Rxjava while loop to get database object and upload to server - android

I'm confused about how to implement this in RxJava.
I would like to
take an object from my database
upload it
delete it from the database
take the next item from the database and repeat 2 and 3
complete when the database has no objects remaining
I know how to do this via loading all objects from the database at first and creating an Observable like this Observable.fromIterable(allMyDbObjects), however I would like to take objects one at a time, in case the database is updated while I'm uploading.
I can't figure out how to do this. I've looked at repeatUntil but it just seems to repeat instantly. Here is pseudocode for what I'm thinking:
getFirstDbObject()
.flatMapCompletable { obj ->
upload(obj)
.doOnComplete {
deleteFromDb(obj)
}
}
.repeatUntil {
// dbIsEmptyLogic.
// This doesn't work. I need to somehow call getFirstDbObject() again
}
Can anyone help?

Assuming getFirstDbObject() returns a Maybe, you can achieve this by mapping the result to a boolean (true if the database is empty, false if not) and then repeating the sequence until getFirstDbObject() returns empty and the stream completes.
getFirstDbObject()
.toObservable()
.flatMapSingle { obj ->
upload(obj)
.doOnComplete { deleteFromDb(obj) } // probably better to use .andThen(deleteFromDb(obj)) if delete also returns a completable
.toSingleDefault(false)
}
.defaultIfEmpty(true)
.repeat()
.takeUntil { isComplete ->
isComplete
}

This is a working solution in my code base.
val source = HashSet<String>()
source.add("a")
source.add("b")
source.add("c")
source.add("d")
source.add("e")
io.reactivex.Observable.just(Unit)
.flatMap { it ->
io.reactivex.Observable.fromCallable {
println("first flatmap print $it")
// uploadObj()
source.first()
}
}.flatMap {
// delete
io.reactivex.Observable.fromCallable {
source.remove(it)
println("second flatmap remove $it")
// delete object
}
}
.repeatUntil { source.isEmpty() }
.subscribe()

Related

Android: collecting a Kotlin Flow inside another not emitting

I have got the following method:
operator fun invoke(query: String): Flow<MutableList<JobDomainModel>> = flow {
val jobDomainModelList = mutableListOf<JobDomainModel>()
jobListingRepository.searchJobs(sanitizeSearchQuery(query))
.collect { jobEntityList: List<JobEntity> ->
for (jobEntity in jobEntityList) {
categoriesRepository.getCategoryById(jobEntity.categoryId)
.collect { categoryEntity ->
if (categoryEntity.categoryId == jobEntity.categoryId) {
jobDomainModelList.add(jobEntity.toDomainModel(categoryEntity))
}
}
}
emit(jobDomainModelList)
}
}
It searches in a repository calling the search method that returns a Flow<List<JobEntity>>. Then for every JobEntity in the flow, I need to fetch from the DB the category to which that job belongs. Once I have that category and the job, I can convert the job to a domain model object (JobDomainModel) and add it to a list, which will be returned in a flow as the return object of the method.
The problem I'm having is that nothing is ever emitted. I'm not sure if I'm missing something from working with flows in Kotlin, but I don't fetch the category by ID (categoriesRepository.getCategoryById(jobEntity.categoryId)) it then works fine and the list is emitted.
Thanks a lot in advance!
I think the problem is that you're collecting infinite length Flows, so collect never returns. You should use .take(1) to get a finite Flow before collecting it, or use first().
The Flows returned by your DAO are infinite length. The first value is the first query made, but the Flow will continue forever until cancelled. Each item in the Flow is a new query made when the contents of the database change.
Something like this:
operator fun invoke(query: String): Flow<MutableList<JobDomainModel>> =
jobListingRepository.searchJobs(sanitizeSearchQuery(query))
.map { jobEntityList: List<JobEntity> ->
jobEntityList.mapNotNull { jobEntity ->
categoriesRepository.getCategoryById(jobEntity.categoryId)
.first()
.takeIf { it.categoryId == jobEntity.categoryId }
}
}
Alternatively, in your DAO you could make a suspend function version of getCategoryById() that simply returns the list.
Get an idea from the code below if your Kotlin coroutine flow gets lost with a continuation approximate peak alloc exception
fun test(obj1: Object,obj2: Object) = flow {
emit(if (obj1 != null) repository.postObj(obj1).first() else IgnoreObjResponse)
}.map { Pair(it, repository.postObj(obj2).first()) }

