使用 ViewPager2获取当前片段

The ViewPagerAdapter is intended to hide all these implementation details which is why there is no straight-forward way to do it.

You could try setting and id or tag on the fragment when you instantiate it in getItem() then use fragmentManager.findFragmentById() or fragmentManager.findFragmentByTag() to retrieve.

Doing it like this, however, is a bit of a code smell. It suggests to me that stuff is being done in the activity when it should be done in the fragment (or elsewhere).

Perhaps there is another approach to achieve what you want but it's hard to give suggestions without knowing why you need to get the current fragment.

I had similar problem when migrating to ViewPager2.

In my case I decided to use parentFragment property (I think you can make it also work for activity) and hope, that ViewPager2 will keep only the current fragment resumed. (i.e. page fragment that was resumed last is the current one.)

So in my main fragment (HostFragment) that contains ViewPager2 view I created following property:

private var _currentPage: WeakReference<MyPageFragment>? = null
val currentPage
get() = _currentPage?.get()


fun setCurrentPage(page: MyPageFragment) {
_currentPage = WeakReference(page)
}

I decided to use WeakReference, so I don't leak inactive Fragment instances

And each of my fragments that I display inside ViewPager2 inherits from common super class MyPageFragment. This class is responsible for registering its instance with host fragment in onResume:

override fun onResume() {
super.onResume()
(parentFragment as HostFragment).setCurrentPage(this)
}

I also used this class to define common interface of paged fragments:

abstract fun someOperation1()


abstract fun someOperation2()

And then I can call them from the HostFragment like this:

currentPage?.someOperation1()

I'm not saying it's a nice solution, but I think it's more elegant than relying on internals of ViewPager's adapter with instantiateItem method that we had to use before.

I was able to get access to current fragment in FragmentStateAdapter using reflection.

Extension function in Kotlin:

fun FragmentStateAdapter.getItem(position: Int): Fragment? {
return this::class.superclasses.find { it == FragmentStateAdapter::class }
?.java?.getDeclaredField("mFragments")
?.let { field ->
field.isAccessible = true
val mFragments = field.get(this) as LongSparseArray<Fragment>
return@let mFragments[getItemId(position)]
}
}

Add Kotlin reflection dependency if needed:

implementation "org.jetbrains.kotlin:kotlin-reflect:1.3.61"

Example call:

val tabsAdapter = viewpager.adapter as FragmentStateAdapter
val currentFragment = tabsAdapter.getItem(viewpager.currentItem)

supportFragmentManager.findFragmentByTag("f" + viewpager.currentItem)

with FragmentStateAdapter in placeFragmentInViewHolder(@NonNull final FragmentViewHolder holder)add Fragment

mFragmentManager.beginTransaction()
.add(fragment, "f" + holder.getItemId())
.setMaxLifecycle(fragment, STARTED)
.commitNow()

was facing same issue now its solved by adding one object in adapter

class MyViewPager2Adapter(fragmentActivity: FragmentActivity) : FragmentStateAdapter(fragmentActivity) {




private val FRAGMENTS_SIZE = 2


var currentFragmentWeakReference: WeakReference<Fragment>? = null


override fun getItemCount(): Int {
return this.FRAGMENTS_SIZE
}


override fun createFragment(position: Int): Fragment {


when (position) {
0 -> {
currentFragmentWeakReference= MyFirstFragment()
return MyFirstFragment()
}
1 -> {
currentFragmentWeakReference= MySecondFragment()
return MySecondFragment()
}
}


return MyFirstFragment() /for default view


}


}

after creating adapter I registered my Viewpager 2 with ViewPager2.OnPageChangeCallback() and overrided its method onPageSelected

now simple did this trick to get current fragment

 private fun getCurrentFragment() :Fragment?{


val fragment = (binding!!.pager.adapter as MyViewPager2Adapter).currentFragmentWeakReference?.get()


retrun fragment
}

I've only tested this with 2 fragments in ViewPager2

cheers guys , hope this mayhelp you.!

