So I have a flow where I need it to emit a value from cache, but at the end it will make an API call to pull values in case there was nothing in cache (or refresh the value it has). I am trying this
override val data: Flow<List<Data>> = dataDao.getAllCachedData()
.onCompletion {
coroutineScope {
launch {
requestAndCacheDataOrEmitError()
}
}
}
.map { entities ->
entities
.map { it.toData() }
.filter { it !is Data.Unknown }
}
.filterNotNull()
.catch { emitRepositoryError(it) }
So the idea is that we emit the cache, and then make an API call to fetch new data regardless of the original mapping. But I do not want it blocking. For example, if we use this flow, I do not ever want the calling function to be blocked by the onCompletion.
I think the problem is that the onCompletion never runs. I set some breakpoints/logs and it never runs at all, even outside of the coroutineScope.
I don't quite understand the work you are doing but I think when you are collecting flow on a certain scope. You end the scope that flow will be put into onCompletion
var job : Job? = null
fun scan() {
job = viewModelScope.launch {
bigfileManager.bigFile.collect {
if (it is ResultOrProgress.Result) {
_bigFiles.value = it.result ?: emptyList()
} else {
_updateProgress.value = (it as ResultOrProgress.Progress).progress ?: 0
}
}
}
}
fun endScreen(){
job?.cancel()
}
Related
I am observing inside a fragment the events of a sharedflow such as this:
myEvent.collectInLifeCycle(viewLifecycleOwner) { event ->
when (state) {
//check the event. The event emited form onStart is never reached here :(
}
}
Whereas in the viewmodel I have
private val _myEvent = MutableSharedFlow<MyEvent>()
val myEvent: SharedFlow<MyEvent> = _myEvent
fun loadData() =
viewModelScope.launch {
getDataUseCase
.safePrepare(onGenericError = { _event.emit(Event.Error(null)) })
.onStart { _event.emit(Event.Loading) }
.onEach { result ->
result.onSuccess { response ->
_event.emit(Event.Something)
}
}
.launchIn(viewModelScope)
}
So the problem is that only the Event.Something is the one being properly collected from the fragment, whereas _event.emit(Event.Loading) is not being collected... If I debug it goes to the onStart, but it is never called in the fragment.
Your SharedFlow needs to have a replay so that collectors always get at least the most recent value. Otherwise, if you emit to the Flow before the collector is registered, it will never see anything emitted. Do this:
private val _myEvent = MutableSharedFlow<MyEvent>(replay = 1)
Personally, unless I'm missing some detail here that would change my mind, I would simplify all your code to avoid having to manually call loadData(). Something like this but I'm guessing a bit because I don't know all your types and functions.
val myEvent: SharedFlow<MyEvent> = flow {
emit(Event.Loading)
emitAll(
getDataUseCase
.transform { result ->
result.onSuccess { response ->
emit(Event.Something)
}
}
.catch { error -> emit(Event.Error(null)) }
)
}.shareIn(viewModelScope, SharingStarted.Lazily, replay = 1)
I am trying to retrieve the base url from my proto datastore to be used to initialize my ktor client instance I know how to get the data from the datastore but I don't know how to block execution until that value is received so the client can be initialized with the base url
So my ktor client service asks for a NetworkURLS class which has a method to return the base url
Here is my property to retrieve terminalDetails from my proto datastore
val getTerminalDetails: Flow<TerminalDetails> = cxt.terminalDetails.data
.catch { e ->
if (e is IOException) {
Log.d("Error", e.message.toString())
emit(TerminalDetails.getDefaultInstance())
} else {
throw e
}
}
Normally when I want to get the values I would do something like this
private fun getTerminalDetailsFromStore() {
try {
viewModelScope.launch(Dispatchers.IO) {
localRepository.getTerminalDetails.collect {
_terminalDetails.value = it
}
}
} catch(e: Exception) {
Log.d("AdminSettingsViewModel Error", e.message.toString()) // TODO: Handle Error Properly
}
}
but in my current case what I am looking to do is return terminalDetails.backendHost from a function and that where the issue comes in I know I need to use a coroutine scope to retrieve the value so I don't need to suspend the function but how to a prevent the function returning until the coroutine scope has finished?
I have tried using async and runBlocking but async doesn't work the way I would think it would and runBlocking hangs the entire app
fun backendURL(): String = runBlocking {
var url: String = "localhost"
val job = CoroutineScope(Dispatchers.IO).async {
repo.getTerminalDetails.collect {
it.backendHost
}
}
url
}
Can anyone give me some assistance on getting this to work?
