Develop Reference

Why google-iosched can save fragment state while navigating with navigation component

Overall look at the code, I don't understand why it can be done.
https://github.com/google/iosched
from apps/iosched/ui/MainActivity.kt, It initialized NavController and NavHostFragment, but seems that there is no special treatment.
https://github.com/google/iosched/blob/main/mobile/src/main/java/com/google/samples/apps/iosched/ui/MainActivity.kt
val appBarConfiguration = AppBarConfiguration(TOP_LEVEL_DESTINATIONS)
private val TOP_LEVEL_DESTINATIONS = setOf(
R.id.navigation_feed,
R.id.navigation_schedule,
R.id.navigation_map,
R.id.navigation_info,
// R.id.navigation_agenda, comment will not stop saving the statement.
R.id.navigation_codelabs,
R.id.navigation_settings
)
from apps/iosched/ui/AgendaFragment.kt: the most simplest fragment, BindingAdapter method will always init AgendaAdapter(), but It can save the position of RecyclerView after init.
https://github.com/google/iosched/blob/main/mobile/src/main/java/com/google/samples/apps/iosched/ui/agenda/AgendaFragment.kt
Why can it save the State of each fragment?

IOSched depends on Navigation 2.4.1. As per the release notes of Navigation 2.4.0:
The NavController ... has been expanded to support saving and restoring the back stack.
As part of this change, the NavigationUI methods of onNavDestinationSelected(), BottomNavigationView.setupWithNavController() and NavigationView.setupWithNavController() now automatically save and restore the state of popped destinations, enabling support for multiple back stacks without any code changes. When using Navigation with Fragments, this is the recommended way to integrate with multiple back stacks.
And IOSched uses setupWithNavController, which means each tab is automatically going to save and restore its state correctly.
That includes the state of a RecyclerView, which has always supported saving and restoring its position automatically.

RecyclerView with StaggeredGridLayoutManager in ViewPager, arranges items automatically when going back to fragment

I am using Navigation component in my App, using google Advanced Sample(here).
my problem is when going back to a fragment, the scrolling position does not lost but it rearranges items and moves highest visible items so that top of those item align to top of recyclerview. please see this:
before going to next fragment:
and after back to fragment:
this problem is matter because some times clicked item goes down and not seen until scroll down.
how to prevent this behavior?
please consider:
this problem exist if using navigation component to change fragment. if start fragment using supportFragmentManager.beginTransaction() or start another activity and then go to this fragment it is OK. but if I navigate to another fragment using navigation component this problem is exist.(maybe because of recreating fragment)
also this problem exist if using fragment in ViewPager. i.e recyclerView is in a fragment that handle with ViewPagerAdapter and viewPager is in HomeFragment that opened with Navigation component. if recyclerView is in HomeFragment there is no problem.
no problem with LinearLayoutManager. only with StaggeredGridLayoutManager.
there is not difference if using ViewPager2 and also FragmentStatePagerAdapter
I try to prevent recreate of fragment(by this solution) but not solved.
UPDATE:
you can clone project with this problem from here

When using Navigation Component + ViewPager + StaggeredGridLayoutManager, wrong recyclerView.computeVerticalScrollOffset() has been returned during Fragment recreate.
In general, all layout managers bundled in the support library already know how to save and restore scroll position, but in this case, we had to take responsibility for this.
class TestFragment : Fragment(R.layout.fragment_test) {
private val testListAdapter: TestListAdapter by lazy {
TestListAdapter()
}
private var layoutManagerState: Parcelable? = null
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
postListView.apply {
layoutManager = StaggeredGridLayoutManager(
2, StaggeredGridLayoutManager.VERTICAL
).apply {
gapStrategy = StaggeredGridLayoutManager.GAP_HANDLING_MOVE_ITEMS_BETWEEN_SPANS
}
setHasFixedSize(true)
adapter = testListAdapter
}
testListAdapter.stateRestorationPolicy = RecyclerView.Adapter.StateRestorationPolicy.PREVENT
}
override fun onPause() {
saveLayoutManagerState()
super.onPause()
}
override fun onViewStateRestored(savedInstanceState: Bundle?) {
super.onViewStateRestored(savedInstanceState)
restoreLayoutManagerState()
}
private fun restoreLayoutManagerState () {
layoutManagerState?.let { postListView.layoutManager?.onRestoreInstanceState(it) }
}
private fun saveLayoutManagerState () {
layoutManagerState = postListView.layoutManager?.onSaveInstanceState()
}
}
Source code: https://github.com/dautovicharis/MyStaggeredListSample/tree/q_65539771

