Multiplatform view model with a backing android VM

gistfile1.txt

class MyMultiPlatformViewModel @Inject constructor(
    private val scope: CoroutineScope,
    private val stateHelper: SavedStateHelper,
) : CoroutineScope by scope, AutoCloseable {
    private var screenId: String by stateHelper.savedState("screenId", default = "")

    fun doAThing() {}

    override fun close() {
        // clean up resources
    }
}
private class PersistentStateDelegate<T: Any>(
    private val holder: SavedStateHandle,
    private val key: String,
    private val default: T
) : ReadWriteProperty<Any, T> {

    override fun getValue(thisRef: Any, property: KProperty<*>): T {
        return holder.get<T>(getKey(thisRef))
            ?: default.also { setValue(thisRef, property, it) }
    }

    override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
        holder.set(getKey(thisRef), value)
    }

    private fun getKey(thisRef: Any) = "${thisRef.javaClass.name}__$key"
}
class BackingAndroidVM(
    private val stateHandle: SavedStateHandle
) : ViewModel(), SavedStateHelper {

    private val persisted = mutableMapOf<Class<*>, Any>()

    fun <T: Any> getOrCreate(clazz: Class<T>, create: (CoroutineScope, SavedStateHelper) -> T) =
        persisted.getOrPut(clazz) { create.invoke(viewModelScope, this) } as T

    override fun <T: Any, VM : Any> savedState(key: String, default: T): ReadWriteProperty<VM, T> {
        return PersistentStateDelegate(stateHandle, key, default)
    }

    override fun onCleared() {
        super.onCleared()
        persisted.keys.filterIsInstance<AutoCloseable>().forEach { it.close() }
    }
}

inline fun <reified T : Any> Fragment.getOrCreatePersisted(noinline create: (CoroutineScope, SavedStateHelper) -> T): T {
    val persistentStorageVM = ViewModelProvider(this, SavedStateViewModelFactory(requireActivity().application, this)).get<BackingAndroidVM>()
    return persistentStorageVM.getOrCreate(T::class.java, create)
}

inline fun <reified T : Any> ComponentActivity.getOrCreatePersisted(noinline create: (CoroutineScope, SavedStateHelper) -> T): T {
    val persistentStorageVM = ViewModelProvider(this, SavedStateViewModelFactory(application, this)).get<BackingAndroidVM>()
    return persistentStorageVM.getOrCreate(T::class.java, create)
}

interface SavedStateHelper {
    fun <T: Any, VM : Any> savedState(key: String, default: T): ReadWriteProperty<VM, T>
}