I have a list of tasks I want to perform in parallell, but I want to display the result of the tasks in the same order as the original list.
In other words, if I have task list [A,B,C], I do not wish to show B-result before I have shown A-result, but nor do I want to wait until A-task is finished before starting B-task.
Additionally, I want to show each result as soon as possible, in other words, if the tasks finish in the order B, then A, then C, I do not want to show anything when I receive B-result, then show A-result immediately followed by B-result when I receive A-result, then show C-result whenever I receive it.
This is of course not terribly tricky to do by making an Observable for each task, combining them with merge, and subscribing on a computation thread pool, then writing a Subscriber which holds a buffer for any results received out of order. However, the Rx rule of thumb tends to be "there's already an operator for that", so the question is "what is the proper RxJava way to solve this?" if indeed there is such a thing.
It seems you need concatEager for this task but it is somewhat possible to achieve it with pre 1.0.15 tools and no need for "creating" Observables. Here is an example for that:
Observable<Long> source1 = Observable.interval(100, 100, TimeUnit.MILLISECONDS).take(10);
Observable<Long> source2 = Observable.interval(100, 100, TimeUnit.MILLISECONDS).take(20);
Observable<Long> source3 = Observable.interval(100, 100, TimeUnit.MILLISECONDS).take(15);
Observable<Observable<Long>> sources = Observable.just(source1, source2, source3);
sources.map(v -> {
Observable<Long> c = v.cache();
c.subscribe(); // to cache all
return c;
})
.onBackpressureBuffer() // make sure all source started
.concatMap(v -> v)
.toBlocking()
.forEach(System.out::println);
The drawback is that it retains all values for the whole duration of the sequence. This can be fixed with a special kind of Subject: UnicastSubject but RxJava 1.x doesn't have one and may not get one "officially". You can, however, look at one of my blog posts and build if for yourself and have the following code:
//...
sources.map(v -> {
UnicastSubject<Long> subject = UnicastSubject.create();
v.subscribe(subject);
return subject;
})
//...
"There's not quite an operator for that". Although, in the 1.0.15-SNAPSHOT build there is an experimental concatEagar() operator those sounds like it does what you're looking for. Pull request for concatEager
repositories {
maven { url 'https://oss.jfrog.org/libs-snapshot' }
}
dependencies {
compile 'io.reactivex:rxjava:1.0.15-SNAPSHOT'
}
If you want to roll your own temporary solution until concatEager() gets the nod of approval. You could try something like this:
public Observable<Result> concatEager(final Observable<Result> taskA, final Observable<Result> taskB, final Observable<Result> taskC) {
return Observable
.create(subscriber -> {
final Observable<Result> taskACached = taskA.cache();
final Observable<Result> taskBCached = taskB.cache();
final Observable<Result> taskCCached = taskC.cache();
// Kick off all the tasks simultaneously.
subscriber.add(
Observable
.merge(taskACached, taskBCached, taskCCached)
.subscribe(
result -> { // Throw away result
},
throwable -> { // Ignore errors
}
)
);
// Put the results in order.
subscriber.add(
Observable
.concat(taskACached, taskBCached, taskCCached)
.subscribe(subscriber)
);
});
}
Note that the above code is totally untested. There are probably better ways to do this but this is what first came to mind...
Related
Observable<List<Stop>> zippedObservable = Observable.zip(observableList, objects -> {
List<Stop> stopList = Collections.emptyList();
for (Object obj : objects) {
stopList.add((Stop) obj);
}
return stopList;
});
I have the zippedObservable variable which was zipped by multiple observables.
disposable.add(zippedObservable.observeOn(AndroidSchedulers.mainThread())
.subscribeWith(new DisposableObserver<List<Stop>>() {
// onNext, onComplete, onError omitted
}));
This function emits the items (zipped stop list) successfully, but I'd like to emit these items every minute. I assumed that interval operator would be perfect for this case, but I couldn't figure out how to mix both zip and interval functionalities.
This is what I tried
zippedObservale.interval() // cannot call interval operator here.
Observable.zip(...).interval() // cannot call interval operator here too.
I am looking for someone to explain how to mix these two operators so that I can emit the items every minute. Thank you.
interval is a static method that creates an Observable<Long> that emits a Long at a given period or interval.
To achieve what you describe, you need to use one such Observable to pace your zipped Observable:
Observable<List<Stop>> zipped = ...;
Observable<Long> interval = Observable.interval(...);
Observable<List<Stop>> everyMinute = zipped.sample(interval);
In that case, it will simply emit at most one result of zipped every minute, dis-regarding whatever else zipped is emitting. I'm not sure that's what you want.
If you want to simply re-emit the same value over and over, you might want to add a repeat() in between.
The Problem
I have two Apis. Api 1 gives me a List of Items and Api 2 gives me more detailed Information for each of the items I got from Api 1. The way I solved it so far results in bad Performance.