The Navigation Component behavior is normal when you navigate from one fragment to another. I mean, onDestroyView() method from the previous fragment is executed, so it means that your view is destroyed, but not the fragment. Remembering that fragment has two lifecycles one for the fragment and another one for the view, There was a video about it.
Also, there were issues registered in issue tracker in order to avoid this behavior in some cases and the GitHub issues:
https://issuetracker.google.com/issues/127932815
https://github.com/android/architecture-components-samples/issues/530
The problem is that when you have fragment that is heavy to recreate, is easier to do not destroy it and just add one fragment. So, when you go back it is not recreated. But, for this behavior is not part of navigation component.
Solutions
The easiest solution is to not use navigation component and work with the tradicional way, as you can see this works perfectly in you use case.
You can use the traditional way just for this use case, and use the navigation component for other cases.
You can inflate this view in an activity. So you are adding un activity
But if the previous tree options is not possible. You can try the following:
If you are using viewModel, you can use SaveState. Basically, it can save the data from your fragment, it is like a map data structure, so you can save positions from your list or recycler view. When go back to this fragment, get the position from this saveState object and use scrollToPosition method in order to add the real position.
Recycler view have methods for restore positions. You can see the uses cases for that, because first you need the data and then add the real position, for more details you can visit this link. This configuration for recycler view is useful also when you lose memory and you need to recreate the recycler view with asynchronous data.
Finally, if you want to understand more about how fragment works with navigation component, you can see this link

Can we use Android Jetpack Navigation Component with Settings preferences?

Is it possible to use Android Jetpack Navigation Component to navigate the fragments of a Settings Preferences? I have a Settings screens with multiple nested Preference Fragments. I was wondering if it is feasible to use Android Jetpack Navigation Component to facilitate my task?

The google documentation states that hierarchical nesting of preferences are now deprecated in favour of inter-fragment navigation; that is the structure <PreferencesScreen> ... <PreferencesScreen ...> ... </PreferenceScreen> ... </PreferenceScreen> is now out.
The documents propose that one replace the nested <PreferenceScreen/> with a preference and set the app:fragment attribute, hence
<xml ...>
<PreferencesScreen ...>
<Preference
android:title="Title"
app:fragment="TLD.DOMAIN.PACKAGE.preferences.SUBPreferenceFragment"
app:destination="#+id/SUBPreferenceFragment" # Not supported by Schema
android:summary="Summary"/>
</PreferenceScreen>
Note : It'd be mighty handy if they allowed app:destination="#+id/SUBPreferenceFragment" to identify the navigation destination; I simply duplicate app:destination from the navigation graph's XML file.
From the documentation it is then possible to navigate between the PREFERENCEFramgents by referencing this preference.fragment as follows
class Activity : AppCompatActivity(),
// Preferences Interface
PreferenceFragmentCompat.OnPreferenceStartFragmentCallback
{
...
override fun onPreferenceStartFragment(caller: PreferenceFragmentCompat, preference: Preference): Boolean
{
// Instantiate the new Fragment
val args = preference.extras
val fragment = supportFragmentManager.fragmentFactory.instantiate(
classLoader,
preference.fragment)
fragment.arguments = args
fragment.setTargetFragment(caller, 0)
// Replace the existing Fragment with the new Fragment
supportFragmentManager.beginTransaction()
.replace(R.id.FRAGMENT, fragment)
.addToBackStack(null)
.commit()
return true
}
...
}
There is however a small Snafoo ! R.id.FRAGMENT in a normal activity represents the placeholder within the activities' view where one would swap the main fragments in an out of existence. When using Navigation R.id.FRAGMENT maps to R.id.NAVIGATION and the code above swaps out the entire navigation fragment with the preference. For an activity using Navigation a reformulation is necessary.
class Activity : AppCompatActivity(),
// Preferences Interface
PreferenceFragmentCompat.OnPreferenceStartFragmentCallback
{
...
override fun onPreferenceStartFragment(caller: PreferenceFragmentCompat, preference: Preference): Boolean {
// Retrieve the navigation controller
val navctrl = findNavController(R.id.navigation)
// Identify the Navigation Destination
val navDestination = navctrl.graph.find { target -> preference.fragment.endsWith(target.label?:"") }
// Navigate to the desired destination
navDestination?.let { target -> navctrl.navigate(target.id) }
}
...
}
While this works I am a bit dubious about the line that determines the navDestination. If you have a better incantation for val navDestination = navctrl.graph.find { target -> preference.fragment.endsWith(target.label?:"") } then please edit the question or leave a comment accordingly so I can update the answer.

