I have an app with a RootFragment containing a ViewPager2, presenting multiple ChildFragment.
The RootFragment has a RootViewModel, and each ChildFragment also has a ChildViewModel.
Running the app, I can see that the root fragment's view model is being created once and preserved through rotations etc - this is exactly what I expected to happen.
However, when I scroll through the child fragments and back again it looks like the ChildVieWModels are being recreated each time the ViewPager2 destroys/recreates the fragments to display. This isn't what I need at all!
How can I get the view models of the child fragment to remain even though the child fragments themselves are being destroyed and recreated by the ViewPager2?
My child fragment is requesting a view model like this:
private val viewModel: ChildViewModel by viewModels()
I have also tried scoping the child view model to the parent fragment but that just means each child fragment gets the same instance of ChildViewModel, which isn't quite right either:
private val viewModel: ChildViewModel by viewModels({ requireParentFragment() })
I've looked at the samples for ViewPager2 but none of them seem to have a view model in their child fragments.
What have I misunderstood here?
How can I get the view models of the child fragment to remain even though the child fragments themselves are being destroyed and recreated by the ViewPager2?
You could set the offscreen page limit. See doc https://developer.android.com/reference/androidx/viewpager2/widget/ViewPager2#setOffscreenPageLimit(int)
If you set the limit to 2, then the fragments that will be kept alive are the current tab, the next 2 tabs and the previous 2 tabs in the viewpager. You can use this to keep all fragments and viewmodels in the viewpager alive if the number of tabs/childfragments are static and small, for example 5 or less.
If the child fragments have some shared state among them or some expensive queries in child fragments, then you can put that in the RootViewModel and use that in your child fragments by doing requireParentFragment as you have pointed out.
There really isn't a better way than to set the offscreen limit or sharing the viewmodel with the parent fragment.
Related
I'm using ViewPager2 which is hosted inside a fragment. On the initial opening of ViewPager2 fragment host everything is loaded and displayed correctly, but when I swipe multiple times (via TabLayout) to the last fragment and go back to the first fragment, its layout is empty/white screen.
The adapter which I'm using is FragmentStateAdapter. Also, I set offscreenPageLimit to a constant value (which is in most cases less than the size of the fragments list in ViewPager2).
This is constructor of my custom FragmentStateAdapter:
pagerAdapter = MyCustomPagerAdapter(requireContext(), childFragmentManager, viewLifecycleOwner.lifecycle)
Since ViewPager2 is using RecyclerView internally, maybe there is some problem with recycling fragments when there are not visible (also considering offscreenPageLimit value).
Problem was in by viewBinding() method for getting binding object.
After using standard way of creating binding object everything is working OK.
When rotating the screen my nested fragment is shown but for some brief moments, the parent fragment is also shown.
I have my MainActivity that has a FrameLayout with ID activity_base_container.
I'm doing this when my activity starts:
Fragment initialFragment = getInitialFragment();
mFragmentManager.beginTransaction()
.add(R.id.activity_base_container, initialFragment, initialFragment.getClass().getSimpleName())
.commit();
That initialFragment initial fragment is responsible to check some conditions and depending them will launch one of two possible fragments:
fragmentManager.beginTransaction().replace(R.id.activity_base_container, fragment, fragment.getClass().getSimpleName()).commit();
Lets assume it launches FragmentF (whit a root FrameLayout with id fragment_f_root). This fragments layout has a set of options. When the user clicks one of those options, the corresponding fragment is created and is launched like this:
//The example here is an option that displays a google map.
fragment = FragmentMapMultipleActivity.newInstance();
fragmentManager.beginTransaction()
.replace(R.id.fragment_f_root, fragment)
.addToBackStack(fragment.getClass().getSimpleName())
.commit();
At this point all is working as expected. The problem is when I rotate the screen. FragmentF appears briefly and then immediately FragmentMapMultipleActivity, the nested fragment, appears.
Is it possible after rotating the screen show only the nested fragment or I should change my "architecture" to something else?
should change my "architecture" to something else?
Probably, you should.
