Hi all :kodee-greetings: , I have a coroutine that...

# coroutines

[Abhimanyu]

Hi all kodee greetings ,
I have a coroutine that has few children coroutines and suspend method calls.
How to rethrow an exception thrown by one of the suspending method calls?
Code example in 🧵

[Abhimanyu]

fun initViewModel(): Job {
  return coroutineScope.launch {
    coroutineScope.launch { // Observes data 1 continuously
      observeData1().collectLatest {
        refreshUiState()
      }
    }
    coroutineScope.launch { // Observes data 2 continuously
      observeData2().collectLatest {
        refreshUiState()
      }
    }
    fetchData1() // Can throw an exception like "IllegalArgumentException"
  }
}

Currently, when fetchData1() throws an exception, initViewModel() throws JobCancellationException instead of the exception thrown by fetchData1() .

How to make initViewModel() throw the actual exception thrown by fetchData1() ?

[Sergey Dmitriev]

Is coroutineScope your custom scope that you created?

[Sergey Dmitriev]

BTW you for observeData1() and observeData2() you can omit coroutinseScope and just use

coroutineScope.launch {
  launch {
    // 1
  }
  launch {
    // 2
  }
}

I believe

[Deeksha Agrawal]

we can run fetchData1() in async and await
something like this

fun initViewModel(): Job {
    return coroutineScope.launch {
        val job1 = launch { observeData1().collectLatest { refreshUiState() } }
        val job2 = launch { observeData2().collectLatest { refreshUiState() } }

        val deferred = async { fetchData1() }
        deferred.await() // exception propagates here
    }
}

[Abhimanyu]

Hi @Sergey Dmitriev,
coroutineScope is injected and used like this.

CoroutineScope(
  dispatcherProvider.mainImmediate + SupervisorJob(),
)

internal class EditAccountScreenViewModel(
    private val coroutineScope: CoroutineScope,
    // 
) : ViewModel(
    viewModelScope = coroutineScope,
)

Can ignore the observing data part it is just a sample example.
The main question I have is how to rethrow exception from fetchData.

[Abhimanyu]

Hi @Deeksha Agrawal,

val deferred = async { fetchData1() }
deferred.await() // exception propagates here

is same as

val deferred = async { fetchData1() }.await() // exception propagates here

So, the async is called followed immediately by await() .
fetchData1() is a suspending method and it seems equivalent to my original code.

I tried this as well to see if it makes any difference in error handling, and I observed that it doesn’t.

[Deeksha Agrawal]

For this scenario either we can wrap siblings in supervisorScope or can something try like this:

runCatching {
            fetchData1()
        }.onFailure { e ->
            
        }