In ViewPager2 the FragmentManager by default have assigned tags to fragments like this:

Fragment in 1st position has a tag of "f0"

Fragment in 2nd position has a tag of "f1"

Fragment in 3rd position has a tag of "f2" and so on... so you can get your fragment's tag and by concatenating the "f" with position of your fragment. To get the current Fragment you can get current position from the viewPager2 position and make your tag like this (For Kotlin):

val myFragment = supportFragmentManager.findFragmentByTag("f" + viewpager.currentItem)

For fragment at a certain position

val myFragment = supportFragmentManager.findFragmentByTag("f" + position)

You can cast the Fragment and always check if it is not null if you are using this technique.

If you host your ViewPager2 in Fragment, use childFragmentManager instead.

REMEMBER

If you have overriden the getItemId(position: Int) in your adapter. Then your case is different. It should be:

val myFragment = supportFragmentManager.findFragmentByTag("f" + your_id_at_that_position)

OR SIMPLY:

val myFragment = supportFragmentManager.findFragmentByTag("f" + adapter.getItemId(position))

If you host your ViewPager2 in Fragment, use childFragmentManager instead of supportFragmentManager.

The solution to find current Fragment by its tag seems the most suitable for me. I've created these extension functions for that:

fun ViewPager2.findCurrentFragment(fragmentManager: FragmentManager): Fragment? {
return fragmentManager.findFragmentByTag("f$currentItem")
}


fun ViewPager2.findFragmentAtPosition(
fragmentManager: FragmentManager,
position: Int
): Fragment? {
return fragmentManager.findFragmentByTag("f$position")
}

• If your ViewPager2 host is Activity, use supportFragmentManager or fragmentManager.
• If your ViewPager2 host is Fragment, use childFragmentManager

Note that:

• findFragmentAtPosition will work only for Fragments that were initialized in ViewPager2's RecyclerView. Therefore you can get only the positions that are visible + 1.
• Lint will suggest you to remove ViewPager2. from fun ViewPager2.findFragmentAtPosition, because you don't use anything from ViewPager2 class. I think it should stay there, because this workaround applies solely to ViewPager2.

Similar to some other answers here, this is what I'm currently using.
I'm not clear on how reliable it is but at least one of these is bound to work 😁.

//Unclear how reliable this is
fun getFragmentAtIndex(index: Int): Fragment? {
return fm.findFragmentByTag("f${getItemId(index)}")
?: fm.findFragmentByTag("f$index")
?: fm.findFragmentById(getItemId(index).toInt())
?: fm.findFragmentById(index)
}

fm is the supportFragmentManager

May as well post my solution to this - it's the same basic approach as @Almighty 's, except I'm keeping the Fragment weak references in a lookup table in the PagerAdapter:

private class PagerAdapter(fm: FragmentManager, lifecycle: Lifecycle) : FragmentStateAdapter(fm, lifecycle) {


// only store as weak references, so you're not holding discarded fragments in memory
private val fragmentCache = mutableMapOf<Int, WeakReference<Fragment>>()


override fun getItemCount(): Int = tabList.size
    

override fun createFragment(position: Int): Fragment {
// return the cached fragment if there is one
fragmentCache[position]?.get()?.let { return it }


// no fragment found, time to make one - instantiate one however you
// like and add it to the cache
return tabList[position].fragment.newInstance()
.also { fragmentCache[position] = WeakReference(it) }
.also { Timber.d("Created a fragment! $it") }
}


// not necessary, but I think the code reads better if you
// can use a getter when you want to... try to get an existing thing
fun getFragment(position: Int) = createFragment(position)
}

and then you can call getFragment with the appropriate page number, like adapter.currentPage or whatever.

So basically, the adapter is keeping its own cache of fragments it's created, but with WeakReferences so it's not actually holding onto them, once the components actually using the fragments are done with them, they won't be in the cache anymore. So you can hold a lookup for all the current fragments.

You could have the getter just return the (nullable) result of the lookup, if you wanted. This version obviously creates the fragment if it doesn't already exist, which is useful if you expect it to be there. This can be handy if you're using ViewPager2.OnPageChangeCallback, which will fire with the new page number before the view pager creates the fragment - you can "get" the page, which will create and cache it, and when the pager calls createFragment it should still be in the cache and avoid recreating it.

It's not guaranteed the weak reference won't have been garbage collected between those two moments though, so if you're setting stuff on that fragment instance (rather than just reading something from it, like a title you want to display) be aware of that!

I had the same problem. I converted from ViewPager to ViewPager2, using FragmentStateAdapter. In my case I have a DetailActivity class (extends AppCompatActivity) which houses the ViewPager2, which is used to page through lists of data (Contacts, Media, etc.) on smaller form-factor devices.