The brightest Android-minds from Square are even advocating to avoid simple fragments everywhere it's possible: Advocating Against Android Fragments
Nested fragemnts, in its turn, increase complexity exponentially. The only good pattern of using them I've seen so far is ViewPager with it's FragmentPagerAdapter. In majority of other cases, consider using Custom Views instead.
It keeps your app's lifecycle cleaner and more predictable.
I don't think you can do much with this blinking you see, apart from:
setRetainInstance(true) and avoid full re-creation of the Fragment in Activity, so you keep you fragment's data during change of the configuration (and then pass same retained fragment to the fragment manager)
keeping layouts as lightweight as possible
avoid re-creation of already initialized variables
keep onViewCreate() as lightweight as possible
Good luck!
I have a Fragment, MainFragment, which can contain two, three, or four nested fragments. The specific fragments that will be shown can be changed by the user in the settings.
There is a different layout for each number of fragments. For instance, layout_3 is used when the user chooses three nested fragments.
What I need to do is dynamically update MainFragment's layout, and which fragments will be nested within that layout, in onResume() (i.e. Once the user comes back from the settings). There are about 10 fragments the user can choose from, and I need to be able to swap them in and out of MainFragment dynamically.
I'm having trouble doing this. The only way to update the layout/view once I return from the settings is to leave MainFragment and then come back (which calls onCreateView()).
Here is an example of what I do in onCreateView() to initialize the layouts (two nested fragments is the default):
mView = mInflater.inflate(R.layout.layout_2, mParent, false);
getChildFragmentManager().beginTransaction().add(R.id.fragmentContainer1, fragment1).commit();
getChildFragmentManager().beginTransaction().add(R.id.fragmentContainer2, fragment2).commit();
return view;
Suppose the user then goes to the settings and chooses to have three nested fragments. This is what I've tried in onResume(), to no effect:
mView = mInflater.inflate(R.layout.layout_3, mParent, false);
getChildFragmentManager().beginTransaction().add(R.id.fragmentContainer1, fragment1)).commit();
getChildFragmentManager().beginTransaction().add(R.id.fragmentContainer2, fragment2).commit();
getChildFragmentManager().beginTransaction().add(R.id.fragmentContainer3, fragment3).commit();
I'm not sure if I'm doing something wrong. Ideally, I would just force MainFragment to call onCreateView() again, but none of the solutions for that problem seem to work.
Any ideas? Thanks for the help.
Edit: I believe the problem is with inflating the new View, rather than replacing the fragments.
For instance, suppose the default screen is layout_4, with four fragment containers. The user then goes to the settings un-checks all four default fragments, and chooses three new fragments. In onResume(), we try to inflate layout_3, and then add the fragments. I think layout_3 never inflates, but because my fragment containers have the same style id across layouts (i.e. fragmentContainer1 - fragmentContainer4), the first three fragment containers are updated. The fourth one remains as it was, since I assumed we were in layout_3 and did not try to update it.
This behavior is confirmed and results in a crash when the user tries to increase the number of fragments, rather than decrease. Above, when the user switched from four fragments to three fragments, there was no crash because all three fragment containers I tried to update exist in layout_4. But if the user is in layout_2 and then goes to the settings to select a third fragment, we'll try to add a fragment to fragmentContainer3 when we resume. This results in a crash because layout_3 fails to inflate.
java.lang.RuntimeException: Unable to resume activity
java.lang.IllegalArgumentException: No view found for id 0x7f0c005f
Any ideas how to fix this? The call to re-inflate mView in onResume() does not seem to have any effect.
Edit 2: I've tried calling mParent.addView(mView) after inflating, but still experience the same behavior as above, for the most part.
When you return from the settings, onResume() should be called in the MainFragment and subsequently any nested fragments that were already loaded in the MainFragment. Can you not include any update logic in the nested fragments' onResume() instead of only in onCreateView()?
Otherwise you can create a different code path and put update logic there: make public methods in the classes for fragment1, fragment2, fragment3 that include all your update logic, and call those methods from somewhere in MainFragment. (You could create an interface and have the nested fragment classes inherit that interface, if they are different classes and you want to have a cleaner design.)