Fetching a Single<List> for all elements of another Single<List> in RxKotlin

A, B, C are objects
All function calls are made to a Rooms DB
This code snippet is inside a ViewModel
repo = Repository
So I'm making an android app (can't provide details) and for a particular screen I need to do the following.
My first call is repo.getInfo, which returns a Single Observable ListOfA: Single<List<A>> //perform some operations
for every element of ListOfA I need to call another function repo.getB(A) which returns a Single Observable ListOfB: Single<List<B>> //perform some operations
for every element of ListOfB I need to call another function repo.getC(B) which returns a Single Observable ListOfC: Single<List<C>> //perform some operations
after I have the required data I need to call another function that combines the data to display on the UI.
Now I can't get this to work. Here's what I've tried. But the flow stops at the line marked THIS LINE and jumps to subscribe block.
Individual calls to the functions work so the data is not the problem.
I'm pretty new at this and quite frankly out of my depth. Any help or hint is appreciated. Thanks
localListOfA = emptyList<A>()
localListOfB = emptyList<B>()
localListOfC = emptyList<C>()
compositeDisposable.add(
getInfo.map{listOfA ->
localListOfA.addAll(listofA)
listOfA.map {elementA -> ////THIS LINE
getB(elementA.id).map{listOfB ->
listOfB.filter {
//some logic to select a few objects
}
}.map { it // filtered list of B
localListofB.addAll(it)
localListOfB.last() //I only need the top element of this list
}.map{elementB ->
getC(elementB.id).map{ listOfC ->
localListOfC.addAll(listOfC)
//do some operations
}
}
}
}
.subscribeOn(DEFAULT_CACHED_SCHEDULERS)
.observeOn(AndroidSchedulers.mainThread())
.doOnError(/*take log*/)
.subscribe{
prepareUi()
}
)
You can flatten a List into an Observable using .flattenAsObservable
getInfo // Single<List<A>>
.doOnSuccess { localListOfA.addAll(it) } // Side effect, adding to localListOfA
.flattenAsObservable { it } // Observable<A>
.flatMapSingle { elementA -> getB(elementA.id) } // Observable<List<B>>
.map { it.filter { true } } // Some logic to select a few objects from B
.doOnNext { localListOfB.addAll(it) } // Side effect, adding to localListOfB
.map { it.last() } // Observable<B>, only the last element
.flatMapSingle { elementB -> getC(elementB.id) } // Observable<List<C>>
.doOnNext { localListOfC.addAll(it) } // Side effect, adding to localListOfC
.flatMapIterable { it } // Observable<C>
Now, you mentioned you need to combine this data somehow. In Rx you can nest chains in order to access the intermediate data. For example, if you have a call that returns a Single<Foo> and you need Foo for the function getBar(foo: Foo): Single<Bar>, one way of achieving this is as follows:
getFoo().flatMap { foo -> // .concatMap, .switchMap
getBar(foo).map { bar ->
// Use both foo and bar
}
}

