Maybe this is a general Kotlin coroutine question....

# compose

[robnik]

Maybe this is a general Kotlin coroutine question. Is every code block passed to viewModelScope.launch { } run in the same thread? I'm wondering how to avoid loading data twice in my ViewModel. I could have a property there, isLoading , but I'm not sure how much I need to worry about thread safety.

[Francesc]

viewmodelScope.launch runs on the Main dispatcher, but the suspend block could be changing dispatchers and run the code on a different thread. You can't know that and should not care either.
launch returns a Job, you could hold a reference to that to know if you've already launched the coroutine

[robnik]

Ah, okay. Thank you. So if use the Job, that transfers my doubts to the scope that calls the ViewModel. Is the Composable fun always running in the same thread? So I can just check if (job != null) { ... } ?

[Francesc]

it would be better if you explained what you are trying to achieve as your question is too abstract and lacking of context to get a proper answer

[robnik]

I have a screen that shows a bunch of info about a car. It starts with a navigation arg, vehicleID: String. From there I need to load some data from a server. The ID and data are properties of the view model. The data is null, until it's loaded from the network. If I just do a simple if (data == null) load() , then load() gets called too many times.

[robnik]

Does that make sense?

[Francesc]

yes, this is what Effects are for. You can use a LaunchedEffect to trigger the load in your viewmodel. This will run once the composable enters the composition for the first time.
See https://developer.android.com/jetpack/compose/lifecycle for details

[Francesc]

if you use the navigation component you can also automatically get the arguments in your viewmodel by using SavedStateHandle

[robnik]

Thanks. I didn't know about SavedStateHandle either. I'll read this. https://developer.android.com/topic/libraries/architecture/viewmodel-savedstate

[Ian Lake]

If you only want to run it once, why not just run it in the init block?

class VehicleViewModel(savedStateHandle: SavedStateHandle) {
  init {
    // Use the same string as you have in your route argument i.e., vehicle/{vehicleID}
    val vehicleId = savedStateHandle.get("vehicleID")
    viewModelScope.launch {
      // Load your data
    }
  }
}

[robnik]

I was creating the ViewModel above my NavHost's composable(route) { } and setting the vehicleID from the nav arg, inside the braces. Now I see that if I call viewModel() inside the braces, I get the nav args in the state as you show. But... I want to navigate to another page and keep this ViewModel. The first page is like "vehicle details" and the next page is drilling down further into more details.

[Ian Lake]

It sounds like what you actually want is a repository that two completely separate ViewModels talk to

[Ian Lake]

That translates IDs -> items

[Ian Lake]

Preferably in a Flow based observable pattern and that reads from a source that properly handles process death and recreation

[robnik]

Okay, I guess I'm trying to put too much "model" in my ViewModel. Does the navigation component have any sense of one destination being "inside" another? Maybe if I have route paths like "foo/vehicleDetails?id={id}" and "foo/vehicleDetails/more?id={id}" ? So that it wouldn't kill and recreate the ViewModel object when navigating from "vehicleDetails" to "vehicleDetails/more"?

[robnik]

I guess I should think of ViewModels as lightweight adapters between my composables and the real model/repository.

[Ian Lake]

Correct, a ViewModel is really just a layer between your domain objects+repository and UI that transforms those domain objects into the minimal set of information needed to fill in your UI

[Ian Lake]

(the most simple translation being an identity translation - just passing it through the layer)