Parent View Model with Contract Interface

Framework.kt

import androidx.annotation.MainThread
import androidx.fragment.app.Fragment
import kotlin.reflect.KClass

interface ViewModelContractProvider {
    fun <T : Any> provideViewModelContract(clazz: KClass<T>): T
    val supportedContracts: Set<KClass<*>>
}

/**
 * Recursively searches parent fragments for it's ViewModelContractProvider
 * If no ViewModelContractProvider is found on parent fragments it will retrieve it from the Activity
 * If there is no ViewModelContractProvider in the scope of this fragment an IllegalArgumentException will be thrown
 */
fun <T : Any> Fragment.findViewModelByContract(clazz: KClass<T>): T {
    if (supportsViewModelContract(clazz)) {
        return (this as ViewModelContractProvider).provideViewModelContract<T>(clazz)
    }
    // Note the version of fragment that arch components is using does not have the method
    // requireParentFragment() so we store into a val
    val parent = parentFragment
    if (parent != null) {
        return parent.findViewModelByContract<T>(clazz)
    }
    if (activity?.supportsViewModelContract(clazz) == true) {
        return (requireActivity() as ViewModelContractProvider).provideViewModelContract<T>(clazz)
    }
    throw IllegalArgumentException("There is no ViewModelContractProvider associated with $this")
}

@MainThread
inline fun <reified T : Any> Fragment.viewModelContract(): Lazy<T> {
    return lazy(LazyThreadSafetyMode.NONE) {
        findViewModelByContract<T>(T::class)
    }
}

private fun <T : Any> Any.supportsViewModelContract(clazz: KClass<T>): Boolean {
    if (this is ViewModelContractProvider) {
        return supportedContracts.contains(clazz)
    }
    return false
}

Implementation.kt

class SomeActivity : AppCompatActivity(): ViewModelContractProvider {

    override fun <T: Any> provideViewModelContract(): T {
        @Suppress("UNCHECKED_CAST") val nullableVm = viewModel as? T
        checkNotNull(nullableVm) {
            "$viewModel does not implement required Contract"
        }
        return nullableVm
    }
    
    override val supportedContracts: Set<KClass<*>> = setOf(SomeViewModelContract::class)
  
}

class SomeActivityViewModel : ViewModel(): SomeViewModelContract {}

class SomeFragmentViewModel : ViewModel() {}

interface SomeFragmentViewModelContract {
}

class SomeFragment : Fragment() {
  
  val myViewModel by viewModels<SomeFragmentViewModel>()
  val contractViewModel by viewModelContract<SomeFragmentViewModelContract>()
  
}