Recurrence or multiple executions in rxjava

I have a specific task to get several packs of data from server by calling same request several times. While answers contains more flag - i have to recall this request.
It seems something like this:
fun getData(some params): Single<Response<DataEntity>>
//
repository.getData(params)
.flatMap {
if (it.body()?.more == false)
more = false
else (
// here i want to repeat my request
// repository.getData(params) to get more data
)
}
.flatMap {// here i want to get all the data from
//previous requests to save to db etc.
}
Maybe i have to use something like repeatWhen or repeautUntil operators but i can't find the solution for now. Please help!)
You can use the concatMap operator in a recursive way, and as exit condition return just the result:
Single<Response<DataEntity>> getDataAndContinue(params) {
return getData(params)
.concatMap(new Func1<Response<DataEntity>, Single<Response<DataEntity>>>() {
#Override
public Single<Response<DataEntity>> call(Response<DataEntity> response) {
if (!response.body().hasMore()) {
return Single.just(response);
}
return Single.just(response)
.concatWith(getDataAndContinue(params));
}
});
}

Apply Service call for every element in the list, and return only one list with RXJava

I have a method that does a call to Firebase. This method accepts a date and returns an observable.
Then I have an array of dates, that will be used as parameter of this firebase call.
I need to call the method once per item in the array, and finally concatenate to a one list.
But I don't know how to achieve it.
I'm trying to do something like:
for (dateToRetrieve in listOfDatesToRetrieve) {
val subscription = FireBaseUtils.getEventsForMap(dateToRetrieve)
.subscribeOn(Schedulers.io())
.subscribe { retrievedEventsForMap ->
val eventsList: MutableList<Event> = retrievedEventsForMap
eventListWithNoDuplicatesTotal.addAll(eventsList)
var eventListWithNoDuplicates = eventListWithNoDuplicatesTotal.distinctBy { it -> it.eventID }
this.presenter.onEventsRetrieved(eventListWithNoDuplicates as MutableList<Event>)
}
this.presenter.addSubscription(subscription)
}
But I know that is not the best solution because I'm sending the calls one by one, and adding to the list.
Is there any possibility to do it and return 1 result with the combination of all the calls?
Thanks
Try this:
val subscription = Observable.fromIterable(listOfDatesToRetrieve)
.flatMap(dateToRetrieve -> FireBaseUtils.getEventsForMap(dateToRetrieve))
.flatMapIterable { item -> item }
.toList()
.distinct { it -> it.eventID }
.subscribeOn(Schedulers.io())
.subscribe { list ->
this.presenter.onEventsRetrieved(list as MutableList<Event>)
}
this.presenter.addSubscription(subscription)

RxJava - Mapping a result of list to another list

I want to convert every object in my list to an another object. But in doing so my code stucks in converting them back to List
override fun myFunction(): LiveData<MutableList<MyModel>> {
return mySdk
.getAllElements() // Returns Flowable<List<CustomObject>>
.flatMap { Flowable.fromIterable(it) }
.map {MyModel(it.name!!, it.phoneNumber!!) }
.toList() //Debugger does not enter here
.toFlowable()
.onErrorReturn { Collections.emptyList() }
.subscribeOn(Schedulers.io())
.to { LiveDataReactiveStreams.fromPublisher(it) }
}
Everything is fine until mapping. But debugger does not even stop at toList or any other below toList. How can I solve this?
Using flatMap() you'll only flatten the Flowable of lists to a single Flowable of the elements. Calling toList() on it requires the Flowable to complete and therefore you'll most likely never get there. If you only want to map the elements in the list and have an item with the new list emitted, you should do the mapping within flatMap() or maybe try using concatMap() to keep the order:
...
.concatMapSingle { list ->
Observable.fromIterable(list).map {
MyModel(it.name!!, it.phoneNumber!!)
}.toList()
}
...
Here is my solution to this. Thanks to Tim for leading me to right answer.
override fun myFunction(): LiveData<MutableList<MyModel>> {
return mySdk
.getAllElements() // Returns Flowable<List<CustomObject>>
.flatMapSingle { Observable.fromIterable(it).map { MyModel(it.name!!, it.phoneNumber!!) }.toList() }
.toFlowable()
.onErrorReturn { Collections.emptyList() }
.subscribeOn(Schedulers.io())
.to { LiveDataReactiveStreams.fromPublisher(it) }
}

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