`async ` throws exception not until ` await ` be called kotlinlang #coroutines

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Title

l

lesincs

07/02/2022, 5:02 AM

async {}

throws exception not until

.await()

be called.

val scope = MainScope()
scope.launch {
    val deferred = async<String> {
        delay(200)
        throw RuntimeException()
    }

    try {
        val result = deferred.await()
        print(result)
    } catch (e: Exception) {
        print("catch exception")
    }
}

In this code snippet, the

try catch

is not gonna work, which confused me that isn’t

async

throwing exceptions only when

await()

is called, so the exception should be catched?

l

louiscad

07/04/2022, 3:18 AM

You need to wrap the code calling

async

with a local

coroutineScope { … }

, and surround that block with the try catch you want. Otherwise, you're breaking structured concurrency, which leads to the surprising behavior you've been witnessing.

Here (video timestamp in the link), I explain structured concurrency, with examples where the

async

function is used:

https://youtu.be/P1eeS6DQ5Uo?t=1367▾

👍 1

I also cover it earlier in the talk if you're curious, but with

launch

in place of

async

(it works exactly the same way):

https://youtu.be/P1eeS6DQ5Uo?t=803▾

l

lesincs

07/04/2022, 11:30 AM

Thanks Louis, I finally wrapped my whole code inside a

supervisorScope

, and then it works as I am expecting, and I think if I use

launch

instead of

async

, I would like to take your suggestion, use

coroutineScope

and then try catch it, thanks again.😁

l

louiscad

07/04/2022, 11:45 AM

My suggestion that doesn't require a

supervisorScope

works for both

async

and

launch

FYI.

✔️ 1

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