Trying to implement graceful handling of the errors with suspend functions that are called from async methods, How to catch the error thrown by a suspend method.
suspend fun findById(id: Long): User? {
throw Exception("my exception") // intentionally throwing to simulate error situation.
return userModel.findById(id) // IO, may throw an error
}
Caller piece, launching with IO thread
GlobalScope.launch(Dispatchers.IO) {
try {
var userAsync: Deferred<User?>? = null
arguments?.getLong("id")?.let {
userAsync = async { viewModel?.findById(it) } // async for efficiency as i've other async methods too.
}
val data = userAsync?.await()
withContext(Dispatchers.Main) {
user = data // data binding, populating UI fields of user
}
} catch (exception: Exception) {
withContext(Dispatchers.Main) { fault(exception) }
}
}
fault method
private fun fault(exception: Exception) {
Log.d("User", "fault: ${exception.localizedMessage}") // expecting output
}
Currently runtime is crashing, want to implement graceful handling of errors.
Attempt 2
Tried placing try catch within the async block but it didn't like it.
var userAsync: Deferred<UserVO?>? = null
arguments?.getLong("id")?.let {
userAsync = async {
try {
delegate?.findById(it)
} catch (e: Exception) {
print(e)
}
}
}
I would use a CoroutineExceptionHandler to make your coroutines fail gracefully:
1) Define the handler:
val exceptionHandler = CoroutineExceptionHandler { context, error ->
// Do what you want with the error
Log.d(TAG, error)
}
2) Refactor your findById function to be executed within an IO context and make your ui code main safe:
suspend fun findById(id : Int) : User? = withContext(Dispatchers.IO){
when(id){
0 -> throw Exception("not valid")
else -> return#withContext User(id)
}
}
Launch your job within MainScope (and so update the ui), passing exceptionHandler to launch coroutine builder in order to catch the exception:
val exceptionHandler = CoroutineExceptionHandler { _, error ->
// Do what you want with the error
Log.d(TAG, error)
}
MainScope().launch(exceptionHandler) {
val user = delegate?.findById(userId)
user?.let {
Timber.v(it.toString())
}
}
Related
I have the below code in my view model class.
class MarketViewModel #Inject constructor(repo: MarketRepository) : ViewModel() {
private val retry = MutableStateFlow(0)
val marketState: LiveData<State<Market>> =
retry.flatMapLatest{repo.refreshMarket()}
.map { State.Success(it) as State<T> }
.catch { error -> emit(State.Error(error)) }
.stateIn(vmScope, SharingStarted.WhileSubscribed(5000), State.Loading())
.asLiveData()
fun retry() {
retry.value++
}
}
MarketRepository.kt:
fun refreshMarket() =
flow { emit(api.getMarkets()) }
.onEach { db.upsert(it) }
.flowOn(dispatchers.IO)
It works fine until a network error occurs in the repository method refreshMarket then when I call the retry() on the view model, it doesn't trigger the flatMapLatest transformer function anymore on the retry MutableStateFlow, why?
Does the flow get complete when it calls a Catch block? how to handle such situation?
You're right, catch won't continue emitting after an exception is caught. As the documentation says, it is conceptually similar to wrapping all the code above it in try. If there is a loop in a traditional try block, it does not continue iterating once something is thrown, for example:
try {
for (i in 1..10) {
if (i == 2) throw RuntimeException()
println(i)
}
} catch (e: RuntimeException) {
println("Error!")
}
In this example, once 2 is encountered, the exception is caught, but code flow does not return to the loop in the try block. You will not see any numbers printed that come after 2.
You can use retryWhen instead of catch to be able to restart the flow. To do it on demand like you want, maybe this strategy could be used (I didn't test it):
class MarketViewModel #Inject constructor(repo: MarketRepository) : ViewModel() {
private val retry = MutableSharedFlow<Unit>()
val marketState: LiveData<State<Market>> =
repo.refreshMarket()
.map { State.Success(it) as State<T> }
.retryWhen { error, _ ->
emit(State.Error(error))
retry.first() // await next value from retry flow
true
}
.stateIn(vmScope, SharingStarted.WhileSubscribed(5000), State.Loading())
.asLiveData()
fun retry() {
retry.tryEmit(Unit)
}
}
I'm trying to implement One Tap, so I have created a function that looks like this:
override suspend fun oneTapSgnInWithGoogle() = flow {
try {
emit(Result.Loading)
val result = oneTapClient.beginSignIn(signInRequest).await()
emit(Result.Success(result))
} catch (e: Exception) {
Log.d(TAG, "oneTapSgnInWithGoogle: ${e.message}")
emit(Result.Error(e.message!!))