Yes, you can use Navigation components inside Preference fragments.
I have also a main preference fragment with multiple nested fragments. For me the best way was to navigate just inside preference fragments without any activity. So I created onPreferenceClickListeners and navigate just like you would from a normal fragment.
findPreference(getString(R.string.PREF_GN_BUTTON_MAIN)).setOnPreferenceClickListener(preference5 -> {
Navigation.findNavController(requireView()).navigate(R.id.another_settings_fragment);
return true;
});
The advantage of this way is that you can use gradle plugin Safe args to navigate between destinations. So it is a little bit safer than to use it only with ids just like Carel from Activity.
I tried to override onPreferenceStartFragment in my activity but I could not see any advantages of it. I use single Activity architecture.

Android ViewModel Sharing Between Fragments [duplicate]

I'm using the navigation component, I want a view model to be shared between a few fragments but they should be cleared when I leave the fragments (hence not scoping them to the activity) I'm trying to take the one activity many fragments approach. I have managed to achieve this using multiple nav hosts and scoping the fragments to it using getParentFragment but this just leads to more issues having to wrap fragments in other parent fragments, losing the back button working seamlessly and other hacks to get something to work that should be quite simple. Does anyone have a good idea on how to achieve this? I wondered if theres anything with getViewModelStore I could be using, given the image below I want to scope a view model to createCardFragment2 and use it in anything after it (addPredictions, editImageFragment, and others i haven't added yet), but then if I navigate back to mainFragment I want to clear the view models.
BTW I cant just call clear on mainFragment view model store as there are other view models here that shouldn't be cleared, I guess i want a way to tell the nav host what the parent fragment should be which I'm aware isn't going to be a thing, or a way to make the view model new if I'm navigating from mainFragment or cardPreviewFragment

Here's a concrete example of Alex H's accepted answer.
In your build.gradle (app)
dependencies {
def nav_version = "2.1.0"
implementation "androidx.navigation:navigation-fragment-ktx:$nav_version"
}
Example of view model
class MyViewModel : ViewModel() {
val name: MutableLiveData<String> = MutableLiveData()
}
In your FirstFlowFragment.kt define
val myViewModel: MyViewModel by navGraphViewModels(R.id.your_nested_nav_id)
myViewModel.name.value = "Cool Name"
And in your SecondFlowFragment.kt define
val myViewModel: MyViewModel by navGraphViewModels(R.id.your_nested_nav_id)
val name = myViewModel.name.value.orEmpty()
Log.d("tag", "welcome $name!")
Now the ViewModel is scoped in this nested fragment, shared state will be destroyed when nested nav is destroyed as well, no need to manually reset them.

Yes, it's possible to scope a viewmodel to a navgraph now starting with androidx.navigation:*:2.1.0-alpha02. See the release notes here and an example of the API here. All you need to give is the R.id for your navgraph. I find it a bit annoying to use, though, because normally viewmodels are initialized in onCreate, which isn't possible with this scope because the nav controller isn't guaranteed to be set by your nav host fragment yet (I'm finding this is the case with configuration changes).
Also, if you don't want your mainFragment to be part of that scope, I would suggest taking it out and maybe using a nested nav graph.

so when i posted this the functionality was there but didn't quite work as expected, since then i now use this all the time and this question keeps getting more attention so thought i would post an up to date example,
using
//Navigation
implementation "androidx.navigation:navigation-fragment:2.2.0-rc04"
// Navigation UI
implementation "androidx.navigation:navigation-ui:2.2.0-rc04"
i get the view model store owner like this
private ViewModelStoreOwner getStoreOwner() {
NavController navController = Navigation
.findNavController(requireActivity(), R.id.root_navigator_fragment);
return navController.getViewModelStoreOwner(R.id.root_navigator);
}
im using the one activity multiple fragments implementation, but using this i can effectively tie my view models to just the scoped fragments and with the new live data you can even limit that too
the first id comes from the nav graphs fragment
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:app="http://schemas.android.com/apk/res-auto">
<fragment
android:id="#+id/root_navigator_fragment"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:name="androidx.navigation.fragment.NavHostFragment"
app:defaultNavHost="true"
app:navGraph="#navigation/root_navigator"/>
</FrameLayout>
and the second comes from the id of the nav graph
<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/root_navigator"
app:startDestination="#id/mainNavFragment">
and then you can use it like so
private void setUpSearchViewModel() {
searchViewModel = new ViewModelProvider(getStoreOwner()).get(SearchViewModel.class);
}

