAL time the method getDeliveriesObs() that will return the final list
We use retrofit to get an Observable of the call
We use flatMap to flattern the resut list and get in the next flatmap a Delivery item, one by one.
Then we get the estimated time of arrival set inside the Delivery object and return it
We sort the list to order by estimated time of arrival.
In case of error we print and retry so the interval does not stop
We subscribe finally to get the list sorted and with ETA inside, then we just return it or whatever you need to do with it.

It's working properly and it's quite nice, I'm starting to like rxjava :)

I like this solution, and the step-by-step is nice too. – Adam S Feb 10 '15 at 13:43 — Adam S, Feb 10 '15 at 13:43

Adam S · Answer 2 · 2015-02-05T15:41:18.857

I haven't spent a lot of time with Java 8 lambdas, but here's an example of mapping each object to a different object, then getting a List<...> out at the other end in plain ol' Java 7:

List<Delivery> deliveries = ...;
Observable.from(deliveries).flatMap(new Func1<Delivery, Observable<ETA>>() {
    @Override
    public Observable<ETA> call(Delivery delivery) {
        // Convert delivery to ETA...
        return someEta;
    }
})
.toList().subscribe(new Action1<List<ETA>>() {
    @Override
    public void call(List<ETA> etas) {

    }
});

Of course, it'd be nice to take the Retrofit response (presumably an Observable<List<Delivery>>?) and just observe each of those. For that we ideally use something like flatten(), which doesn't appear to be coming to RxJava anytime soon.

To do that, you can instead do something like this (much nicer with lambdas). You'd replace Observable.from(deliveries) in the above example with the following:

apiService.getDeliveries().flatMap(new Func1<List<Delivery>, Observable<Delivery>>() {
    @Override
    public Observable<Delivery> call(List<Delivery> deliveries) {
        return Observable.from(deliveries);
    }
}).flatMap(...)

See my answer. I use your advice to made it. Thank you @Adam S — Marcel, Feb 10 '15 at 10:21

RxJava + retrofit, get a List and add extra info for each item

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