Hi i am trying to implement a Single observable that chains two requests together.
In between the two requests i make, i notify a callback to update the UI with the response from request one and then launch the next request in the Schedulaers.io thread.
The issue i am having is that it tries to update the UI from the schedulars.io thread too and results to nothing being updated in the ui thread.
i cold wrap the calback on RunOnUiThread code block in android but wondering if there is a more elegant way of doing it?
i checked couroutines and it seems to just deal with putting a block of code in a seperate thread.
Here is my current code
override fun getHomeScreenInformation() {
delegator.requestOne()
.flatMap { responseOne->
homeScreenCallBack.onResponseOneRecieved(responseOne)
delegator.requestTwo()
}
.observeOn(AndroidSchedulers.mainThread())
.subscribeOn(Schedulers.io())
.subscribe(
{responseTwo-> homeScreenCallBack.onResponseTwoRecieved(responseTwo)},
{error -> homeScreenCallBack.onError()}
)
}
Apply observeOn(AndroidSchedulers.mainThread()) as many times as necessary:
delegator.requestOne()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread()) // <----------------------
.flatMap { responseOne ->
homeScreenCallBack.onResponseOneRecieved(responseOne)
delegator.requestTwo()
.subscribeOn(Schedulers.io()) // <----------------------
}
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{responseTwo-> homeScreenCallBack.onResponseTwoRecieved(responseTwo)},
{error -> homeScreenCallBack.onError()}
)
Related
I have the following code:
Single.create { emitter ->
// I/O thread here
ThirdPartySDK.doSomeAction {
// Main thread here
emitter.onSuccess(someValue)
}
}
.flatMap {
someOtherSingle(it) // Executes on main thread
}
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe({},{})
The ThirdPartySDK.doSomeAction callback posts on main thread, so the emitter will emit on the main thread too, not on the subscribe thread (and if I have some network interactions further in the flatMap, chain will fail).
If I add observeOn(Schedulers.io()) after the first Single, it switches to the correct thread, but is there any way to emit on right thread? I can't modify ThirdPartySDK behaviour.
subscribeOn
The subscribeActual lambda will be invoked on given scheduler
observeOn
Switch thread to given scheduler. Every upstream-onNext call will be called from an ObserveOn-Scheduler-Thread
As you already said, subscribeOn will only invoke the subscribeActual method call on subscribe on given Scheduler-Thread. This does not mean, that the downstream emit will be on the same thread. In your case the onSuccess emit will be called from a different thread (e.g. Database/ Http-ThreadPool etc.).
onSuccess will be called from a unknown thread (in your case main thread). The downstream call will be called from the main-thread. Therefore flatMap is called from the main-thread. Network-calls on the main-thread in the flatMap will probably fail, because it is not allowed to "block" the main-thread.
How to solve this issue?
Just place a observeOn after the Single#create. The main-thread calls onSucess. The observeOn-subscriber will get called from the main-thread. The observeOn-subscriber re-directs onSuccess downstream-call (e.g. flatMap) to given ObserveOn-Scheduler-Thread. Therefore it is given, that flatMap is called from a non main-loop thread.
Example:
#Test
fun wurst() {
val thirdPartySDKImpl = ThirdPartySDKImpl()
Single.create<String> { emitter ->
thirdPartySDKImpl.doSomeAction {
emitter.onSuccess(it)
}
}
// .subscribeOn(Schedulers.computation())
// move emit from unknown thread to computation thread
.observeOn(Schedulers.computation())
// Single.just will be subscribe from a computation thread
.flatMap { Single.just(123) }
// move onSucess/ onError emit from computation thread to main-thread
.observeOn(AndroidSchedulers.mainThread())
// subscribe onNext / onError will be called from the main-android-thread
.subscribe({}, {})
}
interface ThirdPartySDK {
fun doSomeAction(callback: (v: String) -> Unit)
}
class ThirdPartySDKImpl : ThirdPartySDK {
override fun doSomeAction(callback: (v: String) -> Unit) {
// <- impl-detail ->
callback("whatever")
}
}
NOTE: You do not need a subscribeOn, if the create-lambda does not block or does some cpu heavy stuff. If it only subscribes to a callback, which will be called from a different thread, you do not need subscribeOn.
but is there any way to emit on right thread?
