Is there a function on `CoroutineScope` which is `onError` S

Question

Is there a function on `CoroutineScope` which is `onError` So in case any of the coroutines in the scope are cancelled or failing it will not cancel the parent scope routines but instead invoke the error handler Or is `try catch` around the `coroutineScope ` currently the only way

elizarov · Answer

There is `CoroutineExceptionHandler` that does it It is explained here <http kotlinlang org docs reference coroutines exception handling html>

lukaswelte · Answer

Thanks but if I have an async function so I wait for an answer I would need need a fallback in addition instead of just catching the exception

dekans · Answer

`async` provides a `Deferred` which can tell you if an exception occured

lukaswelte · Answer

As an example <https pl kotl in rk9ZlXAcQ> I want that `startFetching` in this example is not cancelling the parent scope and would like to not have `try catch` around the `startFetching` call but rather something like `val x = startFetching ifError emptyList `

dekans · Answer

`CoroutineExceptionHandler` seems a good tool for you then

elizarov · Answer

You can easily write this `ifError` function as an extension

elizarov · Answer

``` fun lt T gt Deferred lt T gt ifError defaultValue gt T Deferred lt T gt = GlobalScope async try await catch e Throwable defaultValue ```

elizarov · Answer

But I highly recommend to reconsider the whole design Writing functions that return `Deferred` is generally a bad idea from the design standpoint You should design your API using suspending functions so that you fetching function looks like `suspend fun fetch T` for example

lukaswelte · Answer

Thank you both I ll rethink and prepare a better example although example does not use `Deferred lt T gt ` but `T` as response

elizarov · Answer

In this case Kotlin 1 3 already has the extensions you need

elizarov · Answer

Write ``` runCatching startFetching getOrElse emptyList ```

elizarov · Answer

But this is not a good design either since it ignores all exceptions So if you have a logic problem somewhere inside your `startFetching` code you ll never know about that

lukaswelte · Answer

`runCatching` is nice Thank you for your thoughts I ll create my own little function which will catch the exception report log it and then return the fallback

kingsley · Answer

Perhaps `supervisorScope` or `SupervisorJob` might also be something to consider for this use case