I have a function that conditionally fetches some data and runs some tasks concurrently on that data. Each task depends on different sets of data and I would like to avoid fetching the data that's not needed. Moreover, some of the data can have already been prefetched and given to the function. See the code I've come up with below.
suspend fun process(input: SomeInput, prefetchedDataX: DataX?, prefetchedDataY: DataY?) = coroutineScope {
val dataXAsync = lazy {
if (prefetchedDataX == null) {
async { fetchDataX(input) }
} else CompletableDeferred(prefetchedDataX)
}
val dataYAsync = lazy {
if (prefetchedDataY == null) {
async { fetchDataY(input) }
} else CompletableDeferred(prefetchedDataY)
}
if (shouldDoOne(input)) launch {
val (dataX, dataY) = awaitAll(dataXAsync.value, dataYAsync.value)
val modifiedDataX = modifyX(dataX)
val modifiedDataY = modifyY(dataY)
doOne(modifiedDataX, modifiedDataY)
}
if (shouldDoTwo(input)) launch {
val modifiedDataX = modifyX(dataXAsync.value.await())
doTwo(modifiedDataX)
}
if (shouldDoThree(input)) launch {
val modifiedDataY = modifyY(dataYAsync.value.await())
doThree(modifiedDataY)
}
}
Any improvements that could be made to this code? One, I don't like having to fakely wrap the prefetched data into a CompletableDeferred. Two, I don't like having to call modifyX, modifyY inside each task, I wish I could apply it at the fetching stage, but I haven't come up with a nice way to do that. Alternatively I could do
val modifiedDataXAsync = lazy {
async { modifyX(prefetchedDataX ?: fetchDataX(input)) }
}
but it feels wasteful to be spawning a new coroutine when the data is already prefetched. Am I over-optimizing?
How about this? This code is pretty similar to yours, I just simplified it a bit.
suspend fun process(input: SomeInput, prefetchedDataX: DataX?, prefetchedDataY: DataY?) = coroutineScope {
val modifiedDataX by lazy {
async { modifyX(prefetchedDataX ?: fetchDataX(input)) }
}
val modifiedDataY by lazy {
async { modifyY(prefetchedDataY ?: fetchDataY(input)) }
}
if (shouldDoOne(input)) launch {
val (dataX, dataY) = awaitAll(modifiedDataX, modifiedDataY)
doOne(dataX, dataY)
}
if (shouldDoTwo(input)) launch {
doTwo(modifiedDataX.await())
}
if (shouldDoThree(input)) launch {
doThree(modifiedDataY.await())
}
}
Related
In Code A, there are two Flows, and I assign two jobs for them, I collect the two Flows in fun beginSoundDensity() and stop collecting the two Flows in fun resetSoundDensity().
I think there are many repeated codes in Code A, so I hope to improve it, but Code B doesn't work.
Can I use one Job in my case?
Code A
private val _soundDensityState = MutableStateFlow(initialMSoundDensity)
val soundDensityState = _soundDensityState.asStateFlow()
private val _timeX = MutableStateFlow(0)
val timeX = _timeX.asStateFlow()
private var myJob1: Job?=null
private var myJob2: Job?=null
val myFlow: Flow<Int> = flow {
var i = 0
while (true) {
emit(i)
i = i + 15
delay(5000)
}
}
fun beginSoundDensity() {
myJob1?.cancel()
myJob2?.cancel()
myJob1 = viewModelScope.launch {
aSoundMeter.startSoundDensity {
pauseSoundDensity()
}.cancellable()
.collect {
_soundDensityState.value = it
}
}
myJob2 = viewModelScope.launch {
myFlow.collect {
_timeX.value = it
}
}
}
}
fun resetSoundDensity(){
myJob1?.cancel()
myJob2?.cancel()
}
Code B
//The same
private var myJob: Job?=null
val myFlow: Flow<Int> = flow {
var i = 0
while (true) {
emit(i)
i = i + 15
delay(5000)
}
}
fun beginSoundDensity() {
myJob?.cancel()
myJob = viewModelScope.launch {
aSoundMeter.startSoundDensity {
pauseSoundDensity()
}.cancellable()
.collect {
_soundDensityState.value = it
}
myFlow.collect {
_timeX.value = it //It will not be launched
}
}
}
}
fun resetSoundDensity(){
myJob?.cancel()
}
Yes and no. You need two separate coroutines running concurrently to collect from two flows. In your Code B myFlow will be collected only after aSoundMeter finishes collecting. Collections need to run at the same time, so you need two concurrent coroutines for this purpose.
