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RxJava simultaneous remove operations

I'm having an issue trying to understand, in a reactive way, how simultaneous operations to the same observable should work.
The scenario is the following:
I have a list of users and a remove button.
Every time I press remove I'm making a call to the API: UsersApi.removeUser. It is possible to remove multiple users at the same time. Which means that multiple UsersApi.removeUser are happening simultaneously.
After each UsersApi.removeUser I need to make a UsersApi.refreshUser call
So in terms of pseudo code what I am doing when clicking remove is the following:
Presenter:
public Observable<User> removeUser(int userId) {
return UsersApi.removeUser(userId)
.flatMap(user -> UsersApi.refreshUser(userId));
}
Fragment:
public void removeUser() {
presenter.removeUser(userId)
.subscribe(user -> {
//remove user from ui
// update number of total users
})
}
The problem with this approach is that because of the asynchronous nature of the remove (multiple removes allowed) I cannot guarantee that what is reaching the subscribe is the latest one. The subscribe will be reached twice, one for each remove, and the user info might not be updated or the latest. Does that make sense?
What I want to happen:
Parallel/Simultaneous remove calls using a reactive approach (triggered by multiple remove clicks from the user)
After a remove call finishes, start the next remove call
Edit: What I would like to know is how to do/if is possible to do the solution I did (see edit2) using Rx operators.
Edit2: My solution for this was to enqueue the user operations (in this case remove) and emit, using a PublishSubject, when the UsersApi.refreshUser(userId) call finishes.
So basically what I did was (pseudo code):
private final PublishSubject<UserOperation> userOperationObs;
private final ConcurrentLinkedQueue<UserOperation> pendingOperations;
private boolean executingOperation;
private void emitUserOperation(final UserOperation operation) {
if (!executingOperation) {
executingOperation = true;
userOperationObs.onNext(operation);
} else {
executingOperation.add(operation);
}
}
public Observable<User> removeUser(UserOperation operation) {
return UsersApi.removeUser(operation.getUserId)
.switchMap(user -> UsersApi.refreshUser(operation.getUserId))
.doOnNext(user -> {
executingOperation = false;
final UserOperation nextOperation = pendingOperations.poll();
if (nextOperation != null) {
userOperationObs.onNext(operation);
}
};
}

You could turn your UI click into Observable (eg. by using RxBinding). After that, you could use concatMap operator to perform api call so it will start next network call once current api call is finished.
// emit clicks as stream
Observable<?> clicks = RxView.clicks(removeView)
// listen clicks then perform network call in sequence
clicks.concatMap(ignored -> usersApi.refreshUser(userId))

How to wait first request finish before start second with Rx?

I have an async method makeRequest() with callback. It called many times from different classes of my application. I need that this calls start one by one and never simultaneously.
I want to implement this using Rx. Like this:
public void execute() { // This method called many times from another classes
Observable.just(true)
// what I need to add here?
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.map(o -> {
internalExecute();
return o;
})
.subscribe();
}
private void internalExecute() { // This method should called only when previous call was finished
makeRequest(this::onRequestFinished);
}
private void onRequestFinished() {
// here is I handle request finish
}
But at now all requests works at parallel. What I need to add here to run requests one by one?

According to comments, you have here separated streams and requests. each client that execute request expect a result from the request. but no requests allowed to run in parallel, in this case I think the easiest way is to limit the Scheduler to an application global background sequential thread Executor, i.e:
Schedulers.from(Executors.newSingleThreadExecutor())
provide somewhere in your app this single thread Executor, in singleton manner of course, it's important that each request stream will use the same object:
private final Scheduler singleThreadScheduler = Schedulers.from(Executors.newSingleThreadExecutor());
public void execute() { // This method called many times from another classes
Observable.just(true)
.map(o -> {
internalExecute();
return o;
})
.subscribeOn(singleThreadScheduler)
.subscribe();
}
private void internalExecute() { // This method should called only when previous call was finished
makeRequest(this::onRequestFinished);
}
private void onRequestFinished() {
//NOTE: you should make sure that the callback execute where you need it (main thread?)
// here is I handle request finish
}
besides that, you're not exposing Observable outside, to the clients, but rather using callback mechanism, you can leverage reactive approach further, by making execute() returning Observable. (and enjoy composition of Obesrvables, operators, proper use of observeOn/subscribeOn, error handling with onError, disposing/unsubscribing etc.), as you're using async api, you can use fromEmitter()/create() (in newer RxJava1 version)), read more here:
private final Scheduler singleThreadScheduler = Schedulers.from(Executors.newSingleThreadExecutor());
public Observable<Result> execute() { // This method called many times from another classes
return Observable.fromEmitter(new Action1<Emitter<? extends Object>>() {
#Override
public void call(Emitter<?> emitter) {
emitter.setCancellation(() -> {
//cancel request on unsubscribing
});
makeRequest(result -> {
emitter.onNext(result);
});
}
})
.subscribeOn(singleThreadScheduler)
}

