如何在 Kotlin 中使用带 Livedata 的 MVVM 模式从子片段访问父片段数据

Question

I have a list Fragment where recyclerview is used to displayed the list when the use presses on the list item user goes to new fragment(parent fragment) where i have setup a tablayout with 2 fragments(detail fragment and second fragment) using viewpager2. Here the data from list item is passed to parent fragment using Bundle. i need to get access this data which is available in parent fragment and use it to display data in child fragment(detail fragment)

i am using hilt as dependency injection.

list fragment code to send data

 private val mylistsViewModel by viewModels<MyListsViewModel>()
    private var mylists: MyListsResponse? = null
.......
`    private fun onItemClicked(mylistsResponse: MyListsResponse) {
        val bundle = Bundle()
        bundle.putString("mylist",Gson().toJson(mylistsResponse))
        findNavController().navigate(R.id.action_mylistsFragment_to_parentFragment,bundle)
    }`

accessing it in parent fragment

 private val mylistsViewModel by viewModels<MyListsViewModel>()
    private var mylists: MyListsResponse? = null
.......

        val jsonMyLists = arguments?.getString("mylist")
        if (jsonMyLists != null) {
            mylists = Gson().fromJson<MyListsResponse>(jsonMyLists, MyListsResponse::class.java)
            mylists?.let {


            }
        }

UI Flow

access data from parent to child fragment

Can any one kindly suggest on how to resolve this issue.

i refered this googleDocs

and tried to access the data in details fragment by refering to this code

class ListFragment: Fragment() {
    // Using the viewModels() Kotlin property delegate from the fragment-ktx
    // artifact to retrieve the ViewModel
    private val viewModel: ListViewModel by viewModels()
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        viewModel.filteredList.observe(viewLifecycleOwner, Observer { list ->
            // Update the list UI
        }
    }
}

class ChildFragment: Fragment() {
    // Using the viewModels() Kotlin property delegate from the fragment-ktx
    // artifact to retrieve the ViewModel using the parent fragment's scope
    private val viewModel: ListViewModel by viewModels({requireParentFragment()})
    ...
}

But when i go to detail fragment the app is crashing and giving error as

"Fragment DetailsFragment is not a child Fragment, it is directly attached to dagger.hilt.android.internal.managers"

Answer 1

The problem is when you are using the Navigation component to handle navigation in your application requireParentFragment will return an instance of the NavHostFragment which is not ListFragment as you expected. Fortunately, there is a way to instantiate a view model using the navigation component backStackEntry and Hilt as follows:

@AndroidEntryPoint
class ListFragment: Fragment() {
    // Using the viewModels() Kotlin property delegate from the fragment-ktx
    // artifact to retrieve the ViewModel
    private val viewModel: ListViewModel by lazy {
        val destination = findNavController().currentBackStackEntry!!.destination

        val backStackEntry = findNavController().getBackStackEntry(destination.id)

        val storeProducer: () -> ViewModelStore = {
            backStackEntry.viewModelStore
        }

        createViewModelLazy(
            ListViewModel::class, storeProducer,
            factoryProducer = {
                HiltViewModelFactory(requireActivity(), backStackEntry)
            }
        ).value
    }
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        viewModel.filteredList.observe(viewLifecycleOwner, Observer { list ->
            // Update the list UI
        }
    }
}

@AndroidEntryPoint
class ChildFragment: Fragment() {
    // Using the viewModels() Kotlin property delegate from the fragment-ktx
    // artifact to retrieve the ViewModel using the parent fragment's scope
    private val viewModel: ListViewModel by lazy {
        val destination = findNavController().previousBackStackEntry!!.destination

        val backStackEntry = findNavController().getBackStackEntry(destination.id)

        val storeProducer: () -> ViewModelStore = {
            backStackEntry.viewModelStore
        }

        createViewModelLazy(
            ListViewModel::class, storeProducer,
            factoryProducer = {
                HiltViewModelFactory(requireActivity(), backStackEntry)
            }
        ).value
    }
    ...
}

Also, don't forget to annotate the ListViewModel with @HiltViewModel .

This way of implementation can work with any fragment and the previous one as you may notice in the ChildFragment we are using previousBackStackEntry instead of currentBackStackEntry in the ListFragment