Compose creating new ViewModel instance

Question

I work in a project that's slowly adopting Jetpack Compose. It's mostly a single Activity multiple Fragments app where we use the Android's navigation component to handle transitions to each screen (Fragment). Whenever we can, we are only replacing the Fragments' XML layout with Composables.

The cases up until now were handling like the following:

A. Fragments show composables and handle navigation:

class ScreenFragment : Fragment() {

    // For observing events that trigger navigation
    private val viewModel by lazyViewModel { ScreenViewModel() }

    override fun onCreateView( ... ): View {
        return ComposeView(requireContext()).apply {

            setViewCompositionStrategy(DisposeOnLifecycleDestroyed(viewLifecycleOwner))

            setContent {
                AppTheme {
                    Screen(onBackPressed = { findNavController().navigateUp() })
                }
            }
        }
    }    
    ...

}

B. Composable that handles everything else UI related:

@Composable
fun CreatePassword(
    onBackPressed: () -> Unit,
) {

    // For observing UI states and events
    val viewModel: CreatePasswordViewModel = viewModel()
    ...
}

As you can see, we have a reference for the screen's ViewModel in both our Fragments and our Composables. Up until now, this has been working fine, the composable viewModel() function was always returning the same existing instance of the Fragment's ViewModel.

The problems came when we needed the reference of a ViewModel scoped to an Activity on the composable:

We create the ViewModel on the Activity:

class MainActivity : AppCompatActivity() {

    private val viewModel by lazyViewModel { MainViewModel() }
    ...
}

Get a reference for the MainActivity's ViewModel on the Fragment like

class MainFragment : Fragment() {

    private val viewModel: MainViewModel by viewModels(::requireActivity)
    ...
}

We get the reference of the ViewModel for composables as shown above (item B)

By doing so the Fragment has the same instance of the Activity's but the composable doesn't.

My questions is, would it be possible to get the reference of the Activity's ViewModel inside the composable? For now I'm simply passing down the Fragment's ViewModel as a parameter to my main composable screen.

`viewModel()` takes a `ViewModelStoreOwner` as a parameter that controls the scope of that ViewModel, just like your `lazyViewModel`. If you want to scope a ViewModel to your activity, why aren't you passing in your activity to `viewModel()`? — ianhanniballake, Feb 11 '22 at 18:16
why you don't pass the viewmodel as an argument to CreatePassword composble ? — Mobin Yardim, Feb 11 '22 at 18:37
@ianhanniballake, I tried passing my `Activity` to `viewModel()` but I ended up getting a new instance of the viewModel too. I was getting my `Activity` as done here: https://stackoverflow.com/a/68423182/8189765 — Arthur Bertemes, Feb 11 '22 at 20:53

score 4 · Accepted Answer · answered Feb 11 '22 at 20:51

I managed to pass down the Activity's ViewModel by providing a ViewModelStoreOwner to my composable as following:

class ScreenFragment : Fragment() {

    // For observing events that trigger navigation
    private val viewModel by lazyViewModel { ScreenViewModel() }

    override fun onCreateView( ... ): View {
        return ComposeView(requireContext()).apply {

            setViewCompositionStrategy(DisposeOnLifecycleDestroyed(viewLifecycleOwner))

            setContent {
                val viewModelStoreOwner =
                    compositionLocalOf<ViewModelStoreOwner> { requireActivity() }
    
                AppTheme {
                    CompositionLocalProvider(
                        LocalViewModelStoreOwner provides viewModelStoreOwner.current
                    ) {
                        Screen(onBackPressed = { findNavController().navigateUp() })
                    }
                }
            }
        }
    }    
    ...

}

42Geek · Answer 2 · 2022-02-12T10:14:39.747

Sounds like a classic dependency problem, at this point I would consider using some sort of Dependency Injection library and make sure to provide the viewModel instance as a singleton.

I can recommend Koin: https://insert-koin.io/

Or, more recently and trendy Hilt: https://developer.android.com/training/dependency-injection/hilt-android

Once using a good DI you can get an instance of the viewModel in your compose functions.

Compose creating new ViewModel instance

2 Answers2