Develop Reference

Given an existing Flux (FluxMap), how can I emit message to this Flux?

Our project use a external library. It has a method return FluxMap (since FluxMap is not completely public so just call it Flux):
Flux<MappedType> aFluxMap = Library.createMappingToMappedType();
I have to emit some objects to aFluxMap to get them converted to MappedType (it has private constructor, few setter), then I can:
aFluxMap.doOnNext(converted -> doJob(converted))
I expect that there is a method on Flux/Mono like:
aFluxMap.emit(myObj);
But I could not find any method like that.
I have searched "how to emit to flux dynamically", then there is a solution:
FluxProcessor p = UnicastProcessor.create().serialize();
FluxSink sink = p.sink();
sink.next(mess);
But seem that it emit to newly created flux (p), not my aFluxMap. So I want to ask is there any way to emit message to a existed Flux (or how to connect a FluxSink to a existed Flux, so that whenever FluxSink .next(mess), then the existed Flux get the message). Thank you
Note: please don't pay much attention to the stupidity of the library. We must use it
==========================================
UPDATE:
As #lkatiforis suggestion:
FluxProcessor p = //see above
Flux<MappedType> aFluxMap = Library.createMappingToMappedType();
p.flatMap(raw -> aFluxMap).subscribe();
I got another issue. Library.createMappingToMappedType() return a subscribed Flux with its source is UnicastProcessor (also subscribed).
When I call p.flatMap(raw -> aFluxMap), then internally aFluxMap get subscribed again cause its source also get subscribed again, so that I got an exception telling that "UnicastProcessor can be subscribe once". Any suggestion?

You can create a new stream and then merge the two streams into one by using one of these methods: merge, concat, zip, and their variants.
Here is an example:
Flux<MappedType> yourFlux = //...
Flux<MappedType> aFluxMap = Library.createMappingToMappedType();
Flux.merge(aFluxMap, yourFlux);
The merge operator executes a merging of the MappedType objects from the two provided publisher sequences.

how to convert Flux<pojo> to ArrayList<String>

In my spring-boot springboot service class, I have created the following code which is not working as desired:
Service class:
Flux<Workspace> mWorkspace = webClient.get().uri(WORKSPACEID)
.retrieve().bodyToFlux(Workspace.class);
ArrayList<String> newmWorkspace = new ArrayList();
newmWorkspace = mWorkspace.blockLast();
return newmWorkspace;
Please someone help me on converting the list of json values to put it into arrayList
Json
[
{
"id:"123abc"
},
{
"id:"123abc"
}
]

Why is the code not working as desired
mWorkspace is a publisher of one or many items of type Workspace.
Calling newmWorkspace.blockLast() will get a Workspace from that Publisher:
which is an object of type: Workspace and not of type ArrayList<String>.
That's why : Type mismatch: cannot convert from Workspace to ArrayList<String>
Converting from Flux to an ArrayList
First of all, in reactive programming, a Flux is not meant to be blocked, the blockxxx methods are made for testing purposes. If you find yourself using them, then you may not need reactive logic.
In your service, you shall try this :
//initialize the list
ArrayList<String> newmWorkspace = new ArrayList<>();
Flux<Workspace> mWorkspace = webClient.get().uri(WORKSPACEID)
.retrieve().bodyToFlux(Workspace.class)
.map(workspace -> {
//feed the list
newmWorkspace.add(workspace.getId());
return workspace;
});
//this line will trigger the publication of items, hence feeding the list
mWorkspace.subscribe();
Just in case you want to convert a JSON String to a POJO:
String responseAsjsonString = "[{\"id\": \"123abc\"},{\"id\": \"123cba\"}] ";
Workspace[] workspaces = new ObjectMapper().readValue(responseAsjsonString, Workspace[].class);

You would usually want to avoid blocking in a non-blocking application. However, if you are just integrating from blocking to non-blocking and doing so step-by-step (unless you are not mixing blocking and non-blocking in your production code), or using a servlet stack app but want to only use the WebFlux client, it should be fine.
With that being said, a Flux is a Publisher that represents an asynchronous sequence of 1..n emitted items. When you do a blockLast you wait until the last signal completes, which resolves to a Workspace object.
You want to collect each resolved item to a list and return that. For this purpose, there is a useful method called collectList, which does this job without blocking the stream. You can then block the Mono<List<Workspace>> returned by this method to retrieve the list.
So this should give you the result you want:
List<Workspace> workspaceList = workspaceFlux.collectList().block();
If you must use a blocking call in the reactive stack, to avoid blocking the event loop, you should subscribe to it on a different scheduler. For the I/O purposes, you should use the boundedElastic Scheduler. You almost never want to call block on a reactive stack, instead subscribe to it. Or better let WebFlux to handle the subscription by returning the publisher from your controller (or Handler).

How to pass Mono<> result from previous step to the next doOnSuccess() method

Let's say that I have a method addVoteToSong like:
public Mono<Map<Song, VoteKind>> addVoteToSong(Principal principal, String songId, VoteKind voteKind) {
return
userRepository.findUserByUsername(principal.getName())
.doOnSuccess(song -> songRepository.findSongById(songId))
.doOnSuccess(vote -> voteRepository.add(Vote.builder().song()))
.//(the rest of the code)
}
I want to pass a result from the line:
userRepository.findUserByUsername(principal.getName())
and
.doOnSuccess(song -> songRepository.findSongById(songId))
to the built object in the line:
.doOnSuccess(vote -> voteRepository.add(Vote.builder().song(here result from findSongById).user(here result from findUserByUsername))
Here comes the question, is it possible to reuse previous API call result in the next doOnSuccess method or I should split find API calls at the same time, giving up on Reactor's cascading operations? On the internet, I have found examples with single save method without basing on the indirect result of the reactive stream and that's why question occurred. I will be grateful for suggestions on how to reach a goal.

