I do have a problem initializing a mutablestate fo...

# compose

[Timo Drick]

I do have a problem initializing a mutablestate for compose.
The following code will show in the composable the text "Init" instead of "Loaded"

class TestViewModel(ctx: Application): AndroidViewModel(ctx) {
    var value by mutableStateOf("Start")

    init {
        value = "Init"
        viewModelScope.launch(Dispatchers.IO) {
            value = "Loaded"
        }
    }
}
@Composable
fun Test(vm: TestViewModel = viewModel()) {
    Text(vm.value)
}

When i change the init to:

init {
    value = "Initialized"
    viewModelScope.launch(Dispatchers.IO) {
        withContext(Dispatchers.Main) {
            value = "Loaded"
        }
    }
}

It shows "Loaded"
Also a delay would make the code work.
Normally i think changing a compose state variable it is not necessary to be on the UI Thread

[Zach Klippenstein (he/him) [MOD]]

This is probably because the coroutine runs before the snapshot that creates the value state object has been applied so as far as that coroutine is concerned the state doesn't exist yet. There are two things that would likely work here.

Probably the better option, since it's good practice for other reasons, is to perform state mutations on a background thread in an explicit snapshot:

value = "Init"
viewModelScope.launch(<http://Dispatchers.IO|Dispatchers.IO>) {
  Snapshot.withMutableSnapshot {
    value = "Loaded"
  }
}

This is nice since it also handles updating multiple states from that coroutine and ensures the updates are all published together at the same time.

Another option is to explicitly tell the snapshot system after your states are initialized:

value = "Init"
Snapshot.notifyObjectsInitialized()
viewModelScope.launch(<http://Dispatchers.IO|Dispatchers.IO>) {
  value = "Loaded"
}

This doesn't handle updating multiple states from a background thread though.

[Timo Drick]

Hi Zach, thank you very much to take time for this answer.
Unfortunately both solutions are not working in my minimal example.
I like to use mutable states in ViewModels because it is much less boilerplate code than using Flows.
And i use it a lot. I only see this problems in the init function. It is really annoying.

[Timo Drick]

Could it be a bug?

[Timo Drick]

In the compose runtime

[Timo Drick]

Btw. as soon as i put a delay(10) into the code it works. In my production code i do read a file from disk. But it looks like it is so fast that i get this race condition with the states.

[Albert Chang]

When you create a mutableStateOf in composition but try to update it in background thread before the composition is completed, the update won't succeed. This is kind of expected due to how the snapshot system works.
This used to result in an exception in older compose versions: https://kotlinlang.slack.com/archives/CJLTWPH7S/p1652084646064769
Now there won't be a crash but as I said the update won't work.

[Zach Klippenstein (he/him) [MOD]]

Oh wait during composition, then yea the composition has its own snapshot so you can’t see anything created in there until composition applies it.