Every LifecycleOwner (i.e. a Fragment or an Activity) keeps its models in a ViewModelStore that has a clear() function. However, the clearing sweeps all the models from the ViewModelStore which in your case is undesirable (Both ViewModel AB and ViewModel CDE would be cleared form the Activity's ViewModelStore). One possible solution to this issue is having a per-ViewModel stores that can be safely cleared when necessary:
class MainActivity : AppCompatActivity() {
val individualModelStores = HashMap<KClass<out ViewModel>, ViewModelStore>()
inline fun <reified VIEWMODEL : ViewModel> getSharedViewModel(): VIEWMODEL {
val factory = object : ViewModelProvider.Factory {
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
//Put your existing ViewModel instantiation code here,
//e.g., dependency injection or a factory you're using
//For the simplicity of example let's assume
//that your ViewModel doesn't take any arguments
return modelClass.newInstance()
}
}
val viewModelStore = [email protected]<VIEWMODEL>()
return ViewModelProvider(this.getIndividualViewModelStore<VIEWMODEL>(), factory).get(VIEWMODEL::class.java)
}
val viewModelStore = [email protected]<VIEWMODEL>()
return ViewModelProvider(this.getIndividualViewModelStore<VIEWMODEL>(), factory).get(VIEWMODEL::class.java)
}
inline fun <reified VIEWMODEL : ViewModel> getIndividualViewModelStore(): ViewModelStore {
val viewModelKey = VIEWMODEL::class
var viewModelStore = individualModelStores[viewModelKey]
return if (viewModelStore != null) {
viewModelStore
} else {
viewModelStore = ViewModelStore()
individualModelStores[viewModelKey] = viewModelStore
return viewModelStore
}
}
inline fun <reified VIEWMODEL : ViewModel> clearIndividualViewModelStore() {
val viewModelKey = VIEWMODEL::class
individualModelStores[viewModelKey]?.clear()
individualModelStores.remove(viewModelKey)
}
}
Use getSharedViewModel() to obtain an instance of ViewModel which is bound to the Activity's lifecycle:
val viewModelCDE : ViewModelCDE = (requireActivity() as MainActivity).getSharedViewModel(/*There could be some arguments in case of a more complex ViewModelProvider.Factory implementation*/)
Later, when it's the time to dispose the shared ViewModel, use clearIndividualViewModelStore<>():
(requireActivity() as MainActivity).clearIndividualViewModelStore<ViewModelCDE>()
In some cases you would want to clear the ViewModel as soon as possible if it's not needed anymore (e.g., in case of it containing some sensitive user data like username or password). Here's a way of logging the state of individualModelStores upon every fragment switching to help you keep track of shared ViewModels:
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
if (BuildConfig.DEBUG) {
navController.addOnDestinationChangedListener { _, _, _ ->
if (individualModelStores.isNotEmpty()) {
val tag = [email protected]
Log.w(
tag,
"Don't forget to clear the shared ViewModelStores if they are not needed anymore."
)
Log.w(
tag,
"Currently there are ${individualModelStores.keys.size} ViewModelStores bound to ${[email protected]}:"
)
for ((index, viewModelClass) in individualModelStores.keys.withIndex()) {
Log.w(
tag,
"${index + 1}) $viewModelClass\n"
)
}
}
}
}
}