Martin,
First of all, be aware that .doOnXXX are just callbacks that will be executed on some archived conditions. You should avoid putting a business logic inside of them.
Coming back to the question, the first idea that comes to my mind is to benefit from zip operator. So you have to put 2 publishers .findUserByUsername and .findSongById and combine the result using BiFunction. So you can try the following:
public Mono<Map<Song, VoteKind>> addVoteToSong(Principal principal, String songId, VoteKind voteKind) {
return Mono
.zip(
userRepository.findUserByUsername(principal.getName()),
songRepository.findSongById(songId),
(user, song) -> voteRepository.add(Vote.builder().song(song).user(user).build())
)
.flatMap(Function.identity())
// your code is here
}

How to get data from more than one json api

I have more than one json api ex: api1 ,api2 and api3
Each one of them has different structure from the others,
I want to get data from these three api and combine them in one recycler view , i searched about these subject but couldn’t get any useful tutorial i am already using retrofit for fetch data from only one api , should i using rxjava withe retrofit to do what i want And how .

I'm guessing you're looking for the zip method. It basically takes many observables, waits for their items - all of them- to arrive and then merges them.
So you can basically do each API call in an observable, whatever they return you'll get it all and do whatever you want with them in order to prepare them to be included in your recyclerview, and finally when the whole operation succeeds you'll populate your recyclerview.
Observable obs1 = Observable.fromCallable(// Callable 1);
Observable obs2 = Observable.fromCallable(// Callable 2);
Observable obs3 = Observable.fromCallable(// Callable 3);
Observable.zip(obs1, obs2, obs3, (o1, o2, o3) -> // something)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(o -> {
// on success
});

How to use switchIfEmpty RxJava

The logic here is that if the ratings in the database are empty, then I want to get them from the API. I have the following code:
Observable.from(settingsRatingRepository.getRatingsFromDB())
.toList()
.switchIfEmpty(settingsRatingRepository.getSettingsRatingModulesFromAPI())
.compose(schedulerProvider.getSchedulers())
.subscribe(ratingsList -> {
view.loadRatingLevels(ratingsList, hideLocks);
}, this::handleError);
The getRatingsFromDB() call returns List<SettingRating>, but the API call returns Observable<List<SettingRating>>.
However, when I unit test this, when I pass an empty list from the database call, it does not execute the API call. Can someone pls help me in this matter. This is my unit test code:
when(mockSettingsRatingsRepository.getRatingsFromDB()).thenReturn(Collections.emptyList());
List<SettingsRating> settingsRatings = MockContentHelper.letRepositoryReturnSettingsRatingsFromApi(mockSettingsRatingsRepository);
settingsParentalPresenter.onViewLoad(false);
verify(mockView).loadRatingLevels(settingsRatings, false);

As #Kiskae mentioned, it's the fact that I am confusing an empty list with an empty Observable. Therefore, I have used the following which is what I want:
public void onViewLoad(boolean hideLocks) {
Observable.just(settingsRatingRepository.getRatingsFromDB())
.flatMap(settingsRatings -> {
if (settingsRatings.isEmpty()) {
return settingsRatingRepository.getSettingsRatingModules();
} else {
return Observable.just(settingsRatings);
}
})
.compose(schedulerProvider.getSchedulers())
.subscribe(ratingsList -> {
view.loadRatingLevels(ratingsList, hideLocks);
}, this::handleError);
}

Observable#toList() returns a single element. If the observable from which it gets its elements is empty, it will emit an empty list. So by definition the observable will never be empty after calling toList().

switchIfEmpty will only be called when your observer completes without emitting any items.
Since you are doing toList it will emit list object. Thats why your switchIfEmpty is never getting called.
If you want to get data from cache and fallback to your api if cache is empty, use concat along with first or takeFirst operator.
For example:
Observable.concat(getDataFromCache(), getDataFromApi())
.first(dataList -> !dataList.isEmpty());

Building on answer by #kiskae, your use of a toList() emits the elements aggregated as a single List.
There is an alternative to the use of Observable.just() + a flatMap here.
Observable.from will iterate over the list returned by your service and emit each individual items, so an Observable<Rating>. If said list is empty, it naturally produces an empty Observable. Your API call also produces an Observable<Rating>, and in both cases you want to reaggregate that back into a List<Rating> for further processing.
So just move the toList() from your original code down one line, after the switchIfEmpty calling the API:
Observable.from(settingsRatingRepository.getRatingsFromDB())
.switchIfEmpty(settingsRatingRepository.getSettingsRatingModulesFromAPI())
.toList()
.compose(schedulerProvider.getSchedulers())
.subscribe(ratingsList -> {
view.loadRatingLevels(ratingsList, hideLocks);
}, this::handleError);
Granted, that solution may produce a bit more garbage (as the db's List is turned into an Observable just to be turned into a new List later on).