The Question
Efficent and fast solution to this Problem with the help of Retrofit and RxJava.
My Approach
At the Moment my Solution Looks like this:
Step 1: Retrofit executes Single<ArrayList<Information>> from Api 1.
Step 2: I iterate through this Items and make a request for each to Api 2.
Step 3: Retrofit Returns Sequentially executes Single<ExtendedInformation> for
each item
Step 4: After all calls form Api 2 completely executed I create a new Object for all Items combining the Information and Extended Information.
My Code
public void addExtendedInformations(final Information[] informations) {
final ArrayList<InformationDetail> informationDetailArrayList = new ArrayList<>();
final JSONRequestRatingHelper.RatingRequestListener ratingRequestListener = new JSONRequestRatingHelper.RatingRequestListener() {
#Override
public void onDownloadFinished(Information baseInformation, ExtendedInformation extendedInformation) {
informationDetailArrayList.add(new InformationDetail(baseInformation, extendedInformation));
if (informationDetailArrayList.size() >= informations.length){
listener.onAllExtendedInformationLoadedAndCombined(informationDetailArrayList);
}
}
};
for (Information information : informations) {
getExtendedInformation(ratingRequestListener, information);
}
}
public void getRatingsByTitle(final JSONRequestRatingHelper.RatingRequestListener ratingRequestListener, final Information information) {
Single<ExtendedInformation> repos = service.findForTitle(information.title);
disposable.add(repos.subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread()).subscribeWith(new DisposableSingleObserver<ExtendedInformation>() {
#Override
public void onSuccess(ExtendedInformation extendedInformation) {
ratingRequestListener.onDownloadFinished(information, extendedInformation);
}
#Override
public void onError(Throwable e) {
ExtendedInformation extendedInformation = new ExtendedInformation();
ratingRequestListener.onDownloadFinished(extendedInformation, information);
}
}));
}
public interface RatingRequestListener {
void onDownloadFinished(Information information, ExtendedInformation extendedInformation);
}
tl;dr use concatMapEager or flatMap and execute sub-calls asynchronously or on a schedulers.
long story
I'm not an android developer, so my question will be limited to pure RxJava (version 1 and version 2).
If I get the picture right the needed flow is :
some query param
\--> Execute query on API_1 -> list of items
|-> Execute query for item 1 on API_2 -> extended info of item1
|-> Execute query for item 2 on API_2 -> extended info of item1
|-> Execute query for item 3 on API_2 -> extended info of item1
...
\-> Execute query for item n on API_2 -> extended info of item1
\----------------------------------------------------------------------/
|
\--> stream (or list) of extended item info for the query param
Assuming Retrofit generated the clients for
interface Api1 {
#GET("/api1") Observable<List<Item>> items(#Query("param") String param);
}
interface Api2 {
#GET("/api2/{item_id}") Observable<ItemExtended> extendedInfo(#Path("item_id") String item_id);
}
If the order of the item is not important, then it is possible to use flatMap only:
api1.items(queryParam)
.flatMap(itemList -> Observable.fromIterable(itemList)))
.flatMap(item -> api2.extendedInfo(item.id()))
.subscribe(...)
But only if the retrofit builder is configured with
Either with the async adapter (calls will be queued in the okhttp internal executor). I personally think this is not a good idea, because you don't have control over this executor.
.addCallAdapterFactory(RxJava2CallAdapterFactory.createAsync()
Or with the scheduler based adapter (calls will be scheduled on the RxJava scheduler). It would my preferred option, because you explicitly choose which scheduler is used, it will be most likely the IO scheduler, but you are free to try a different one.
.addCallAdapterFactory(RxJava2CallAdapterFactory.createWithScheduler(Schedulers.io()))
The reason is that flatMap will subscribe to each observable created by api2.extendedInfo(...) and merge them in the resulting observable. So results will appear in the order they are received.
If the retrofit client is not set to be async or set to run on a scheduler, it is possible to set one :
api1.items(queryParam)
.flatMap(itemList -> Observable.fromIterable(itemList)))
.flatMap(item -> api2.extendedInfo(item.id()).subscribeOn(Schedulers.io()))
.subscribe(...)
This structure is almost identical to the previous one execpts it indicates locally on which scheduler each api2.extendedInfo is supposed to run.
It is possible to tune the maxConcurrency parameter of flatMap to control how many request you want to perform at the same time. Although I'd be cautious on this one, you don't want run all queries at the same time. Usually the default maxConcurrency is good enough (128).
Now if order of the original query matter. concatMap is usually the operator that does the same thing as flatMap in order but sequentially, which turns out to be slow if the code need to wait for all sub-queries to be performed. The solution though is one step further with concatMapEager, this one will subscribe to observable in order, and buffer the results as needed.