EDIT: Here is my temporary solution, I do not intend on keeping it this way, The issue with runBlocking{} turned out to be the Flow<T> does not finish so runBlocking{} continues to block the app.
fun backendURL(): String {
val details = MutableStateFlow<TerminalDetails>(TerminalDetails.getDefaultInstance())
val job = CoroutineScope(Dispatchers.IO).launch {
repo.getTerminalDetails.collect {
details.value = it
}
}
runBlocking {
delay(250L)
}
return details.value.backendHost
}
EDIT 2: I fully fixed my issue. I created a method with the same name as my val (personal decision) which utilizes runBlocking{} and Flow<T>.first() to block while the value is retrieve. The reason I did not replace my val with the function is there are places where I need the information as well where I can utilize coroutines properly where I am not initializing components on my app
val getTerminalDetails: Flow<TerminalDetails> = cxt.terminalDetails.data
.catch { e ->
if (e is IOException) {
Log.d("Error", e.message.toString())
emit(TerminalDetails.getDefaultInstance())
} else {
throw e
}
}
fun getTerminalDetails(): TerminalDetails = runBlocking {
cxt.terminalDetails.data.first()
}
I currently have a piece of logic as follows:
interface anotherRepository {
fun getThings(): Flow<List<String>>
}
interface repository {
suspend fun getSomeThings(): AsyncResult<SomeThings>
}
when (val result = repository.getSomeThings()) {
is AsyncResult.Success -> {
anotherRepository.getThings().collectLatest {
// update the state
}
else -> { }
}
}
The problem I am having is that, if repository.getSomeThings has been triggered multiple times before, anotherRepository.getThings is getting triggered for the amount of all the pre-loaded values from repository.getSomeThings. I was wondering what is the proper way to use these repositories, one a suspend function, the other a Flow together. The equivalent behaviour that is combineLatest{} in Rx.
Thank you.
There are a couple of ways to solve your problem. One way is just to call
repository.getSomeThings() in the collectLatest block and cache last result:
var lastResult: AsyncResult<SomeThings>? = null
anotherRepository.getThings().collectLatest {
if (lastResult == null) {
lastResult = repository.getSomeThings()
}
// use lastResult and List<String>
}
Another approach is to create a Flow, which will be calling repository.getSomeThings() function, and combine two Flows:
combine(
anotherRepository.getThings(),
flow {emit(repository.getSomeThings())}
) { result1: List<String>, result2: AsyncResult<SomeThings> ->
...
}
The following code is from the project.
The function of tasksRepository.refreshTasks() is to insert data from remote server to local DB, it's a time consuming operation.
In class TasksViewModel, asksRepository.refreshTasks() is wrapped with viewModelScope.launch{}, it means launch and careless.
1: How can I guarantee tasksRepository.observeTasks().distinctUntilChanged().switchMap { filterTasks(it) } to return the latest result?
2: I don't know how distinctUntilChanged() work, will it keep listening to return the latest result in whole Lifecycle ?
3: What's happened if I use tasksRepository.observeTasks().switchMap { filterTasks(it) } instead of tasksRepository.observeTasks().distinctUntilChanged().switchMap { filterTasks(it) }
Code
class TasksViewModel(..) : ViewModel() {
private val _items: LiveData<List<Task>> = _forceUpdate.switchMap { forceUpdate ->
if (forceUpdate) {
_dataLoading.value = true
viewModelScope.launch {
tasksRepository.refreshTasks()
_dataLoading.value = false
}
}
tasksRepository.observeTasks().distinctUntilChanged().switchMap { filterTasks(it) }
}
...
}
class DefaultTasksRepository(...) : TasksRepository {
override suspend fun refreshTask(taskId: String) {
updateTaskFromRemoteDataSource(taskId)
}
private suspend fun updateTasksFromRemoteDataSource() {
val remoteTasks = tasksRemoteDataSource.getTasks()
if (remoteTasks is Success) {
tasksLocalDataSource.deleteAllTasks()
remoteTasks.data.forEach { task ->
tasksLocalDataSource.saveTask(task)
}
} else if (remoteTasks is Result.Error) {
throw remoteTasks.exception
}
}
override fun observeTasks(): LiveData<Result<List<Task>>> {
return tasksLocalDataSource.observeTasks()
}
}
switchMap - The returned LiveData delegates to the most recent LiveData created by calling switchMapFunction with the most recent value set to source, without changing the reference. Doc
Yes, it'll keep listening to return the latest result in whole Lifecycle. distinctUntilChanged creates a new LiveData object that does not emit a value until the source LiveData value has been changed. The value is considered changed if equals() yields false.
