RxJava2 Flowable onErrorReturn not called - kotlin

Say I need to wrap BroadcastReceiver with Flowable:
Flowable
.create<Boolean>({ emitter ->
val broadcastReceiver = object : BroadcastReceiver() {
override fun onReceive(context: Context?, intent: Intent?) {
throw RuntimeException("Test exception")
}
}
application.registerReceiver(broadcastReceiver, IntentFilter(LocationManager.PROVIDERS_CHANGED_ACTION))
}, BackpressureStrategy.MISSING)
.onErrorReturn { false }
Then I need to catch any exceptions thrown inside Flowable in one single place.
I supposed onErrorReturn should be able to catch that throw RuntimeException("Test exception") inside broadcastReceiver but it doesn't catch that exception and app crashes.
Certainly, I can wrap anything inside BroadcastReceiver with try/catch. But actually, I have a lot of source code there so that adding try/catch makes source code quite messy.
Is there any way to catch all the exceptions thrown in any line inside Flowable in one single place?

In case of Flowable#create() to follow contract of Flowable if you have error and want to pass it through stream, you need to catch it and call emitter.onError(). If you do that, Flowable.onErrorReturn() starts work as expected.
To properly register/unregister BroadcastReceiver and handle exceptions you can use that approach
Flowable
.create<Boolean>({ emitter ->
val broadcastReceiver = object : BroadcastReceiver() {
override fun onReceive(context: Context?, intent: Intent?) {
try {
throw RuntimeException("Test exception")
} catch(e: Throwable) {
emitter.tryOnError(e)
}
}
}
try {
application.registerReceiver(broadcastReceiver, IntentFilter(LocationManager.PROVIDERS_CHANGED_ACTION))
emitter.setCancellable {
application.unregisterReceiver(broadcastReceiver)
}
} catch (e: Throwable) {
emitter.tryOnError(e)
}
}, BackpressureStrategy.MISSING)
.onErrorReturn { false }

Related

How to catch an error in Jetpack compose?

I tried to catch application errors in this way:
class MainActivity : ComponentActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContent {
try {
MyApp()
}
catch (t: Throwable) {
Text(t.message ?: "Error", fontSize = 24.sp)
Text(t.stackTraceToString(), fontFamily = FontFamily.Monospace)
}
}
}
}
But it does not compile:
Try catch is not supported around composable function invocations.
WTF?
What else should I use?
#Composable
fun MyScreen() {
val data = try {
loadData()
} catch (error: Throwable) {
// Handle the error here
null
}
if (data != null) {
// Render the data
} else {
// Render an error message or retry button
}
}
To use the catch operator, you can call it on a Composable function and pass in a lambda function to handle the error. The lambda function should take a single parameter of type Throwable, which represents the error that occurred. Inside the lambda function, you can perform any necessary error handling, such as displaying an error message or retrying the failed operation.

Launch coroutine in try catch block crashes

I could not understand the difference between these two functions. Why the func2 crashes program while func1 can caught the exception ?
fun main() {
runBlocking {
func1() //Prints exception
func2() //Program crashes
}
}
fun CoroutineScope.func1() {
launch {
try {
throw IllegalArgumentException("error")
} catch (t: Throwable) {
println(t)
}
}
}
fun CoroutineScope.func2() {
try {
launch {
throw IllegalArgumentException("error")
}
} catch (t: Throwable) {
println(t)
}
}
The code inside "launch" block runs on a separate Coroutine with a different context. The external try/catch cannot catch the exception happening. You need to have try/catch in one block, like your func1.