You should not use any concurrency in operators. You would think, you could just do something like:
Single.create<String> { emitter ->
thirdPartySDKImpl.doSomeAction {
Schedulers.io().scheduleDirect {
emitter.onSuccess(it)
}
}
}
But this is not recommended, because you could break the serialized onNext contract^1. This example would make sure, that the onSucess downstream call would happen on expected thread, but cancellation/ unsubscription is not handled and there might be other pitfalls.
If you have a non reactive API and you want to enforce some threading-model I would suggest to wrap the sync. API with an async one and provide proper observeOn/ subscribeOn operators. Later on only use the async API.
interface ThirdPartySDKAsync {
fun doSomeAction(): Single<String>
}
class ThirdPartySDKAsyncImpl(private val sdk: ThirdPartySDK, private val scheduler: Scheduler) :
ThirdPartySDKAsync {
override fun doSomeAction(): Single<String> {
return Single.create<String> { emitter ->
sdk.doSomeAction {
emitter.onSuccess(it)
}
}.observeOn(scheduler)
}
}
Further reading: https://tomstechnicalblog.blogspot.com/2016/02/rxjava-understanding-observeon-and.html
^1 Only one thread a time is allowed to call onNext/onSuccess/onError/onComplete
I am trying to chain two network calls in my Android app. I am using Retrofit. Basically I want to do :
Make API Call to login
Wait for the response of login, save the token to SharedPrefs
Make another API call right after I've saved the token
Wait for the response, save the data
I think I have chained the stream in the right way, the only thing is I want to update the UI in between. For example once the call starts I want to display a progressDialog ( I do that in doOnSubscribe ), or dismiss the Dialog once the call has completed ( I do that in doOnComplete ). However I get the exception Only the original thread that created a view hierarchy can touch its views. I subscribe on the io thread and observe on the mainThread so that I can make the changes to the UI, however I must be missing something.
I tried adding .subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
lower in the stream, but I still get the same error message.
getView().onLoginAction().subscribe(aVoid -> Observable.combineLatest(
getView().userNameObservable().map(CharSequence::toString),
getView().passwordObservable().map(CharSequence::toString),
Pair::new)
.first()
.subscribe(usernamePasswordPair -> {
User user = User.create(usernamePasswordPair.first, usernamePasswordPair.second, "");
RetrofitClientInstance.createService(AuthenticationNetworkApi.class).login(new Login(user.username(), user.password()))
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.doOnNext(loginResponse -> {
AuthorizationResponse responseBody = loginResponse.body();
if (responseBody != null && responseBody.getAccessToken() != null && !responseBody.getAccessToken().isEmpty()) {
if (localStorage.getAccessToken().isEmpty()) {
localStorage.saveAccessToken(responseBody.getAccessToken());
}
}
}
).
doOnSubscribe( action -> getView().showProgressDialog())
.doOnError(error -> {
getView().dismissProgressDialog();
getView().showErrorMessage("Login Unsuccessful");
}).doOnComplete(() -> getView().dismissProgressDialog()
)
.flatMap(response -> RetrofitClientInstance.createService(ActivitiesApi.class).getUserActivities())
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.doOnNext(activities -> {
for (UserActivityApiModel useractivity : activities
) {
activityService.addActivity(Activity.create(Integer.parseInt(useractivity.getId()), useractivity.getActivityName(), useractivity.getDate(),
Integer.parseInt(useractivity.getValue()), Integer.parseInt(useractivity.getSubCategory().getId())));
}
}).doOnError(error -> getView().showErrorMessage(error.getMessage()))
.doOnComplete(() -> getView().redirectToHomeScreen())
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe();
}));
The error occurs here :
.doOnError(error -> {
getView().dismissProgressDialog();
getView().showErrorMessage("Login Unsuccessful");
})
It seems you are using a different thread to execute your backend. In that case, you can't touch the main UI thread from the second one. You need to execute first runOnUiThread { //your code }
In //your code, call the two lines of code that you put on doOnError.