Be careful about whether or not the nested fragments have been resumed yet—calling methods on View objects when nested fragments' onResume() hasn't been called yet could be problematic.
I have one fragment that contains two fragments inside.
Every fragment is loading some names from database and displays them as a list with a limit of 50.
At start my adapter has 50 elements, when user scrolls down another 50 is beign loaded, then adapter is with 100 elements.
Now whenever I rotate the device, my data in adapter is messed up, that is my question is there any way to save my state of childFragment? without using parceable and parcel the whole 100 elements together with current position?
You should use Fragment's setRetainInstance(boolean) feature.
Setting
setRetainInstance(true);
to a Fragment containing the child Fragments should do.
Control whether a fragment instance is retained across Activity re-creation (such as from a configuration change). This can only be used with fragments not in the back stack. If set, the fragment lifecycle will be slightly different when an activity is recreated:
Understanding Fragment's setRetainInstance(boolean)
Having searched regarding this issue beforehand, I can find many discussions regarding dynamically adding and removing selected Fragments from a ViewPager. What I'm actually concerned about here however is how I can programmatically remove an entire ViewPager 'cleanly' from its containing ViewGroup, when that ViewPager has been used to display Fragments via a FragmentPagerAdapter, and ensure that the contained Fragments are destroyed properly.
To expand on the question a bit more, I have a landscape two-pane layout where a selection is made from a list within a Fragment on the left-hand-side, and chosen content is then placed on the right within a FrameLayout. The key thing is that the content may or may not be paginated. Therefore, the content must either be displayed in a ViewPager format, or if it is not paginated then it shall be represented by a single Fragment directly.
To show a single Fragment, I simply perform a FragmentTransaction as you normally would in order to place the Fragment into the FrameLayout container. If on the other hand it's paginated content to be shown, then instead I create a ViewPager and add it as a child of the FrameLayout.
When I need to change the content, then if the previous content was a stand-alone Fragment then I can simply remove it via FragmentTransaction .remove(). When I do this, the Fragment goes through the onPause() ... onDestroy() cycle as expected. If the previous content was a ViewPager then I remove it from the FrameLayout using .removeAllViews(). Here I come to the problem: I don't see any of the onPause() ... onDestroy() methods being called in any of the Fragments that were held within that ViewPager via the FragmentPagerAdapter.
From a user point of view, the application works fine. After several rounds of ViewPager being removed, I can see the GC reclaiming memory. However, I don't like the fact that those Fragments' end of life methods aren't called as I can't do any cleanup within them, and it just doesn't seem 'right'.
Is there a method I can hook into in order to remove the ViewPager's Fragments when the ViewPager is detached from its parent, perhaps? In other words, when I know that the ViewGroup is no longer in used, I would perform FragmentTransactions somewhere (perhaps in the FragmentPagerAdapter) to remove those Fragments.
Alternatively, I realise that I could just keep the ViewPager on the right permanently, and dynamically swap the Fragments within it. Of course it simply would not matter that at certain times it would only hold one page. If this would be a better way to go then I shall refactor my code to do this, but I would appreciate opinions.
However, I don't like the fact that those Fragments' end of life methods aren't called as I can't do any cleanup within them, and it just doesn't seem 'right'.
They should get cleaned up when the activity is destroyed, if that is not too late for you (e.g., heap issues).
In other words, when I know that the ViewGroup is no longer in used, I would perform FragmentTransactions somewhere (perhaps in the FragmentPagerAdapter) to remove those Fragments.
You did not execute the transactions to put the fragments there. Hence, you cannot readily execute the transactions to remove the fragments. If you switch to FragmentStatePagerAdapter, and call setAdapter(null), it should cause all existing fragments in the pager to be destroyed, by my reading of the source code. FragmentPagerAdapter never uses remove(), but FragmentStatePagerAdapter does, from its destroyItem() method, and all extant fragments are destroyed via destroyItem() when a new adapter (or null) is supplied to setAdapter().