}
}
If I use flow and try to emit the result, my app crashed with the following message:
Flow exception transparency is violated:
StandaloneCoroutine has completed normally; but then emission attempt of value 'Error(message=StandaloneCoroutine has completed normally)' has been detected.
However, if change the code to:
override suspend fun oneTapSgnInWithGoogle() = channelFlow {
try {
send(Result.Loading)
val result = oneTapClient.beginSignIn(signInRequest).await()
send(Result.Success(result))
} catch (e: Exception) {
Log.d(TAG, "oneTapSgnInWithGoogle: ${e.message}")
send(Result.Error(e.message!!))
}
}
And I use channelFlow and try to send the result, the app isn't crashing but I still get the error message saying:
StandaloneCoroutine has completed normally
How can I emit the result correctly and get rid of this error message?
P.S. In my ViewModel class I use:
fun oneTapSgnInWithGoogle() = liveData(Dispatchers.IO) {
viewModelScope.launch {
repo.oneTapSgnInWithGoogle().collect { result ->
emit(result)
}
}
}
This is not a good practice to launch a coroutine in liveData block. liveData block is a suspend lambda, you can collect values directly in it without launching a coroutine:
fun oneTapSgnInWithGoogle() = liveData(Dispatchers.IO) {
repo.oneTapSgnInWithGoogle().collect { result ->
emit(result)
}
}
In your case liveData block has already finished execution (and corresponding coroutine, in which liveData block is executed) when you try to emit a value to LiveData. The solution above should solve the problem.
I have an android app that I have built up an architecture similar to the Google IO App. I use the CoroutineUseCase from that app (but wrap results in a kotlin.Result<T> instead).
The main code looks like this:
suspend operator fun invoke(parameters: P): Result<R> {
return try {
withContext(Dispatchers.Default) {
work(parameters).let {
Result.success(it)
}
}
} catch (e: Throwable) {
Timber.e(e, "CoroutineUseCase Exception on ${Thread.currentThread().name}")
Result.failure<R>(e)
}
}
#Throws(RuntimeException::class)
protected abstract suspend fun work(parameters: P): R
Then in my view model I am invoking the use case like this:
viewModelScope.launch {
try {
createAccountsUseCase(CreateAccountParams(newUser, Constants.DEFAULT_SERVICE_DIRECTORY))
.onSuccess {
// Update UI for success
}
.onFailure {
_errorMessage.value = Event(it.message ?: "Error")
}
} catch (t: Throwable) {
Timber.e("Caught exception (${t.javaClass.simpleName}) in ViewModel: ${t.message}")
}
My problem is even though the withContext call in the use case is wrapped with a try/catch and returned as a Result, the exception is still thrown (hence why I have the catch in my view model code - which i don't want). I want to propagate the error as a Result.failure.
I have done a bit of reading. And my (obviously flawed) understanding is the withContext should create a new scope so any thrown exceptions inside that scope shouldn't cancel the parent scope (read here). And the parent scope doesn't appear to be cancelled as the exception caught in my view model is the same exception type thrown in work, not a CancellationException or is something unwrapping that?. Is that a correct understanding? If it isn't what would be the correct way to wrap the call to work so I can safely catch any exceptions and return them as a Result.failure to the view model.
Update:
The implementation of the use case that is failing. In my testing it is the UserPasswordInvalidException exception that is throwing.
