`async {}` throws exception not until `.await()` be called.

# coroutines

[lesincs]

async {} throws exception not until .await() be called.

[lesincs]

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?

[louiscad]

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.

[louiscad]

Here (video timestamp in the link), I explain structured concurrency, with examples where the async function is used:

https://youtu.be/P1eeS6DQ5Uo?t=1367▾

[louiscad]

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▾

[lesincs]

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.😁

[louiscad]

My suggestion that doesn't require a supervisorScope works for both async and launch FYI.