Assuming retrofit clients are async or ran on a specific scheduler :
api1.items(queryParam)
.flatMap(itemList -> Observable.fromIterable(itemList)))
.concatMapEager(item -> api2.extendedInfo(item.id()))
.subscribe(...)
Or if the scheduler has to be set locally :
api1.items(queryParam)
.flatMap(itemList -> Observable.fromIterable(itemList)))
.concatMapEager(item -> api2.extendedInfo(item.id()).subscribeOn(Schedulers.io()))
.subscribe(...)
It is also possible to tune the concurrency in this operator.
Additionally if the Api is returning Flowable, it is possible to use .parallel that is still in beta at this time in RxJava 2.1.7. But then results are not in order and I don't know a way (yet?) to order them without sorting after.
api.items(queryParam) // Flowable<Item>
.parallel(10)
.runOn(Schedulers.io())
.map(item -> api2.extendedInfo(item.id()))
.sequential(); // Flowable<ItemExtended>
the flatMap operator is designed to cater to these types of workflows.
i'll outline the broad strokes with a simple five step example. hopefully you can easily reconstruct the same principles in your code:
#Test fun flatMapExample() {
// (1) constructing a fake stream that emits a list of values
Observable.just(listOf(1, 2, 3, 4, 5))
// (2) convert our List emission into a stream of its constituent values
.flatMap { numbers -> Observable.fromIterable(numbers) }
// (3) subsequently convert each individual value emission into an Observable of some
// newly calculated type
.flatMap { number ->
when(number) {
1 -> Observable.just("A1")
2 -> Observable.just("B2")
3 -> Observable.just("C3")
4 -> Observable.just("D4")
5 -> Observable.just("E5")
else -> throw RuntimeException("Unexpected value for number [$number]")
}
}
// (4) collect all the final emissions into a list
.toList()
.subscribeBy(
onSuccess = {
// (5) handle all the combined results (in list form) here
println("## onNext($it)")
},
onError = { error ->
println("## onError(${error.message})")
}
)
}
(incidentally, if the order of the emissions matter, look at using concatMap instead).
i hope that helps.
Check below it's working.
Say you have multiple network calls you need to make–cals to get Github user information and Github user events for example.
And you want to wait for each to return before updating the UI. RxJava can help you here.
Let’s first define our Retrofit object to access Github’s API, then setup two observables for the two network requests call.
Retrofit repo = new Retrofit.Builder()
.baseUrl("https://api.github.com")
.addConverterFactory(GsonConverterFactory.create())
.addCallAdapterFactory(RxJavaCallAdapterFactory.create())
.build();
Observable<JsonObject> userObservable = repo
.create(GitHubUser.class)
.getUser(loginName)
.subscribeOn(Schedulers.newThread())
.observeOn(AndroidSchedulers.mainThread());
Observable<JsonArray> eventsObservable = repo
.create(GitHubEvents.class)
.listEvents(loginName)
.subscribeOn(Schedulers.newThread())
.observeOn(AndroidSchedulers.mainThread());
Used Interface for it like below:
public interface GitHubUser {
#GET("users/{user}")
Observable<JsonObject> getUser(#Path("user") String user);
}
public interface GitHubEvents {
#GET("users/{user}/events")
Observable<JsonArray> listEvents(#Path("user") String user);
}
After we use RxJava’s zip method to combine our two Observables and wait for them to complete before creating a new Observable.
Observable<UserAndEvents> combined = Observable.zip(userObservable, eventsObservable, new Func2<JsonObject, JsonArray, UserAndEvents>() {
#Override
public UserAndEvents call(JsonObject jsonObject, JsonArray jsonElements) {
return new UserAndEvents(jsonObject, jsonElements);
}
});
Finally let’s call the subscribe method on our new combined Observable:
combined.subscribe(new Subscriber<UserAndEvents>() {
...
#Override
public void onNext(UserAndEvents o) {
// You can access the results of the
// two observabes via the POJO now
}
});
No more waiting in threads etc for network calls to finish. RxJava has done all that for you in zip().
hope my answer helps you.