I have simple job which is selecting data using GreenDao. Also i want show ProgressWheel, but this job blocks UI thread.
showLoader();
DataManager.getInstance(this).getQuestions(0, 800)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.map(QuestionUtils::shuffleAnswers)
.subscribe(this::onFinishLoad);
And
public Observable<List<Question>> getQuestions(int offset, int limit) {
return Observable.fromCallable(() -> daoSession.getQuestionDao().queryBuilder()
.offset(offset)
.limit(limit)
.build().list());
}
It blocks because of the order of your operations.
DataManager.getInstance(this)
.getQuestions(0, 800)
.map(QuestionUtils::shuffleAnswers)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(this::onFinishLoad);
should solve it. Else you would subscribe data and map them after.
If you want to finish the loader onec your data are emitted you can use action in your subscribe method and process the loading in your emitted nextData.
.subscribe (nextData -> {}, error -> {}, () -> doCompletedStuff())
I'm trying to create an Observable such that it will load some data from the network on an interval, and whenever the user refreshes the page. This is the gist of what I have so far:
PublishSubject<Long> refreshSubject = PublishSubject.create();
Observable<MyDataType> observable = Observable.merge(
Observable.interval(0, 3, TimeUnit.SECONDS),
refreshSubject
)
.flatMap(t -> {
// network operations that eventually return a value
// these operations are not observables themselves
// they are fully blocking network operations
})
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(data -> {
// update ui with data
}, error -> {
// do something with error
});
Later in a refresh callback I have:
refreshSubject.onNext(0L);
It runs on the interval fine, however when I refresh, it explodes with a NetworkOnMainThreadException. I thought that I handled this with subscribeOn/observeOn. What am I missing? Also, why doesn't this cause a crash when the Observer is triggered from the interval?
You have to change your subscribeOn(Schedulers.io()) to observeOn(Schedulers.io()) and move it over your flatMap.
The reason for this is that your refreshSubject is a PublishSubject, which is an Observable and an Observer.
Since the onNext() of this PublishSubject is called inside the intern Observable first before the result gets delivered to your subscription.
This is also the reason that it works when you just use your Observable(and the fact that interval always subscribes to the computation thread by default).
Just check the output of those two snippets:
Observable.merge(
Observable.interval(0, 3, TimeUnit.SECONDS),
refreshSubject
)
.observeOn(Schedulers.io())
.doOnNext(aLong -> Log.d("Thread", Thread.currentThread().toString()))
.observeOn(AndroidSchedulers.mainThread())
.subscribe(data -> {
Log.d("Subscribe Thread", Thread.currentThread().toString());
}, error -> {
// do something with error
});
vs
Observable.merge(
Observable.interval(0, 3, TimeUnit.SECONDS),
refreshSubject
)
.doOnNext(aLong -> Log.d("Thread", Thread.currentThread().toString()))
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(data -> {
Log.d("Subscribe Thread", Thread.currentThread().toString());
}, error -> {
// do something with error
});
In android app i have this case:
Listen to my editText with observable:
WidgetObservable.text(myEditText, false)
.map { it.text().toString() }
.debounce(800, TimeUnit.MILLISECONDS, Schedulers.io())
Then i need to send network request with string emitted by observable:
.flatMap { networkObservable.subscribeOn(Schedulers.io()) }
My question is: what is the best possible way to write infinite stream of these network results.
Errors handled by UI.
Unsubscription done with AppObservable.bindActivity() wrapper
I ended up attaching materialize() operator to network observable, and then handling it like:
.subscribe{
when (it.getKind()) {
Kind.OnNext -> text.setText(it.getValue())
Kind.OnError -> text.setText(it.getThrowable().getMessage())
}
}
Do you know better way, or its just fine?
At least it works.
P.S. another useful case will be Refresh button clicks flatMap'ed to network calls
You can use onErrorResumeNext to recovery your Observable from a failure. E.g.,
WidgetObservable.text(myEditText, false)
.map { it.text().toString() }
.debounce(800, TimeUnit.MILLISECONDS, Schedulers.io())
.flatMap {
networkObservable.subscribeOn(Schedulers.io())
.onErrorResumeNext(t -> t.getMessage())
}