Hi! I ran into error handling behaviour I don’t un...

# coroutines

[Vita Sokolova]

Hi! I ran into error handling behaviour I don’t understand, can someone explain it to me? I prepared a repo with simplified example. I have an async call in on a ViewModel scope, if I use try/catch, catch code is hit, but app crashes

fun loadData() {
        viewModelScope.launch {
            try {
                greeting.value = async { getGreetingUseCase() }.await()
            }
            catch (e: CancellationException){
                throw e
            }
            catch (e: Exception) {
                greeting.value = "Error occurred!"
            }
        }
    }

But with CoroutineExceptionHandler it works without crashes.

fun loadDataWithCoroutineExceptionHandler() {
        val exceptionHandler = CoroutineExceptionHandler { _, exception ->
            greeting.value = "Error occurred!"
        }
        viewModelScope.launch(exceptionHandler) {
            greeting.value = async { getGreetingUseCase() }.await()
        }
    }

[Dmitry Khalanskiy [JB]]

async reports its error in two places:
• The exception is propagated to the parent coroutine (in this case, to viewModelScope.launch);
• The exception is also stored in the Deferred as the result of the computation.
When you catch the exception thrown by await (the second place where the exception is stored), this doesn't stop the launch from learning that one of its children failed, and so it fails, too.

[Vita Sokolova]

I didn’t expect it to work that way, thanks for explanation 🙌