```supervisorScope { async { throw Exception() } async { throw Exceptio...

# coroutines

[mingkangpan]

supervisorScope {
    async {
        throw Exception()
    }

    async {
        throw Exception()    
    }
}

^ app doesn't crash

supervisorScope {
    launch {
        throw Exception()
    }

    launch {
        throw Exception()    
    }
}

^ app crashes
what am I doing wrong?

[Manuel Vivo]

When async has a SupervisorJob as a parent (which is in that case), it won’t crash until you call .await() . launch automatically throws the exceptions as soon as it happens but that’s not the case of async . It’ll hold it until you call await

[Manuel Vivo]

Notice, that if you change supervisorScope for coroutineScope, now the parent Job of async is of type Job (not SupervisorJob) and the exception will be propagated as soon as it happens

[Manuel Vivo]

That’s because a SupervisorJob let’s the coroutine handle the exception (and crash if it hasn’t been handled)

[Manuel Vivo]

An article about this is planned to be published next Wednesday; based on Florina Muntenescu and my talk at KotlinConf

[mingkangpan]

@Manuel Vivo could you hep me woth this one?

override fun onChanged(t: AccountInfo) {
        fetchDataAfterAccountInfo?.cancel()
        fetchDataAfterAccountInfo = viewModelScope.launch {
            try {
                supervisorScope {
                    launch {
                        val purchases = appService.fetchPurchasesWithPickupData().purchases
                        pickPurchasesLiveData.postValue(purchases)
                    }
                    launch {
                        val paymentPlan = appService.fetchPaymentPlan()
                        paymentPlanLiveData.postValue(paymentPlan.instalments)
                    }
                }
            } catch (e : Exception) {
                Timber.e(e, "loading data after accountInfo failed")
            }
        }
    }

[Manuel Vivo]

supervisorScope is not going to propagate the exception up to the try/catch, if you want that behavior, use coroutineScope

[mingkangpan]

everytime after a new accountInfo, I want to load 2 different set if data and put them into livedata
if one fails, it should NOT influence the other one
but I want to be able to cancel both at same time

[mingkangpan]

how would you code?

[Manuel Vivo]

Then use supervisorScope but try/catch the inner launches instead of try/catching the supervisorScope call

[mingkangpan]

override fun onChanged(info: AccountInfo) {
        fetchDataAfterAccountInfo?.cancel()
        fetchDataAfterAccountInfo = viewModelScope.launch(CoroutineExceptionHandler { _, e -> Timber.e(e, "loading data after accountInfo failed") }) {
            supervisorScope {
                launch {
                    val purchases = appService.fetchPurchasesWithPickupData().purchases
                    pickPurchasesLiveData.postValue(purchases)
                }
                launch {
                    val paymentPlan = appService.fetchPaymentPlan()
                    paymentPlanLiveData.postValue(paymentPlan.instalments)
                }
            }
        }
    }

[mingkangpan]

this should do it right?! 😅

[Manuel Vivo]

https://medium.com/androiddevelopers/exceptions-in-coroutines-ce8da1ec060c

[Manuel Vivo]

It’s the point of the section. You should never do that

[Manuel Vivo]

because it’s pointless, it’s not doing anything

[mingkangpan]

why does the method has this argument then?

[Evan R.]

The argument is more for changing the coroutine dispatcher so you can run in a different thread pool, for instance. The proper way to deal with the SupervisorJob would be to define job as CoroutineScope(SupervisorJob()) and then perform multiple launch {} invocations off of that so they can still run independently.

[Evan R.]

I think you can also add a CoroutineExceptionHandler but I could be wrong