RxJava pattern for returning cold results, doing more work, then returning hot results

I'm learning RxJava so please be gentle. I've watched the tutorials, done the reading, searched SO, however, I'm still having some problems transforming my AsyncTaskLoader. For some reason, I can't find a pattern of operators to achieve my task (although I think it's a common one). What I'm trying to do is the following: return an Observable my fragment could subscribe to. The observable should do the following on subscribe:
1) Fetch data from the local database by doing 2 queries, running some logic and returning results;
2) Fetching data from API;
3) Synchronising the new API data with the database;
4) Repeating step one and returning results;
So far I've transformed my db calls and my API calls to return observables. I'm trying to understand how I can emit the cold results and continue with the chain. I could probably keep the two operations separately, and use the same subscriber to subscribe to both? But I'm not sure how that would work if my new loader-replacement class returns an observable... Also I don't really need to process the results from the second observable - I just need for the first one to replay when the second one finished.
So far I have the following:
public Observable<StuffFetchResult> getColdStuff() {
return Observable.zip(mDataSource.listStuff(), mDataSource.listOtherStuff(),
(stuff, moreStuff) -> {
List<Stuff> mergedList = new ArrayList<>();
// do some merging stuff
return new StuffFetchResult(mergedList);
}).subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}
Assume I also have getHotStuff() that will do the API call and the synchronisation with the database, if that's the right approach, and return the same Observable. However, I'm stuck on the next step - how can I restart the first observable to replay once hotStuff has completed, without adding another subscriber?
EDIT:
I've made some progress and I think all I need now is to join it all up. I have my two methods:
1) getColdStuff() is pretty much as described above
2) getHotStuff() will do call to the API, synchronise with the database, and return an Observable. The idea was to call getColdStuff() again after getHotStuff() has finished in order to refresh the UI, so actual result returned from getHotStuff() can be ignored. All it needs to do is to trigger getColdStuff() once done.
I've tried the suggestion in the answer to and created the following:
BehaviorRelay<Observable<StuffFetchResult>> callSequence = BehaviorRelay.create();
Observable<StuffFetchResult> valueSequence = Observable.switchOnNextDelayError(callSequence.toSerialized());
valueSequence.subscribe(new Subscriber<StuffFetchResult>() {
#Override
public void onCompleted() {}
#Override
public void onError(Throwable e) {}
#Override
public void onNext(StuffFetchResult result) {
// UI stuff
}
});
callSequence.call(loader.getColdStuff());
I can subscribe to valueSequence here and use callSequence.call(loader.getColdStuff());, which will run the first method and produce results in onNext() of my subscription, which I can use for my UI. However, I'm not sure how to run getHotStuff() in parallel and also do a different action on it when it returns. Also getHotStuff() returns a different type of Observable so I can't really use the same callSequence?
EDIT 2
Using two subscribers, I can achieve the required behaviour I think. Not really sure if that's the right way to go about it though.
loader.getHotStuff()
.subscribeOn(Schedulers.io())
.subscribe( new Subscriber<Object>() {
#Override
public void onCompleted() {}
#Override
public void onError(Throwable e) {}
#Override
public void onNext(Object stuffWeDontCareAbout) {
callSequence.call(loader.getColdStuff());
}
});

if i understand your scenario correctly, you may want something like that -
BehaviorSubject<Observable<T> callSequence = BehaviorSubject.create();
Observable<T> valueSequence = Observable.swithOnNextDelayError(callSequence.toSerialized());
your subscriber will be listening to the valueSequence, and whenever you need to "restart", you will call this -
callSequence.onNext(call.cache()); // *call* is Observable<T>
(i leave the .subscribeOn/.observeOn configuration to you)

