If I do something like the below, am I right in thinking I cannot use preview it, because `ViewModel...

# compose

[adjpd]

If I do something like the below, am I right in thinking I cannot use preview it, because `ViewModel`s aren't supported in @Preview?

@Composable
private fun TodoActivityScreen(todoViewModel: TodoViewModel) {
  ...
}

[Ian Lake]

Your preview code can certainly construct an object to pass to your Composable, ViewModels aren't any different from any other object in that regard

[Ian Lake]

Now if your ViewModel is constructed via Hilt/something else, you'd need to figure out how to construct all of those objects and on and on up the tree, which is where passing in just the specific state you need or an interface you can more easily mock that your ViewModel implements are all valid alternatives

[adjpd]

java.lang.IllegalStateException: ViewModels creation is not supported in Preview

[adjpd]

I get that when I try to do:

todoViewModel = object : TodoViewModel() { }

[Ian Lake]

That looks like the error when you call viewModel(). You sure you aren't calling that?

[adjpd]

ViewModel() as in the constructor? A capital V?

[adjpd]

Because I'm definitely calling the ViewModel constructor. My ViewModel extends that, after all.

[Ian Lake]

No, the composable method that constructs ViewModels and hooks it into the actual ViewModelStoreOwner
https://developer.android.com/jetpack/compose/libraries#viewmodel

[Ian Lake]

You can't call that or hiltViewModel() in something you want to Preview

[adjpd]

No. I'm not calling that. I'll try to make a more concrete and contained example and post it. Give me a couple of minutes.

[adjpd]

My mistake. It's nothing to do with /my/ view model. But there's a problem with NavHost and @Preview. It gives the above error about view models.

@Composable
fun ExampleComp() {
  val controller = rememberNavController()
  NavHost(controller, "main") {
    composable("main") {
      Text("hi")
    }
  }
}
@Preview
@Composable
fun PreviewExampleComp() {
  ExampleComp()
}

[Ian Lake]

Yes, NavHost uses viewModel() internally, that's expected.

[Ian Lake]

Which is why the testing guide focuses on not using a `NavHost`/`NavController` in the components you want to preview at all: https://developer.android.com/jetpack/compose/navigation#testing

[adjpd]

Ah, okay. I guess this error will crop up a lot. A warning when introducing Preview may help others to know this sooner. Thanks for you help, anyhow Ian.

[Ian Lake]

Can you see the full stack trace? I would have expected that it would have included NavHost in it