Yes you can use that too but it'll keep emitting the values even the values are the same as the last emitted value.
e.g. first emitted value is ["aman","bansal"] and the second is the same ["aman","bansal"] which you don't want to emit since the values are same. So you use distinctUntilChanged to make sure it won't emit the same value until changed.
I hope this helped.
Update Coroutines 1.3.0-RC
Working version:
#FlowPreview
suspend fun streamTest(): Flow<String> = channelFlow {
listener.onSomeResult { result ->
if (!isClosedForSend) {
offer(result)
}
}
awaitClose {
listener.unsubscribe()
}
}
Also checkout this Medium article by Roman Elizarov: Callbacks and Kotlin Flows
Original Question
I have a Flow emitting multiple Strings:
#FlowPreview
suspend fun streamTest(): Flow<String> = flowViaChannel { channel ->
listener.onSomeResult { result ->
if (!channel.isClosedForSend) {
channel.sendBlocking(result)
}
}
}
After some time I want to unsubscribe from the stream. Currently I do the following:
viewModelScope.launch {
beaconService.streamTest().collect {
Timber.i("stream value $it")
if(it == "someString")
// Here the coroutine gets canceled, but streamTest is still executed
this.cancel()
}
}
If the coroutine gets canceled, the stream is still executed. There is just no subscriber listening to new values. How can I unsubscribe and stop the stream function?
A solution is not to cancel the flow, but the scope it's launched in.
val job = scope.launch { flow.cancellable().collect { } }
job.cancel()
NOTE: You should call cancellable() before collect if you want your collector stop when Job is canceled.
You could use the takeWhile operator on Flow.
flow.takeWhile { it != "someString" }.collect { emittedValue ->
//Do stuff until predicate is false
}
For those willing to unsubscribe from the Flow within the Coroutine scope itself, this approach worked for me :
viewModelScope.launch {
beaconService.streamTest().collect {
//Do something then
this.coroutineContext.job.cancel()
}
}
With the current version of coroutines / Flows (1.2.x) I don't now a good solution. With onCompletion you will get informed when the flow stops, but you are then outside of the streamTest function and it will be hard to stop listening of new events.
beaconService.streamTest().onCompletion {
}.collect {
...
}
With the next version of coroutines (1.3.x) it will be really easy. The function flowViaChannel is deprecated in favor for channelFlow. This function allows you to wait for closing of the flow and do something in this moment, eg. remove listener:
channelFlow<String> {
println("Subscribe to listener")
awaitClose {
println("Unsubscribe from listener")
}
}
When a flow runs in couroutin scope, you can get a job from it to controls stop subscribe.
// Make member variable if you want.
var jobForCancel : Job? = null
// Begin collecting
jobForCancel = viewModelScope.launch {
beaconService.streamTest().collect {
Timber.i("stream value $it")
if(it == "someString")
// Here the coroutine gets canceled, but streamTest is still executed
// this.cancel() // Don't
}
}
// Call whenever to canceled
jobForCancel?.cancel()
For completeness, there is a newer version of the accepted answer. Instead of explicitly using the launch coroutine builder, we can use the launchIn method directly on the flow:
val job = flow.cancellable().launchIn(scope)
job.cancel()
Based on #Ronald answer this works great for testing when you need to make your Flow emits again.
val flow = MutableStateFlow(initialValue)
flow.take(n).collectIndexed { index, _ ->
if (index == something) {
flow.value = update
}
}
//your assertions
We have to know how many emissions in total we expect n and then we can use the index to know when to update the Flow so we can receive more emissions.
If you want to cancel only the subscription being inside it, you can do it like this:
viewModelScope.launch {
testScope.collect {
return#collect cancel()
}
}
There are two ways to do this that are by design from the Kotlin team:
As #Ronald pointed out in another comment:
Option 1: takeWhile { //predicate }
Cancel collection when the predicate is false. Final value will not be collected.
flow.takeWhile { value ->
value != "finalString"
}.collect { value ->
//Do stuff, but "finalString" will never hit this
}
Option 2: transformWhile { //predicate }
When predicate is false, collect that value, then cancel
flow.transformWhile { value ->
emit(value)
value != "finalString"
}.collect { value ->
//Do stuff, but "finalString" will be the last value
}
https://github.com/Kotlin/kotlinx.coroutines/issues/2065