Replacing GlobalScope.launch with something better

I'm refactoring the following bit of code that's wrapping a CompletableFuture API into something that can be used with Coroutines, but it's using GlobalScope.launch { ... } which is discouraged:
suspend fun <T> transaction(f: suspend (Connection) -> T): T {
val cf = CompletableFuture<T>()
try {
this.connectionPool.inTransaction { connection ->
GlobalScope.launch {
try {
cf.complete(f(connection))
} catch (e: Throwable) {
cf.completeExceptionally(e)
}
}
cf
}
} catch (e: Throwable) {
log.error(e.message ?: "", e)
cf.completeExceptionally(e)
}
return cf.await()
}
Getting rid of the CompletableFuture and replacing it with CompletableDeferred is the easy part:
suspend fun <T> transaction(f: suspend (Connection) -> T): T {
val cdf = CompletableDeferred<T>()
try {
connectionPool.inTransaction { connection ->
GlobalScope.launch {
try {
cdf.complete(f(connection))
} catch (e: Throwable) {
cdf.completeExceptionally(e)
}
}
cdf.asCompletableFuture()
}
} catch (e: Throwable) {
log.error(e.message ?: "", e)
cdf.completeExceptionally(e)
}
return cdf.await()
}
The inTransaction API expects a CompletableFuture, I'm assuming this is for backwards compatibility with Java
override fun <A> inTransaction(f: (Connection) -> CompletableFuture<A>):
CompletableFuture<A> =
objectPool.use(configuration.executionContext) { it.inTransaction(f) }
Since I'm outside a CoroutineScope, I can't just call launch { ... } and since f is a suspend function, that section needs to be inside a CoroutineScope
Wrapping the connectionPool.inTransaction inside a coroutineScope in order to replace the GlobalScope locks up when it runs ...
try {
coroutineScope {
connectionPool.inTransaction { connection ->
launch {
try {
cdf.complete(f(connection))
} catch (e: Throwable) {
cdf.completeExceptionally(e)
}
}
cdf.asCompletableFuture()
}
}
} catch (e: Throwable) {
Similarly with
try {
coroutineScope {
connectionPool.inTransaction { connection ->
async {
try {
cdf.complete(f(connection))
} catch (e: Throwable) {
cdf.completeExceptionally(e)
}
}
cdf.asCompletableFuture()
}
}
} catch (e: Throwable) {
Adding some good ol' println debugging:
suspend fun <T> transaction(f: suspend (Connection) -> T): T {
val cdf = CompletableDeferred<T>()
println("1")
try {
println("2")
coroutineScope {
println("3")
connectionPool.inTransaction { connection ->
println("4")
launch {
println("5")
try {
println("6")
cdf.complete(f(connection))
println("7")
} catch (e: Throwable) {
println("8")
cdf.completeExceptionally(e)
println("9")
}
}
println("10")
cdf.asCompletableFuture()
}
}
} catch (e: Throwable) {
log.error(e.message ?: "", e)
cdf.completeExceptionally(e)
}
println("11")
return cdf.await()
}
Outputs:
1
2
3
11
4
10
followed by a stacktrace that the query timed out
timeout query item <postgres-connection-2> after 73751 ms and was not cleaned by connection as it should, will destroy it - timeout is 30000
This means the code inside the launch is never executing, similarly with async, I'm assuming some thread is getting blocked somewhere.
Replacing the GlobalScope.launch with CoroutineScope(Dispatchers.IO).launch { ... } works (similarly with Dispatchers.Unconfined)), but is this the correct solution here?
If CompletableFuture is thread-blocking, then this is probably better than the GlobalScope.launch solution ...
suspend fun <T> transaction(f: suspend (Connection) -> T): T {
val cdf = CompletableDeferred<T>()
try {
connectionPool.inTransaction { connection ->
CoroutineScope(Dispatchers.IO).launch {
try {
cdf.complete(f(connection))
} catch (e: Throwable) {
cdf.completeExceptionally(e)
}
}
cdf.asCompletableFuture()
}
} catch (e: Throwable) {
log.error(e.message ?: "", e)
cdf.completeExceptionally(e)
}
return cdf.await()
}
Any suggestions on what the correct way is to get rid of that GlobalScope.launch?
It's pretty complicated, but I believe the cause of launch()/async() not executing is that their parent coroutineScope() already finished at this point in time. Notice "11" happens before "4", meaning that you invoked launch() after going out of coroutineScope(). Which makes sense, because inTransaction() starts asynchronous operation, so it returns immediately, without waiting for the inner code. To fix this, you just need to move cdf.await() inside coroutineScope().
Another thing that concerns me is that you await on completable that you created by yourself and not the one returned from inTransaction(). Note that it may be a totally different CompletableFuture and in that case you actually return before the operation completes.
Also, I'm not sure if this manual exception handling for completable is really necessary. async() already performs exception handling and wraps the result as CompleteableDeferred, then it is converted to CompletableFuture which also wraps exceptions. The only thing we have to do is to replace coroutineScope() with supervisorScope(). Otherwise, async() would automatically signal coroutineScope() to fail, so the exception handling would totally bypass inTransaction() function.
Try this code:
suspend fun <T> transaction(f: suspend (Connection) -> T): T {
return supervisorScope {
connectionPool.inTransaction { connection ->
async { f(connection) }.asCompletableFuture()
}.await()
}
}

After a coroutine scope is canceled, can it still be used again?

