Make your app work perfectly off— Connection Interrupted

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@jeremybbonze Android Engineer at WorkWell @xgouchet Lead Android Engineer at WorkWell

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“Do not treat Oﬄine as an error condition. Oﬄine is the default state, Online is icing on the cake.”

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accessibility /əksɛsɪˈbɪlɪti/ n. ● the quality of being able to be reached or entered. ● the quality of being easy to obtain or use.

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✓

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✗

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✓

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✓

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✓

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data class Item ( // … val lastModified: Long, val timeToLive: Long )

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interface ItemRemoteSource { fun get(id: String): Single fun getAll(): Single> } interface ItemRemoteSink { fun create(item: Item): Single fun update(item: Item): Single fun delete(id: String): Single }

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interface ItemLocalSource { fun get(id: String): Single> fun getAll(): Single>> } interface ItemLocalSink { fun create(item: Item): Single fun update(item: Item): Single fun delete(id: String): Single }

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sealed class LocalResult { abstract fun get(): T class Present(private val value: T): LocalResult() { override fun get(): T = value } class Obsolete(private val value: T): LocalResult() { override fun get(): T = value } class NotFound: LocalResult() { override fun get(): T = throw Resources.NotFoundException("No value") } }

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interface ItemGlobalSource { fun get(id: String): Single fun getAll(): Single> } interface ItemGlobalSink { fun create(item: Item): Single fun update(item: Item): Single fun delete(id: String): Single }

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fun Observable>.flatMapOfflineFirst( fallback: RemoteSource.() -> Observable, update: LocalSink.(O) -> Observable ): Observable { return flatMap { result -> when (result) { // … } } }

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return flatMap { result -> when (result) { is LocalResult.Present -> Observable.just(result.get()) // … } }

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return flatMap { result -> when (result) { is LocalResult.Obsolete -> { if (networkWatcher.isNetworkAvailable()) { remoteSource.fallback() .flatMap { localSink.update(it) } .startWith(result.get()) } else { Observable.just(result.get()) } } // … } }

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return flatMap { result -> when (result) { // … is LocalResult.NotFound -> { if (networkWatcher.isNetworkAvailable()) { remoteSource.fallback() .flatMap { localSink.update(it) } } else { Observable.empty() } } } }

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fun List>.keepPresentOnly() : LocalResult> { return LocalResult.Present( filter { it is LocalResult.Present } .map { it.get() } ) }

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fun LocalResult>.notFountIfEmpty() : LocalResult> { return if (this is LocalResult.NotFound || get().isNotEmpty()) { This } else { LocalResult.NotFound() } }

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fun List>.allPresentOrNone() : LocalResult> { return if (isNotEmpty() && all { it is LocalResult.Present }) { LocalResult.Present(map { it.get() }) } else { LocalResult.Obsolete (mapNotNull { if (it is LocalResult.NotFound) { null } else { it.get() } }) } }

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abstract class PaginationScrollListener( private val srLayout: SwipeRefreshLayout, private val onLoadNextPage: () -> Unit ) : RecyclerView.OnScrollListener() { override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) { super.onScrolled(recyclerView, dx, dy) if (isLastItemVisible() && !isFirstItemVisible() && !srLayout.isRefreshing) { onLoadNextPage() } } }

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override fun loadNextPage(context: ContextC, currentListSize: Int) { val pageToLoad = computePageToLoad(currentListSize) if (pageToLoad >= 0) { loadPage(context, pageToLoad, false) } } private fun computePageToLoad(currentListSize: Int): Int { val remainder = currentListSize % PAGE_SIZE val pageCount = (currentListSize - remainder) / PAGE_SIZE return if (remainder > 0) -1 else pageCount + 1 }

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● Same pagination logic on Remote and Local source

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● Balance between disk usage / available data ● Only deals with part of the problem

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● They do actually mean something ● 404 (NOT FOUND) or 410 (GONE) → delete locally

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● Request (once in a while) recently deleted items ● Use a Hash / Timestamp to only get the relevant changeset

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● The main solution to handle oﬄine actions is to schedule jobs

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● JobScheduler ● Android-Job (Evernote) ● JobDispatcher (Firebase) [Deprecated] ● WorkManager

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● Set syncStatus in local DB ● Launch sync job as an internet connection is back ● Once remote and local DB are synced, update syncStatus

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data class Item ( // … val lastModified: Long, val timeToLive: Long, val syncStatus: SyncStatus ) enum class SyncStatus { TO_BE_SYNCED, SYNCED }

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✓

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“Do not treat Oﬄine as an error condition. Oﬄine is the default state, Online is icing on the cake.”

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● Measure your network usage ● Test your own application with lousy network conditions ● Test your own application in airplane mode ● Use your own application

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● Offline First Slack Community ○ http://offlinefirst.org/chat/ ● Offline resources and articles ○ https://medium.com/offline-camp/ ○ http://offlinefirst.org/

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