using countDownLatch.await() to make sure result is delivered

Question

Full source code can be found here : https://github.com/alirezaeiii/SavingGoals-Cache

This is LocalDataSource class :

@Singleton
class QapitalLocalDataSource @Inject constructor(
    private val goalsDao: GoalsDao
) : LocalDataSource {

    override fun getSavingsGoals(): Single<List<SavingsGoal>> =
        Single.create { singleSubscriber ->
            goalsDao.getGoals()
                .subscribe {
                    if (it.isEmpty()) {
                        singleSubscriber.onError(NoDataException())
                    } else {
                        singleSubscriber.onSuccess(it)
                    }
                }
        }
}

Above Method has been used in Repository class :

@Singleton
class GoalsRepository @Inject constructor(
    private val remoteDataSource: QapitalService,
    private val localDataSource: LocalDataSource,
    private val schedulerProvider: BaseSchedulerProvider
) {

    private var cacheIsDirty = false

    fun getSavingsGoals(): Observable<List<SavingsGoal>> {
        lateinit var goals: Observable<List<SavingsGoal>>
        if (cacheIsDirty) {
            goals = getGoalsFromRemoteDataSource()
        } else {
            val latch = CountDownLatch(1)
            var disposable: Disposable? = null
            disposable = localDataSource.getSavingsGoals()
                .observeOn(schedulerProvider.io())
                .doFinally {
                    latch.countDown()
                    disposable?.dispose()
                }.subscribe({
                    goals = Observable.create { emitter -> emitter.onNext(it) }
                }, { goals = getGoalsFromRemoteDataSource() })
            latch.await()
        }
        return goals
    }
}

As you see I am using countDownLatch.await() to make sure result is emmited in subscribe or error block. Is there any better solution than using CountDownLatch while using RxJava?

I'm not sure what you want to achieve. You want to wait until at least one item (no matter if local or remote) is emitted? — skywall, Nov 28 '20 at 20:42
Yes exactly, when result is emitted, in finally block, I *countDown()* — Ali, Nov 28 '20 at 20:48

score 1 · Accepted Answer · edited Dec 15 '20 at 07:12

latch.await() blocks the thread which kinda defeats the whole point of using an async API like RxJava.

RxJava has APIs like onErrorResumeNext to handle exceptions and toObservable to convert a Single result to an Observable result.

Also, RxJava types like this are typically intended to be cold (they don't run or figure anything out until you subscribe) so I'd recommend not checking cacheIsDirty until the subscription happens.

I'd go with something like:

    fun getSavingsGoals(): Observable<List<SavingsGoal>> {
        return Observable
            .fromCallable { cacheIsDirty }
            .flatMap {
                if (it) {
                    getGoalsFromRemoteDataSource()
                } else {
                    localDataSource.getSavingsGoals()
                        .toObservable()
                        .onErrorResumeNext(getGoalsFromRemoteDataSource())
                }
            }
    }

Btw, if you are already using Kotlin, I highly recommend coroutines. Then you async code ends up reading just like regular sequential code.

using countDownLatch.await() to make sure result is delivered

1 Answers1