I need to know the currently shown fragment (which is my own class DetailFragment which extends androidx.fragment.app.Fragment), because that class contains the string I use to update the title on the DetailActivity toolbar.

I first started down the road of registering an onPageChangeCallback listener as suggested by some, but I quickly ran into problems:

1. I initially created tags during the ViewPager2's adapter.createFragment() call as suggested by some with the idea to add the newly created fragment to a Bundle object (using FragmentManager.put()) with that tag. This way I could then save them across config changes. The problem here is that during createFragment(), the fragment isn't actually yet part of the FragmentManager, so the put() calls fail.
2. The other problem is that if the fundamental idea is to use the onPageSelected() method of the OnPageChangeCallback to find the fragment using the internally generated "f"+position tag names - we again have the same timing issue: The very first time in, onPageSelected() is called PRIOR to the createFragment() call on the adapter - so there are no fragments yet created and added to the FragmentManager, so I can't get a reference to that first fragment using the "f0" tag.
3. I then tried to find a way where I could save the position passed in to onPageSelected, then try and reference that somewhere else in order to retrieve the fragment after the adapter had made the createFragment() calls - but I could not identify any type of handler within the adapter, the associated recyclerview, the viewpager etc. that allows me to surface the list of fragments that I could then reference that to the position identified within that listener. Strangely, for example, one adapter method that looked very promising was onViewAttachedToWindow() - however it is marked final so can't be overridden (even though the JavaDoc clearly anticipates it being used this way).

So what I ended up doing that worked for me was the following:

1. In my DetailFragment class, I created an interface that can be implemented by the hosting activity:

    public interface DetailFragmentShownListener {
// Allows classes that extend this to update visual items after shown
void onDetailFragmentShown(DetailFragment me);
}

2. Then I added code within onResume() of DetailFragment to see if the associated activity has implemented the DetailFragmentShownListener interface within this class, and if so I make the callback:

    public void onResume() {
super.onResume();
View v = getView();
if (v!=null && v.getViewTreeObserver().isAlive()) {
v.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
v.getViewTreeObserver().removeOnGlobalLayoutListener(this);
// Let our parent know we are laid out
if ( getActivity() instanceof DetailFragmentShownListener ) {
((DetailFragmentShownListener) getActivity()).onDetailFragmentShown(DetailFragment.this);
}
}
});
}
}

2. Then where I need to know when this fragment is shown (such as within DetailActivity), I implement the interface and when it receives this callback I know that this is the current fragment:

    @Override
public void onDetailFragmentShown(DetailFragment me) {
mCurrentFragment = me;
updateToolbarTitle();
}

mCurrentFragment is a property of this class as its used in various other places.

If you want the current Fragment to only perform some action in it, you can use a SharedViewModel which is shared between ViewPager container and its Fragments and pass an Identifier to each fragment and observe to a LiveData in SharedViewModel. Set value of that LiveData to an object which consists of Identifier of the fragment you want to update (i.e. Pair<String, MyData> which SharedViewModel0 is type of the Identifier). Then inside your observers check if the current emitted SharedViewModel2 is as same as the fragment's Identifier or not and consume data if it is equal.
Its not as simple as using fragment's tags to find them, But it least you do not need to worry about changes to how ViewPager2 create tag for each fragment.

Solution for get fragment by position:

class ClubPhotoAdapter(
private val dataList: List<MyData>,
fm: FragmentManager,
lifecycle: Lifecycle
) : FragmentStateAdapter(fm, lifecycle) {


private val fragmentMap = mutableMapOf<Int, WeakReference<MyFragment>>()


override fun getItemCount(): Int = dataList.size


override fun createFragment(position: Int): Fragment {
val fragment = MyFragment[dataList.getOrNull(position)]


fragmentMap[position] = WeakReference(fragment)


return fragment
}


fun getItem(position: Int): MyFragment? = fragmentMap[position]?.get()
}

You can get the current fragment from ViewPager2 like following,

adapter.getRegisteredFragment(POSITION);