When we have a coroutine scope, when it is canceled, can it be used again?
e.g. for the below, when I have scope.cancel, the scope.launch no longer work
#Test
fun testingLaunch() {
val scope = MainScope()
runBlocking {
scope.cancel()
scope.launch {
try {
println("Start Launch 2")
delay(200)
println("End Launch 2")
} catch (e: CancellationException) {
println("Cancellation Exception")
}
}.join()
println("Finished")
}
}
Similarly, when we have scope.cancel before await called,
#Test
fun testingAsync() {
val scope = MainScope()
runBlocking {
scope.cancel()
val defer = scope.async {
try {
println("Start Launch 2")
delay(200)
println("End Launch 2")
} catch (e: CancellationException) {
println("Cancellation Exception")
}
}
defer.await()
println("Finished")
}
}
It will not execute. Instead, it will crash with
kotlinx.coroutines.JobCancellationException: Job was cancelled
; job=SupervisorJobImpl{Cancelled}#39529185
at kotlinx.coroutines.JobSupport.cancel(JobSupport.kt:1579)
at kotlinx.coroutines.CoroutineScopeKt.cancel(CoroutineScope.kt:217)
at kotlinx.coroutines.CoroutineScopeKt.cancel$default(CoroutineScope.kt:215)
at com.example.coroutinerevise.CoroutineExperiment$testingAsync$1.invokeSuspend(CoroutineExperiment.kt:241)
at |b|b|b(Coroutine boundary.|b(|b)
at kotlinx.coroutines.DeferredCoroutine.await$suspendImpl(Builders.common.kt:101)
at com.example.coroutinerevise.CoroutineExperiment$testingAsync$1.invokeSuspend(CoroutineExperiment.kt:254)
Caused by: kotlinx.coroutines.JobCancellationException: Job was cancelled; job=SupervisorJobImpl{Cancelled}#39529185
at kotlinx.coroutines.JobSupport.cancel(JobSupport.kt:1579)
at kotlinx.coroutines.CoroutineScopeKt.cancel(CoroutineScope.kt:217)
at kotlinx.coroutines.CoroutineScopeKt.cancel$default(CoroutineScope.kt:215)
at com.example.coroutinerevise.CoroutineExperiment$testingAsync$1.invokeSuspend(CoroutineExperiment.kt:241)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
Is it true, a canceled coroutine scope cannot be used for launch or async anymore?
Instead of using CoroutineScope for cancelling all of the launched jobs in it, you may want to use the underlying CoroutineContext with its cancelChildren() method which doesn't affect the Job state (which is not true for plain cancel() method) and allows to continue launching new coroutines after being invoked.
Following up on #Alex Bonel response.
For example you have a method like
fun doApiCall() {
viewModelScope.launch {
// do api call here
}
}
You can call doApiCall() again and again
viewModelScope.coroutineContext.cancelChildren()
doApiCall()
Calling doApiCall() will not have any effect.
viewModelScope.coroutineContext.cancel()
doApiCall() // would not call
I am using that in compose and it is slightly different.
Define coroutine scope in compose
val coroutineScope = rememberCoroutineScope()
Launch coroutine
coroutineScope.launch(Dispatchers.Main) { //Perform your operation }
Cancel all coroutines that belong to coroutineScope
coroutineScope.coroutineContext.cancelChildren()
Related to lifecycleScope comment above, I'm not sure how common cancellation like this would be in practice? fwiw something like following will work:
val scope = MainScope()
runBlocking {
val job1 = scope.launch {
try {
println("Start Launch 1")
delay(200)
println("End Launch 1")
} catch (e: CancellationException) {
println("Cancellation Exception")
}
}
job1.cancel()
val job2 = scope.launch {
try {
println("Start Launch 2")
delay(200)
println("End Launch 2")
} catch (e: CancellationException) {
println("Cancellation Exception")
}
}
job2.join()
println("Finished")
}
This particular example will print
Start Launch 1
Cancellation Exception
Start Launch 2
End Launch 2
Finished
I needed to collect the event just once and then stop listening. I came up with this solution:
/**
* Collect the value once and stop listening (one-time events)
*/
suspend fun <T> Flow<T>.collectOnce(action: suspend (value: T) -> Unit) {
try {
coroutineScope {
collectLatest {
action(it)
this#coroutineScope.cancel()
}
}
} catch (e: CancellationException) { }
}
You can use it like this:
viewModelScope.launch {
myStateFlow.collectOnce {
// Code to run
}
}

Does the scope handle exceptions/failures silently under the hood?

I have the following snippet for testing purposes;
fun main() {
val myScope = CoroutineScope(Dispatchers.Default) + Job()
myScope.launch {
val job = async {
delay(1000)
throw RuntimeException("shiiiet")
}
try {
job.await()
} catch (ret: RuntimeException){
throw RuntimeException("yooo!")
}
}
try {
Thread.sleep(5000)
} catch(e: Exception){
}
println("wohoooo!")
}
I thought the flow would never reach the last "wohooo!" line but I was wrong. I see it's printed on the screen. The reason I had in my mind that launch would propagate the exception to the parent scope and since the parent scope does not handle it, it would crash the JVM by the time it reaches the print statement.
Is this because the parent scope got cancelled once it's child failed, received a CancellationException and it was ignored?
You tried multiple throwand catch approaches in your example.
async works as expected - when you await for it, you can catch the exception. But if you launch a co-routine, the default Thread.uncaughtExceptionHandler just prints the result to console.
Even if you do
myScope.launch {
....
}.invokeOnCompletion { e -> println("Exception: $e") }
you still get the result additionally on console.
The propagation rules and the different types of calls to handle the exceptions are explained here.
An example on how to catch an exception in the "main" co-routine:
fun main() = runBlocking {
try {
GlobalScope.launch {
delay(10)
throw Exception("You don't catch me in main.")
}
launch {
delay(10)
throw Exception("You catch me in main.")
}
delay(100)
println("Never reached.")
} catch(e: Exception) {
println("Caught in main: ${e.cause}")
}
println("The end")
}