So based on the answers here I made a function that lazily returns a ViewModel scoped to the current navigation graph.
private val scopedViewModel by lazy { getNavScopedViewModel(arg) }
/**
* The [navGraphViewModels] function is not entirely lazy, as we need to pass the graph id
* immediately, but we cannot call [findNavController] to get the graph id from, before the
* Fragment's [onCreate] has been called. That's why we wrap the call in a function and call it lazily.
*/
fun getNavScopedViewModel(arg: SomeArg): ScopedViewModel {
// The id of the parent graph. If you're currently in a destination within this graph
// it will always return the same id
val parentGraphScopeId = findNavController().currentDestination?.parent?.id
?: throw IllegalStateException("Navigation controller should already be initialized.")
val viewModel by navGraphViewModels<ScopedViewModel>(parentGraphScopeId) {
ScopedViewModelFactory(args)
}
return viewModel
}
It's not the prettiest implementation but it gets the job done

Shared ViewModel to help communication between fragments and parent activity

While Navigation component of JetPack looks pretty promising I got to a place where I could not find a way to implement something I wanted.
Let's take a look at a sample app screen:
The app has one main activity, a top toolbar, a bottom toolbar with fab attached.
There are 2 challenges that I am facing and I want to make them the right way.
1. I need to implement fragment transactions in order to allow replacing the fragment on the screen, based on the user interaction.
There are three ways I can think of and have this implemented:
the callbacks way. Having a interface onFragmentAction callback in fragment and have activity implement it. So basically when user presses a button in FragmentA I can call onFragmentAction with params so the activity will trigger and start for example transaction to replace it with FragmentB
implement Navigation component from JetPack. While I've tried it and seems pretty straightforward, I had a problem by not being able to retrieve the current fragment.
Use a shared ViewModel between fragment and activity, update it from the fragment and observe it in the activity. This would be a "replacement" of the callbacks
2. Since the FAB is in the parent activity, when pressed, I need to be able to interact with the current visible fragment and do an action. For instance, add a new item in a recyclerview inside the fragment. So basically a way to communicate between the activity and fragment
There are two ways I can think of how to make this
If not using Navigation then I can use findFragmentById and retrieve the current fragment and run a public method to trigger the action.
Using a shared 'ViewMode' between fragment and activity, update it from activity and observe it in the fragment.
So, as you can see, the recommended way to do navigation would be to use the new 'Navigation' architecture component, however, at the moment it lacks a way to retrieve the current fragment instance so I don't know how to communicate between the activity and fragment.
This could be achieved with shared ViewModel but here I have a missing piece: I understand that fragment to fragment communication can be made with a shared ViewModel. I think that this makes sense when the fragments have something in common for this, like a Master/Detail scenarion and sharing the same viewmodel is very useful.
But, then talking between activity and ALL fragments, how could a shared ViewModel be used? Each fragment needs its own complex ViewModel. Could it be a GeneralViewModel which gets instantiated in the activity and in all fragments, together with the regular fragment viewmodel, so have 2 viewmodels in each fragment.
Being able to talk between fragments and activity with a viewmodel will make the finding of active fragment unneeded as the viewmodel will provide the needed mechanism and also would allow to use Navigation component.
Any information is gladly received.
Later edit. Here is some sample code based on the comment bellow. Is this a solution for my question? Can this handle both changes between fragments and parent activity and it's on the recommended side.
private class GlobalViewModel ():ViewModel(){
var eventFromActivity:MutableLiveData<Event>
var eventFromFragment:MutableLiveData<Event>
fun setEventFromActivity(event:Event){
eventFromActivity.value = event
}
fun setEventFromFragment(event:Event){
eventFromFragment.value = event
}
}
Then in my activity
class HomeActivity: AppCompatActivity(){
onCreate{
viewModel = ViewModelProviders.of(this, factory)
.get(GlobalViewModel::class.java)
viewModel.eventsFromFragment.observe(){
//based on the Event values, could update toolbar title, could start
// new fragment, could show a dialog or snackbar
....
}
//when need to update the fragment do
viewModel.setEventFromActivity(event)
}
}
Then in all fragments have something like this
class FragmentA:Fragment(){
onViewCreated(){
viewModel = ViewModelProviders.of(this, factory)
.get(GlobalViewModel::class.java)
viewModel.eventsFromActivity.observe(){
// based on Event value, trigger a fun from the fragment
....
}
viewModelFragment = ViewModelProviders.of(this, factory)
.get(FragmentAViewModel::class.java)
viewModelFragment.some.observe(){
....
}
//when need to update the activity do
viewModel.setEventFromFragment(event)
}
}