hello, i need some help with setting the `main th...

# mvikotlin

[trashcoder]

hello,

i need some help with setting the main thread for jvm and dispaching from it.

(detailed description in slack thread)

[trashcoder]

i have working multiplatform implementations with mvikotlin and decompse for android and jvm (windows).

i used to have some "not on main thread" warnings on android which i could fix with observing (and dispatching) on mainscheduler. but i never got any warnings for jvm.

now i noticed this message in the jvm log for the first time:

[MVIKotlin]: Main thread id is undefined, main thread assert is disabled

when i set the main thread in the jvm main function using setMainThreadId(Thread.currentThread().id) it breaks my implementation (which works fine on android).

i cannot dispatch from within scope.launch{} anymore. but disatching before launching works.

works without setting the main thread on jvm and `android`:

private inner class ExecutorImpl : CoroutineExecutor<Intent, Unit, State, Msg, Nothing>() {
    override fun executeAction(action: Unit, getState: () -> State) {
        scope.launch {
            dispatch(Msg.Booting)

works on jvm when i set the main thread:

private inner class ExecutorImpl : CoroutineExecutor<Intent, Unit, State, Msg, Nothing>() {
    override fun executeAction(action: Unit, getState: () -> State) {
        dispatch(Msg.Booting)
        scope.launch {

[trashcoder]

i do not change the scope's context btw. it is still the default (main).

[Arkadii Ivanov]

Hello. What kind of main dispatcher you are using on JVM? Is it Swing?

[trashcoder]

Erm... I don't know 🙈 I use Jetbrains Compose for UI... How can I determine my main dispatcher?

[Arkadii Ivanov]

In this case it should be Swing - https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-swing/

How do you set the main thread id on JVM? Where do you call the setMainThreadId method?

[trashcoder]

i set it directly at the beginning of the main function:

fun main() {
    setMainThreadId(Thread.currentThread().id)

    val lifecycle = LifecycleRegistry()
    val root = root(DefaultComponentContext(lifecycle = lifecycle))

    application {
        val windowState = rememberWindowState()
        val icon = painterResource("ic_launcher-desktop.png")

        LifecycleController(lifecycle, windowState)

        Tray(
            icon = icon,
            menu = {
                Item("Beenden", onClick = ::exitApplication)
            }
        )

        Window(
            onCloseRequest = ::exitApplication,
            state = windowState,
            title = "Trashboard",
            icon = icon
        ) {
            App(root)
        }
    }
}

[Arkadii Ivanov]

Right, this is a mistake. The main function runs not on the UI thread. You can try the following code:

SwingUtilities.invokeAndWait {
  setMainThreadId(Thread.currentThread().id)
}

[trashcoder]

https://github.com/arkivanov/MVIKotlin/blob/ae716edd787ce657b84c8cb5987d6bb3e77ff8[…]otlin/com/arkivanov/mvikotlin/timetravel/client/desktop/Main.kt

i also have to create my components in that block, i guess?

[Arkadii Ivanov]

I would say yes, if you want the check to be enabled. Otherwise it may crash. You can use that code as an example. Let me know, if there are any issues!

[trashcoder]

this works for me:

fun main() {
    val lifecycle = LifecycleRegistry()

    lateinit var root: Root
    SwingUtilities.invokeAndWait {
        setMainThreadId(Thread.currentThread().id)

        root = root(DefaultComponentContext(lifecycle = lifecycle))
    }

    application {
        val windowState = rememberWindowState()
        val icon = painterResource("ic_launcher-desktop.png")

        LifecycleController(lifecycle, windowState)

        Tray(
            icon = icon,
            menu = {
                Item("Beenden", onClick = ::exitApplication)
            }
        )

        Window(
            onCloseRequest = ::exitApplication,
            state = windowState,
            title = "Trashboard",
            icon = icon
        ) {
            App(root)
        }
    }
}

thanks a lot! 🙂

[trashcoder]

hello again,

i ran into 2 follow up issues after setting the main thread for jvm.

1. [FYI/SOLVED] i observe some server sent and push events with observables. i used to observeOn(mainScheduler) and then call my callback/event in subcribe which is implemented in the store, so the store can dispatch the new state directly. this does not work anymore because the (swing awt) main thread is not the same as the mainScheduler. i solved this by removing observeOn(mainScheduler) and wrapping the dispatch in the store with scope.launch{}.
before:

observable<String> { emitter ->
    ...
}.observeOn(mainScheduler).subscribe {
    onServerSentEvent(it)
}

onServerSentEvent = sse -> {
    dispatch(Msg.SSE(sse))
}

after:

observable<String> { emitter ->
    ...
}.subscribe {
    onServerSentEvent(it)
}

onServerSentEvent = sse -> {
    scope.launch {
        dispatch(Msg.SSE(sse))
    }
}

2. I use an mqtt client to subscribe to a server (my roomba). the client has callbacks for listening to subscription messages. however, when i set the main thread, i can still connect successfully and subscribe but i never receive a subscription message in the subscription listener. so something seems to be blocking this...?
i tried suspending at several locations with several contexts/dispatchers but it never worked.
if i remove setMainThreadId, i receive the messages again.

any idea/hint what i could do about it? (other than not setting the main thread of course 😉)

[trashcoder]

btw: both problems only occured on jvm after setting the main thread. android still works fine.

[Arkadii Ivanov]

Re issue 1: you should definitely use observeOn(mainScheduler), and it should switch to the right thread. Most likely your mainScheduler implementation uses another thread for dispatching. If you are using the default one, then it's just a random single background thread. You can either implement the mainScheduler using Swing UI thread, or just wrap the coroutines main dispatcher as I did here - https://github.com/arkivanov/MVIKotlin/blob/8d06462eff0225a99111633cc99f024f8de2b52f/mvikotlin-timetravel-client/app-desktop/src/jvmMain/kotlin/com/arkivanov/mvikotlin/timetravel/client/desktop/Main.kt#L38

[Arkadii Ivanov]

Re issue 2: unfortunately I'm not familiar with mqtt. This sounds like you are not receiving messages if you are subscribing on the UI thread. I would refer to a documentation for mqtt or something.

[Arkadii Ivanov]

Btw, you can omit observeOn(mainScheduler) if you are sure that the upstream already emits on the main thread. This will avoid unnecessary re-dispatches. But you definitely have to use mainScheduler for switching from background threads.

[trashcoder]

issue 1 is solved in a better way now, thanks! 🙂

[trashcoder]

FYI:
issue 2 is also a "not on main thread" error on dispatching the mqtt client's connection result. unfortunately, the client catches it (it happens in the clients callback) and so it was never seen in console... 🙈

[Arkadii Ivanov]

So, did you solve it?

[trashcoder]

yes. i don't use an obervable there, so i just wrapped it with `MainScope().launch{}`:

MainScope().launch {
    onConnectionResult(true)
}

MainScope().launch {
    onMessage(robotId, robotUpdateMessage)
}