override suspend fun work(parameters: CreateAccountParams): Account {
val tokenClient = with(parameters.serviceDirectory) {
TokenClient(tokenAuthorityUrl, clientId, clientSecret, moshi)
}
val response = tokenClient.requestResourceOwnerPassword(
parameters.newUser.emailAddress!!,
parameters.newUser.password!!,
"some scopes offline_access"
)
if (!response.isSuccess || response.token == null) {
response.statusCode?.let {
if (it == 400) {
throw UserPasswordInvalidException("Login failed. Username/password incorrect")
}
}
response.exception?.let {
throw it
}
throw ResourceOwnerPasswordException("requestResourceOwnerPassword() failed: (${response.message} (${response.statusCode})")
}
// logic to create account
return acc
}
}
class UserPasswordInvalidException(message: String) : Throwable(message)
class ResourceOwnerPasswordException(message: String) : Throwable(message)
data class CreateAccountParams(
val newUser: User,
val serviceDirectory: ServiceDirectory
)
Update #2:
I have logging in the full version here is the relevant details:
2020-09-24 18:12:28.596 25842-25842/com.ipfx.identity E/CoroutineUseCase: CoroutineUseCase Exception on main
com.ipfx.identity.domain.accounts.UserPasswordInvalidException: Login failed. Username/password incorrect
at com.ipfx.identity.domain.accounts.CreateAccountsUseCase.work(CreateAccountsUseCase.kt:34)
at com.ipfx.identity.domain.accounts.CreateAccountsUseCase.work(CreateAccountsUseCase.kt:14)
at com.ipfx.identity.domain.CoroutineUseCase$invoke$2.invokeSuspend(CoroutineUseCase.kt:21)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:571)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:738)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:678)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:665)
2020-09-24 18:12:28.598 25842-25842/com.ipfx.identity E/LoginViewModel$createAccount: Caught exception (UserPasswordInvalidException) in ViewModel: Login failed. Username/password incorrect
The full exception is logged inside the catching in CoroutineUseCase.invoke. And then again the details logged inside the catch in the view model.
Update #3
#RKS was correct. His comment caused me to look deeper. My understanding was correct on the exception handling. The problem was in using the kotlin.Result<T> return type. I am not sure why yet but I was somehow in my usage of the result trigger the throw. I switched the to the Result type from the Google IO App source and it works now. I guess enabling its use as a return type wasn't the smartest.
try/catch inside viewModelScope.launch {} is not required.
The following code is working fine,
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.delay
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.withContext
class TestCoroutines {
private suspend fun work(): String {
delay(1000)
throw Throwable("Exception From Work")
}
suspend fun invoke(): String {
return try {
withContext(Dispatchers.Default) {
work().let { "Success" }
}
} catch (e: Throwable) {
"Catch Inside:: invoke"
}
}
fun action() {
runBlocking {
val result = invoke()
println(result)
}
}
}
fun main() {
TestCoroutines().action()
}
Please check the entire flow if same exception is being thrown from other places.
i just created an app where my function getdata() call every second to fetch new data from server and updateui() function will update view in UI i don't use any asynctask or coroutine in my app i wants to do this please tell me how i can do that.
here's my code...
private fun getdata(){
try {
val api = RetroClient.getApiService()
call = api.myJSON
call!!.enqueue(object : Callback<ProductResponse> {
override fun onResponse(
call: Call<ProductResponse>,
response: Response<ProductResponse>
) {
if (response.isSuccessful) {
productList = response.body()!!.data
for (list in productList) {
if (list.BB.equals("AAA")) {
aProductList.add(list)
}
}
if (recyclerView.adapter != null) {
eAdapter!!.updatedata(aProductList)
}
updateui()
}
}
override fun onFailure(call: Call<ProductResponse>, t: Throwable) {
println("error")
}
})
} catch (ex: Exception) {
} catch (ex: OutOfMemoryError) {
}
Handler().postDelayed({
getdata()
}, 1000)
}
private fun updateui() {
try {
//some code to handel ui
} catch (e: NumberFormatException) {
} catch (e: ArithmeticException) {
} catch (e: NullPointerException) {
} catch (e: Exception) {
}
}
To run a function every second with coroutines:
val scope = MainScope() // could also use an other scope such as viewModelScope if available
var job: Job? = null
fun startUpdates() {
stopUpdates()
job = scope.launch {
while(true) {
getData() // the function that should be ran every second
delay(1000)
}
}
}
fun stopUpdates() {
job?.cancel()
job = null
}
However, if getData() only starts a network request and doesn't wait for its completion, this might not be a very good idea. The function will be called a second after it finished, but because the network request is done asynchronously it may be scheduled way too much.
For example if the network request takes 5 seconds, it will have been started 4 more times before the first one even finished!
To fix this, you should find a way to suspend the coroutine until the network request is done.
This could be done by using a blocking api, then pass Dispatchers.IO to the launch function to make sure it's done on a background thread.
Alternatively you could use suspendCoroutine to convert a callback-based api to a suspending one.
