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MVVM: Communicate child Fragment to parent Fragment

I have a parent Fragment that has ViewPager which holds another Fragments(child). Some child Fragment can have list. If the user press a back button the list will scroll up if can scroll vertically, another press on back button will move the ViewPager to first item (Fragment).
While I can create an approach like this on child Fragment.
if (adapter.currentList.isNotEmpty() && recyclerView.canScrollVertically(-1))
recyclerView.smoothScrollToPosition(0)
else {
try {
val parentFragment: HomeFragment = parentFragment as HomeFragment
parentFragment.onBackPressed()
} catch (e: Exception) {
FirebaseCrashlytics.getInstance().recordException(e)
}
}
And a public method on parent Fragment like this
fun onBackPressed() {
if (viewPager.currentItem != 0)
viewPager.setCurrentItem(0, true)
else
requireActivity().onBackPressed()
}
I am not sure if this is the best thing to do since I read that Fragments should better not communicate directly with each other, instead communication should be handle by the host Activity or shared ViewModel. But doing so seems an overkill and I do not feel the use of LiveData just for this case.

I would go with the SharedViewModel approach, its kinda built for this. Your child Fragments are effecting the ViewPager that belongs to the ParentFragment. They are effectively part of the same View but the child Fragments don't have access to the View that contains them. So using a SharedViewModel can bridge the gap.
What I would do is make a well defined interface that manipulates the ViewPager and its Adapter as you see fit for your application. That interface should then be accessible through the SharedViewModel.
In the interface you could have a function called navigateToFragment(Class fragmentClass); In this function you would then need to find a fragment via its class inside the Adapter, and then use the ViewPager to go to it. Ie navigateToFragment(HomeFragment.class);

How to share ViewModel between Parent fragment and fragment inside child fragment

I have parent Fragment that contains child fragment. Inside child fragment I have ViewPager with fragments. My question is how can I share ViewModel between parent child and fragments in viewpager and makeing Viewmodel visible only on ParentFragment scope?

what do you mean when you say "visible only on ParentFragment scope"?
According Google's document, there is one way that you can share ViewModel.
Check this document: https://developer.android.com/topic/libraries/architecture/viewmodel#sharing
Shortly, your parent fragment and child fragment will use the same ViewModel. Your parent fragment will call the function of ViewModel to change the data, your child fragment just observer the LiveData of ViewModel.

You can place your sharedViewModel inside your Activity. Then you can access it from every fragment which in attached to this activity with this code:
(requireActivity() as MainActivity).viewModel;
With this approach, you can set data from one fragment and observe data from another fragment. So, you enable communication between two fragments.
Setting data:
viewModel.liveDataObject.value = value
Observing data:
viewModel.liveDataObject.observe(viewLifecycleOwner) {}

Can a Dialog have a view model in android?

I need to do an API call from a Dialog. Do I need to go back to the fragment for doing so, or is there any way to refer to the fragment view model?

Yes, it is possible and I was able to do that because class DialogFragment extends Fragment. So I added a view model just like any other fragment.
like below where BaseDialog class extends DialogFragment

You can try with this:
Use interface, implement it in fragment so you have callback funtion.
Pass high ordered function, declare it like this in dialog:
var click: (() -> Unit)? = null;
Then you can set it from fragment when you instantiate your dialog.
Use shared view model, for example make view model in your activity and then you can access it from every fragment or dialog like this:
(requireActivity() as MainActivity).viewModel
Like this you can set value in view model variable (liveData often) inside your dialog and observe changes in fragment

I think you can pass a high ordered function to dialog and handle it in fragment using viewModel inside.

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)
}
}

Why is this fragment's fields null?

In my parent Fragment's onCreateView method, I invoke the Child Fragment Manager to insert a Fragment into the layout. This child fragment contains a few FloatingActionButtons.
After I commit the transaction, I then check the contents of my underlying list (powering a RecyclerView), and based on whether any values are present, I either hide/unhide some of the FloatingActionButtons.
However it's telling me that the buttons are null! Does committing a Fragment transaction not call all of its typical lifecycle events first? Is there a way to force it to wait? Is there a better practice for this?

The commit is not synchronous. There's no guarantee when the fragment's life cycle will execute.
If you need to know when the fragment is "ready", implement a callback from the fragment to the hosting activity letting it know.
class CustomB extends CustomA {
interface Listener {
void onViewCreated();
}
public View onCreateView() {
View v = super.onCreateView();
if (getActivity() instanceof Listener) {
((Listener)getActivity()).onViewCreated();
}
return v;
}
}
Generally speaking you should try to design your fragments to be self-contained. If the fragment is tightly bound to the activity, consider implementing the functionality as a custom View instead.