Desktop: Updating composable lambda from compose scope throw CompletionHandlerException

[G0xilla]

Describe the bug When I update composable lambda from compose scope I get CompletionHandlerException

Affected platforms Select one of the platforms below:

• Desktop

Versions

• Kotlin version*: 1.9.10
• Compose Multiplatform version*: 1.5.1
• OS version(s)* (required for Desktop and iOS issues): Linux 6.1.53
• OS architecture (x86 or arm64): arm64
• JDK (for desktop issues): OpenJDK 18.0.2

To Reproduce Steps and/or the code snippet to reproduce the behavior:

fun main() {
    val content = MutableStateFlow<(@Composable () -> Unit)?>(null)

    fun updateContent() {
        // Simulate some time to update content
        Thread.sleep(1000)
        content.value = {
            Box(
                contentAlignment = Alignment.Center,
                modifier = Modifier.fillMaxSize()
            ) {
                Text("Second Screen")
            }
        }
    }

    // init content
    content.value = {
        val scope = rememberCoroutineScope()

        Box(
            contentAlignment = Alignment.Center,
            modifier = Modifier.fillMaxSize()
        ) {
            Column(
                horizontalAlignment = Alignment.CenterHorizontally
            ) {
                Text("First Screen")
                Button(
                    onClick = {
                        scope.launch { updateContent() }
                    }
                ) {
                    Text("Update Content")
                }
            }
        }
    }

    application {
        Window(onCloseRequest = ::exitApplication) {
            val contentState = content.collectAsState()
            contentState.value!!.invoke()
        }
    }
}

To reproduce behavior click Upadate Content several times in a row.

Expected behavior Normally open Second Screen without exception.

Screenshots

https://github.com/JetBrains/compose-multiplatform/assets/62667055/a9c6ebb4-140f-48ea-8cbf-57b797b0168f

[dima-avdeev-jb]

Also reproduces on MacOS: https://github.com/dima-avdeev-jb/issue-3715-exception-desktop and JDK 17

[eymar]

rememberCoroutineScope is tied to the particular point of composition.

Return a CoroutineScope bound to this point in the composition using the optional CoroutineContext provided by getContext. getContext will only be called once and the same CoroutineScope instance will be returned across recompositions. This scope will be cancelled when this call leaves the composition.

This scope will be cancelled

it can't be cancelled in your case because you use blocking Thread.sleep():

Running updateContent in scope.launch { updateContent() } still blocks the UI because rememberCoroutineScope will by default use the dispatcher of Recomposer.applier.

if you want to run a heavy task in onClick callback, you can use a different dispatcher:

val scope = rememberCoroutineScope {
            Dispatchers.IO // just an example
        }

or if you want to have a simple delay:

• make updateContent a suspend fun
• replace Thread.sleep(...) with delay(...)

suspend fun updateContent() {
        // Simulate some time to update content
       delay(1000)
       ....
}

making any of these changes makes your code snippet work. (Prefer using suspend function if possible)

__

please let us know if this helps and if you have any more questions.

[G0xilla]

Yes, I should not do heavy task in Recomposer.applier, but if i do it should not end up with Exception. If you run same project on android, only UI will block and exception will not throw. Here is a example project for android. This issue can be duplicate of other issue.

[eymar]

@Gorry077 thank you!

[okushnikov]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.