Sample FragmentStateAdapter,

public class ViewPagerAdapterV2 extends FragmentStateAdapter {
private final FragmentActivity context;
private final HashMap<Integer, Fragment> mapFragments;


public ViewPagerAdapterV2(FragmentActivity fm) {
super(fm);
this.context = fm;
this.mapFragments = new HashMap<>();
}


public Context getContext() {
return context;
}


@Override
public int getItemCount() {
return NUMBER_OF_PAGES;
}


public Fragment getRegisteredFragment(int position) {
return mapFragments.get(position);
}


@NonNull
@Override
public Fragment createFragment(int position) {
MyFragment fragment = new MyFragment();
mapFragments.put(position, fragment);
return fragment;
}
}

I found this works for me even after the activity gets destroyed and re-created. The key can be any type. Assume you have ViewModel used for your fragment.

fun findFragment(fragmentId: String?): YourFragment? {
fragmentId?: return null
return supportFragmentManager.fragments.filterIsInstance(YourFragment::class.java).find { it.viewModel.yourData.value.id == fragmentId}
}

To avoid finding fragment with the hard-coded tag f$id set internally which might be changed by Google in any future release:

Method 1: filtering the page fragments with the resumed one

Assuming the page fragment of the ViewPager2 is PageFragment, then you can find the current ViewPager fragment in the current fragmentManager fragments and check if it is in the resumed state (currently displayed on the screen)

val fragment = supportFragmentManager.fragments
.find{ it is PageFragment && it.isResumed } as PageFragment

Note: supportFragmentManager should be replaced with childFragmentManager if the ViewPager is a part of a Fragment.

For java (API 24+):

Fragment fragment =
getSupportFragmentManager().getFragments().stream()
.filter(it -> it instanceof PageFragment && it.isResumed())
.collect(Collectors.toList()).stream().findFirst().get();

Method 2: setting some argument to the PageFragment, and filter fragments based on it

Kotlin:

Adapter

class MyAdapter(fragmentManager: FragmentManager, lifecycle: Lifecycle) :
FragmentStateAdapter(fragmentManager, lifecycle) {


//.... omitted


override fun createFragment(position: Int): Fragment
= PageFragment.getInstance(position)


}

PageFragment:

class PageFragment : Fragment() {


//.... omitted


companion object {
const val POSITION = "POSITION";
fun getInstance(position: Int): PageFragment {
return PageFragment().apply {
arguments = Bundle().also {
it.putInt(POSITION, position)
}
}
}
}


}

And filter with the position argument to get the needed PageFragment:

val fragment = supportFragmentManager.fragments.firstOrNull {
it is PageFragment && it.arguments?.getInt("POSITION") == id }  // id from the viewpager >> for instance `viewPager.currentItem`

Java (API 24+):

Adapter:

class MyAdapter extends FragmentStateAdapter {
    

// .... omitted


@NonNull
@Override
public Fragment createFragment(int position) {
return PagerFragment.newInstance(position);
}


}

PageFragment:

public class PagerFragment extends Fragment {


// .... omitted


public static Fragment newInstance(int position) {
PagerFragment fragment = new PagerFragment();
Bundle args = new Bundle();
args.putInt("POSITION", position);
fragment.setArguments(args);
return fragment;
}
}

And to get a fragment of a certain viewPager item:

Optional<Fragment> optionalFragment = getSupportFragmentManager().getFragments()
.stream()
.filter(it -> it instanceof PagerFragment && it.getArguments() != null && it.getArguments().getInt("POSITION") == id)
.findFirst();


optionalFragment.ifPresent(fragment -> {
// This is your needed PageFragment
});

There is no need to rely on tags. ViewPager2 hides implementation details, but we all know it has an inner recycler so the layout manager would do the trick. We can write something like this:

fun ViewPager2.getCurrentView(): View? {
return (getChildAt(0) as RecyclerView).layoutManager?.getChildAt(currentItem)
}


fun ViewPager2.getCurrentFragment(): Fragment? {
return getCurrentView().findFragment()
}

It would also works for for any given position not only the first one if you want to.

Solutions with WeakReference did not worked for me because Android restores the state and createFragment is not always called, so I was always getting nulls.

Here is my try and seems to be woking fine:

val fragment = childFragmentManager.fragments.first {
it.lifecycle.currentState == Lifecycle.State.RESUMED
}

this will iterate over the fragments and return one which is RESUMED, meaning it will be the current selected fragment

I simply use this in my tab fragment

fun currentFragment() = childFragmentManager.fragments.find { it.isResumed }