I solved a similar problem with RxJava2. Execution of requests for Api 2 in parallel slightly speeds up the work.
private InformationRepository informationRepository;
//init....
public Single<List<FullInformation>> getFullInformation() {
return informationRepository.getInformationList()
.subscribeOn(Schedulers.io())//I usually write subscribeOn() in the repository, here - for clarity
.flatMapObservable(Observable::fromIterable)
.flatMapSingle(this::getFullInformation)
.collect(ArrayList::new, List::add);
}
private Single<FullInformation> getFullInformation(Information information) {
return informationRepository.getExtendedInformation(information)
.map(extendedInformation -> new FullInformation(information, extendedInformation))
.subscribeOn(Schedulers.io());//execute requests in parallel
}
InformationRepository - just interface. Its implementation is not interesting for us.
public interface InformationRepository {
Single<List<Information>> getInformationList();
Single<ExtendedInformation> getExtendedInformation(Information information);
}
FullInformation - container for result.
public class FullInformation {
private Information information;
private ExtendedInformation extendedInformation;
public FullInformation(Information information, ExtendedInformation extendedInformation) {
this.information = information;
this.extendedInformation = extendedInformation;
}
}
Try using Observable.zip() operator. It will wait until both Api calls are finished before continuing the stream. Then you can insert some logic by calling flatMap() afterwards.
http://reactivex.io/documentation/operators/zip.html
I can think of two ways to get the value from Single
Single<HotelResult> observableHotelResult =
apiObservables.getHotelInfoObservable(requestBody);
final HotelResult[] hotelResults = new HotelResult[1];
singleHotelResult
.subscribe(hotelResult -> {
hotelResults[0] = hotelResult;
});
Or
final HotelResult hotelResult = singleHotelResult
.toBlocking()
.value();
It's written in the documentation that we should avoid using .toBlocking method.
So is there any better way to get value
Even it is not recommended to block it (you should subscribe), in RxJava v2 the method for blocking is blockingGet(), it returns the object immediately.
When we use toBlocking then we get result immediately. When we use subscribe then result is obtained asynchronously.
Single<HotelResult> observableHotelResult =
apiObservables.getHotelInfoObservable(requestBody);
final HotelResult[] hotelResults = new HotelResult[1];
singleHotelResult.subscribe(hotelResult -> {
hotelResults[0] = hotelResult;
});
// hotelResults[0] may be not initialized here yet
// println not show result yet (if operation for getting hotel info is long)
System.out.println(hotelResults[0]);
For blocking case:
final HotelResult hotelResult = singleHotelResult.toBlocking().value();
// hotelResult has value here but program workflow will stuck here until API is being called.
toBlocking helps in the cases when you are using Observables in "normal" code where you need to have the result in place.
subscribe helps you for example in Android application when you can set some actions in subscribe like show result on the page, make button disabled etc.
can someone explain me this loop?
private Observable<ArrayList<RateItem>> generateRates(List<Rates> rates){
rateItemArrayList = new ArrayList<RateItem>();
Observable<ArrayList<RateItem>> observable = Observable.from(rates)
.map(new Func1<Rates, ArrayList<RateItem>>() {
#Override
public ArrayList<RateItem> call(Rates r) {
for(Rates rat : rates)
rateItemArrayList.add(new RateItem(rat.getName(),rat.getCode(),rat.getValue()));
return rateItemArrayList;
}
});
return observable;
}
My List has 35(checked) items after response from server, then i want to make different ArrayList from this list. Why this loop iterate about 1000 times instead of 35? After that my new ArrayList has 1350 items o.O.
you code contains a side effect : a method update a object outside of his context (here, the map method update a list outside of it).
Then your for loop is quite strange as you loop over rates variables, which is used to build the Observable too. (Which I think is not expected)
Your code can be simplier, using non side effect code :
private Observable<List<RateItem>> generateRates(List<Rates> rates){
return Observable.from(rates)
.map(r -> new RateItem(r.getName(), r.getValue())
.toList();
}
it should be quite close to what you want to achieve.
#dwursteisen covered the side effects you should avoid but the code was not correct. This will work I think.
Observable<List<RateItem>> =
Observable
.from(rates)
.flatMap(r ->
Observable.from(r)
.map(rat ->
new RateItem(rat.getName(),
rat.getCode(),
rat.getValue()));
Using RxJava I have an Observable<A> and an Observable<B>. I want to start subscription on B as soon as the first (and only) element of A is emitted. I know I can chain it like this:
final Observable<A> obsOfA;
final Observable<B> obsOfB;
obsOfA.subscribe(new Action1<A>() {
#Override
public void call(A a) {
obsOfB.subscribe(...)
}
});
..But this will cause a nesting syntax which gets ugly as soon as we introduce Observable<C>. How can I "unwrap" the syntax to a more fluent one - getting one that is more like the javascript Promise.then()-flow?
You should use flatMap:
obsOfA.flatMap(new Func1<A, Observable<B>>() {
#Override
public Observable<B> call(A a) {
return obsOfB;
}
})
.subscribe(/* obsOfB has completed */);
Every time obsOfA calls onNext(a), call will be executed with this value a.
You can use switch, combined with map in switchMap:
obsOfA.switchMap(i -> obsOfB)
.subscribe(/* obsOfB has completed */);
This does almost the same as merge in flatMap as long as obsOfA only yield 1 value, but when it yield more values, flatmap will combine them, while switch will only be subscribed to the last instance of obsOfB. This might be useful when you need to switch to a different stream.