Update - Lifecycle scope
Inside a component with a Android Lifecycle you could use the following code to automate repeating ui updates:
fun startUpdates() {
val lifecycle = this // in Activity
val lifecycle = viewLifecycleOwner // in Fragment
lifecycle.lifecycleScope.launch {
lifecycle.repeatOnLifecycle(Lifecycle.State.STARTED) {
// this block is automatically executed when moving into
// the started state, and cancelled when stopping.
while (true) {
getData() // the function to repeat
delay(1000)
}
}
}
}
This code requires the current androidx.lifecycle:lifecycle-runtime-ktx dependency.
The above remark about async, blocking or suspending code inside getData() still applies.
it's not advisable to hit the server every second. if you need to get data continuously try the socket. Because some times your server takes more than a few seconds to respond to your request. Then all your requests will be in a queue..if you still need to try with this.
fun repeatFun(): Job {
return coroutineScope.launch {
while(isActive) {
//do your network request here
delay(1000)
}
}
}
//start the loop
val repeatFun = repeatRequest()
//Cancel the loop
repeatFun.cancel()
For those who are new to Coroutine
add Coroutine in Build.gradle
implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.4.2'
To create a repeating Job
/**
* start Job
* val job = startRepeatingJob()
* cancels the job and waits for its completion
* job.cancelAndJoin()
* Params
* timeInterval: time milliSeconds
*/
private fun startRepeatingJob(timeInterval: Long): Job {
return CoroutineScope(Dispatchers.Default).launch {
while (NonCancellable.isActive) {
// add your task here
doSomething()
delay(timeInterval)
}
}
}
To start:
Job myJob = startRepeatingJob(1000L)
To Stop:
myJob .cancel()
I ended up doing like this with an extension function:
fun CoroutineScope.launchPeriodicAsync(repeatMillis: Long, action: () -> Unit) = this.async {
while (isActive) {
action()
delay(repeatMillis)
}
}
then call it like:
val fetchDatesTimer = CoroutineScope(Dispatchers.IO)
.launchPeriodicAsync(TimeUnit.MINUTES.toMillis(1)) {
viewModel.fetchDeliveryDates()
}
and cancel it like:
fetchDatesTimer.cancel()
My solution in Kotlin inside MainViewModel
fun apiCall() {
viewModelScope.launch(Dispatchers.IO) {
while(isActive) {
when(val response = repository.getServerData()) {
is NetworkState.Success -> {
getAllData.postValue(response.data)
}
is NetworkState.Error -> this#MainViewModel.isActive = false
}
delay(1000)
}
}
}
sealed class NetworkState<out R> {
data class Success<out T>(val data: T): NetworkState<T>()
data class Error(val exception: String): NetworkState<Nothing>()
object Loading: NetworkState<Nothing>()
}
My solution for one time running a code after check for something is successful and checking for that periodically, function is:
fun CoroutineScope.launchPeriodic(repeatMillis: Long, action: () -> Unit) : Job {
return launch {
while (!enabled) {
action()
delay(repeatMillis)
}
}
}
and start periodic function here (in which action runs every 2 seconds), which automatically ends up when something is enabled and some code run:
CoroutineScope(Dispatchers.IO).launchPeriodic(TimeUnit.SECONDS.toMillis(2)) {
if(checkIfSomethingIsEnabledCodeIsHere) {
enabled = true
//some code here to run when it is enabled
}
}
I have the following code using Coroutine to perform network fetching on background
try {
networkJob = CoroutineScope(Dispatchers.IO).launch {
val result = fetchOnBackground(searchText)
withContext(Dispatchers.Main) {
showResult("Count is $result")
}
}
} catch (exception: Throwable) {
showResult(exception.localizedMessage)
}
When the network is there, all is good. However, when either the host is not correct, or no network, it crashes. The catch can't catch it.
When I code using RxJava
disposable = Single.just(searchText)
.map{fetchOnBackground(it)}
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{ showResult("Count is $it") },
{ showResult(it.localizedMessage) })
All works fine. Even in no network situation, the error is caught in the error callback.
What did I miss in the Coroutine code, that I can't catch the error on my end when using the Coroutine?
Note: the network fetching is using OkHttp.
Seems like I need to put the try-catch within the CouroutineScope(Dispatchers.IO).launch
networkJob = CoroutineScope(Dispatchers.IO).launch {
try {
val result = fetchOnBackground(searchText)
showResult("Count is $result")
} catch (exception: Throwable) {
showResult(exception.localizedMessage)
}
}
And I change my showResult to be a suspend function, so that it could include the withContext(Dispatchers.Main)
private suspend fun showResult(result: String) {
withContext(Dispatchers.Main) {
// Code that show the result
}
}