Rxjava onNext not firing via retrofit observable

I have this simple retrofit2 api interface which contains
interface Api {
#GET(BuildConfig.END_POINT) Observable<Response> fetchData();
}
So everything is fine when I'm doing a fresh request
but let say I fire a request and I un-subscribe immediately and then I try to fire new request it returns nothing.
So, in code it looks something like this:
in Activity::onPause I perform un-subscription and in Activity::onResume I fire the request again.
My request looks something like this::
api.fetchData()
.timeout(30,TimeUnit.SECONDS)
.doOnNext(new Action1<Response>() {
#Override public void call(Response response) {
list = response.getDataForList();
}
}).flatMap(new Func1<Response, Observable<List<Object>>>() {
#Override public Observable<Object>> call(Response response) {
return Observable.just(list);
}
});
When I tried debugging it, the call is made but doOnNext() is not called. None of the lifecycle methods are called.
And just for clarification from here I'm just returning the observable which I'm using it somewhere else where I'm observing on main thread and subscribing on IO.

Don't use doOnNext, just map. And try to get use to use lambdas, make your code much readable.
api.fetchData()
.timeout(30,TimeUnit.SECONDS)
.map(response -> response.getDataForList())
.flatMap(list -> Observable.just(list));
Now as concept, every time that some observer subscribe to this observable consume the items, then observable automatically unsubscribe the observer. So you don't have to worry about unsubscribe anything.
You can see some practical examples here. https://github.com/politrons/reactive

UI blocking occurs in Android despite RxJava

I am using RxJava to move network access to a separate thread in Android, but my UI still blocks.
I am not using the wrong observable as shown here: Android RxJava, Non Blocking?
The codepoints [A], [B] and [C] in below code are passed in the order [A] -> [C] -> [B] so the current thread is processed fine and RxJava calls [C] once it had a result. This is fine.
Also, blocking is much better compared to doing the network call on the UI thread, but I still have minor blocking. The UI stays fluent after the call is made, but if the server does not respond in a matter of milliseconds, it blocks.
private search; // search is an instance variable in the same class
// [A]
Observable.just(search.find("something")) // search.find calls the REST endpoint
.subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread())
.subscribe(new Action1<Search>() {
#Override public void call(Search search) {
// further processing // [B]
}
}, new Action1<Throwable>() {
#Override public void call(Throwable throwable) {
// error handler
}
});
// [C]
Could it be a problem that search is an instance variable in the same class where the Observable uses it, but the endpoint call is performed from a separate library? It shouldn't matter, right?
Am I doing anything bad that I shouldn't be doing?
--
Find looks like this (removed exception handling for brevity):
public Search find(String searchtext) {
setSearchtext(searchtext);
SearchEndpoint.find(Session.getUser().getId(), searchtext);
return this;
}
SearchEndpoint like this:
public static Search find(final Long userId, final String searchtext) throws IOException {
return ApiService.api().searches().find(userId).setFind(searchtext).execute();
}
and makes a call to the generated Google cloud endpoint library.

Try this:
Observable.create(new Observable.OnSubscribe<Search>() {
#Override
// method signature is from memory - I hope I am correct...
public void call(Subscriber<? super Search> subscriber) {
try {
Search search = search.find("something");
subscriber.onNext(search);
subscriber.onCompleted();
} catch (SomeException e) {
subscriber.onError(e);
}
}
})
// and then continue with your .subscribeOn(...)
To clarify, maybe this makes the problem with your code more obvious:
Observable.just(search.find("something"))
is clearly equivalent to
Search search = search.find("something");
Observable.just(search)
And this makes it obvious that search.find is executed before we ever hand the control over to rxjava and it is executed on whatever thread you are currently on - then the construction of an Observable from the pre-computed value and the delivery of the value happen on another thread but that does not help you much...

I know this is a few months old-- but instead of createing an entirely new Observable (which is relatively error-prone), you can use the map operator to run the search:
String search_input = "something"; // this is where you can specify other search terms
Observable.just(search_input)
.map(s -> search.find(s)) // search.find calls the REST endpoint
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe( // your subscriber goes here
If not using lambdas, that map function should look like:
.map(new Func1<String, Search>() {
#Override
public Search call(String s) {
return search.find(s)
}
})