Is there any way to make a catch operator which do...

# coroutines

[Vaibhav Jaiswal]

Is there any way to make a catch operator which does not cancel the upstream flow when an exception occurs
What currently happens is, it cancels the flow

val flow = flowOf(1, 2, 3, 4, 5)
    .onEach { if(it % 3 == 0) throw Exception("Failed") }
    .catch { println(it) }

fun main() {
    runBlocking {
        flow.collect { println(it) }
    }
}

Output:
1
2
java.lang.Exception: Failed

How can i make catch just skip 3
So the final output becomes

1 2 4 5

I know i can just use a tryCatch,
but in my usecase, the failure can happen in any flow operator, and i dont want to cancel the flow

My usecase:
I have a flow which listens to isTyping, isOnline events in a messaging scenario
Now if there is any error in any flow operator the upstream flow is cancelled,
What i want is to just ignore that emission and let the flow continue as usual

I dont want to add a try catch to every operator, but use something like the inbuilt catch operator which will catch any exception in the upstream flow but not cancel the upstream flow

[Sam]

No, it's not possible. You must think of the upstream flow the same way you think of an ordinary function. When it throws an exception, it exits.

[Dmitry Khalanskiy [JB]]

It's not possible. The code in your example becomes this after the operators are applied:

val flow = flow {
  for (it in listOf(1, 2, 3, 4, 5)) {
    if(it % 3 == 0) throw Exception("Failed")
    emit(it)
  }
}.catch { println(it) }

Once the exception is thrown, we can't just continue executing the code anyway, that's against Kotlin's rules.

What you can do is restart the whole upstream, though: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/retry.html From the description of your use case, it looks like it might work.