However, if you always start and cancel both collections together, then I think it would be better to group them into a single coroutine like this:
fun beginSoundDensity() {
myJob?.cancel()
myJob = viewModelScope.launch {
coroutineScope {
launch {
aSoundMeter.startSoundDensity {
pauseSoundDensity()
}.cancellable()
.collect {
_soundDensityState.value = it
}
}
launch {
myFlow.collect {
_timeX.value = it //It will not be launched
}
}
}
}
}
fun resetSoundDensity(){
myJob?.cancel()
}
Let me use a simple image to illustrate what I want to get:
I don't want to use SharedFlow's replayCache to achieve this because if a new observer observes that SharedFlow, it will get 2 emissions instead of one latest emission.
Or if I write it in code:
val sharedFlow = MutableSharedFlow(replay = 1)
val theFlowThatIWant = sharedFlow.unknownOperator { … }
sharedFlow.emit(1)
sharedFlow.emit(2)
sharedFlow.collect {
println(it)
}
Expected output:
2
theFlowThatIWant.collect {
println(it)
}
Expected output:
1
We can create such operator by ourselves. We can generalize it to more items than only the last one and use circular buffer to keep postponed items:
suspend fun main() {
val f = flow {
repeat(5) {
println("Emitting $it")
emit(it)
delay(1000)
}
}
f.postponeLast()
.collect { println("Collecting $it") }
}
fun <T> Flow<T>.postponeLast(count: Int = 1): Flow<T> = flow {
val buffer = ArrayDeque<T>(count)
collect {
if (buffer.size == count) {
emit(buffer.removeFirst())
}
buffer.addLast(it)
}
}
Note that this solution never emits postponed items. If you like to emit them at the end, just add this after collect { }:
while (buffer.isNotEmpty()) {
emit(buffer.removeFirst())
}
I have a method in my Kotlin app that looks like this:
coroutineScope{
val aFetcher = async { a.fetch()}
val bFetcher = async { b.fetch()}
val cFetcher = async { c.fetch()}
val dFetcher = async { d.fetch()}
Merged(a.await(),b.await(),c.await(),d.await())
}
The problem I am having is that I can't find a way to make one request depend on the other. In my case, I need for cFetcher to wait until bFetcher ended it's work before starting.
What's the right way to do that in Kotlin?
Just make the part that should be synchronous, synchronous.
coroutineScope{
val aFetcher = async { a.fetch() }
val dFetcher = async { d.fetch() }
val bResult = b.fetch()
val cFetcher = async { c.fetch(bResult) }
Merged(aFetcher.await(), bResult, cFetcher.await(), dFetcher.await())
}
If you have more than one of these dependencies and want to run them in parallel you could do something like this I suppose:
coroutineScope{
val aFetcher = async { a.fetch() }
val dFetcher = async { d.fetch() }
val bAndCFetcher = async {
val bResult = b.fetch()
bResult to c.fetch(bResult)
}
Merged(aFetcher.await(), bAndCFetcher.await().first, bAndCFetcher.await().second, dFetcher.await())
}
I need to run a task, which emits some data. I want to subscribe to this data like PublishSubject. But I can't solve a problem of one-instance flow. If I try to call it again, it will create another instance and the job will be done twice.
I tried to run the flow internally and post values to the BroadcastChannel, but this solution doesn't seem correct.
What is the best practice for such a task?
This will do the magic:
fun <T> Flow<T>.refCount(capacity: Int = Channel.CONFLATED, dispatcher: CoroutineDispatcher = Dispatchers.Default): Flow<T> {
class Context(var counter: Int) {
lateinit var job: Job
lateinit var channel: BroadcastChannel<T>
}
val context = Context(0)
fun lock() = synchronized(context) {
if (++context.counter > 1) {
return#synchronized
}
context.channel = BroadcastChannel(capacity)
context.job = GlobalScope.async(dispatcher) {
try {
collect { context.channel.offer(it) }
} catch (e: Exception) {
context.channel.close(e)
}
}
}
fun unlock() = synchronized(context) {
if (--context.counter == 0) {
context.job.cancel()
}
}
return flow {
lock()
try {
emitAll(context.channel.openSubscription())
} finally {
unlock()
}
}
}
I would like to convert my rxJava Code to Kotlin CoRoutine.
Below is the code makes both the api and db call and returns the data to UI whatever comes first. Let us say if DB response happens to be quicker than the api. In that case still, the api response would continue until it receives the data to sync with db though it could have done the UI update earlier.
How Would I do it?
class MoviesRepository #Inject constructor(val apiInterface: ApiInterface,
val MoviesDao: MoviesDao) {
fun getMovies(): Observable<List<Movie>> {
val observableFromApi = getMoviesFromApi()
val observableFromDb = getMoviesFromDb()
return Observable.concatArrayEager(observableFromApi, observableFromDb)
}
fun getMoviesFromApi(): Observable<List<Movie>> {
return apiInterface.getMovies()
.doOnNext { it ->
it.data?.let { it1 -> MoviesDao.insertAllMovies(it1) }
println("Size of Movies from API %d", it.data?.size)
}
.map({ r -> r.data })
}
fun getMoviesFromDb(): Observable<List<Movie>> {
return MoviesDao.queryMovies()
.toObservable()
.doOnNext {
//Print log it.size :)
}
}
}
As the first step you should create suspend funs for your ApiInterface and MovieDao calls. If they have some callback-based API, you can follow these official instructions.
You should now have
suspend fun ApiInterface.suspendGetMovies(): List<Movie>
and
suspend fun MoviesDao.suspendQueryMovies(): List<Movie>
Now you can write this code:
launch(UI) {
val fromNetwork = async(UI) { apiInterface.suspendGetMovies() }
val fromDb = async(UI) { MoviesDao.suspendQueryMovies() }
select<List<Movie>> {
fromNetwork.onAwait { it }
fromDb.onAwait { it }
}.also { movies ->
// act on the movies
}
}
The highlight is the select call which will simultaneously await on both Deferreds and act upon the one that gets completed first.
If you want to ensure you act upon the result from the network, you'll need some more code, for example:
val action = { movies: List<Movie> ->
// act on the returned movie list
}
var gotNetworkResult = false
select<List<Movie>> {
fromNetwork.onAwait { gotNetworkResult = true; it }
fromDb.onAwait { it }
}.also(action)
if (!gotNetworkResult) {
action(fromNetwork.await())
}
This code will act upon the DB results only if they come in before the network results, which it will process in all cases.
Something along those lines should work:
data class Result(val fromApi: ???, val fromDB: ???)
fun getMovies(): Result {
val apiRes = getMoviesFromApiAsync()
val dbRes = getMoviesFromDbAsync()
return Result(apiRes.await(), dbRes.await())
}
fun getMoviesFromApiAsync() = async {
return apiInterface.getMovies()
.doOnNext { it ->
it.data?.let { it1 -> MoviesDao.insertAllMovies(it1) }
println("Size of Movies from API %d", it.data?.size)
}
.map({ r -> r.data })
}
fun getMoviesFromDbAsync() = async {
return MoviesDao.queryMovies()
}
I don't know what you're returning, so I just put ??? instead.