How to get Fragment by tag in ViewPager2

Question

I have been told to use the createFragment override of the FragmentStateAdapter to get the current fragment by index.

This is how my adapter appears to be .

class AddRestaurantPagerAdapter(fragmentManager: FragmentManager, lifecycle: Lifecycle) :
    FragmentStateAdapter(fragmentManager, lifecycle) {
    private var fragments = listOf<Fragment>(
        FragmentA.newInstance(),
        FragmentB.newInstance(),
        FragmentC.newInstance(),
        FragmentD.newInstance()
    )

    override fun getItemCount() = 4
    override fun createFragment(position: Int) =
        fragments[position]

}

This works fine until we force an onSaveInstance/OnRestoreInstance on this activity/fragment.

The ViewPager2 reloads the already existing fragments on it , and we need a way to access that. This is evidenced by the following LOC in the FragmentStateAdapter class

private void ensureFragment(int position) {
        long itemId = getItemId(position);
        if (!mFragments.containsKey(itemId)) {
            // TODO(133419201): check if a Fragment provided here is a new Fragment
            Fragment newFragment = createFragment(position);
            newFragment.setInitialSavedState(mSavedStates.get(itemId));
            mFragments.put(itemId, newFragment);
        }
    }

After activity/fragment recreation , you'd find that the this If block never executes because mFragments would already contain the fragment added before the activity was collected.

So I was wondering how should be extract the fragments already in the viewpager2

Answer 1

If you take a look at this method,

/**
 * @param holder that has been bound to a Fragment in the {@link #onBindViewHolder} stage.
 */
@SuppressWarnings("WeakerAccess") // to avoid creation of a synthetic accessor
void placeFragmentInViewHolder(@NonNull final FragmentViewHolder holder) {
    Fragment fragment = mFragments.get(holder.getItemId());
    if (fragment == null) {
        throw new IllegalStateException("Design assumption violated.");
    }
    FrameLayout container = holder.getContainer();
    View view = fragment.getView();

    /*
    possible states:
    - fragment: { added, notAdded }
    - view: { created, notCreated }
    - view: { attached, notAttached }

    combinations:
    - { f:added, v:created, v:attached } -> check if attached to the right container
    - { f:added, v:created, v:notAttached} -> attach view to container
    - { f:added, v:notCreated, v:attached } -> impossible
    - { f:added, v:notCreated, v:notAttached} -> schedule callback for when created
    - { f:notAdded, v:created, v:attached } -> illegal state
    - { f:notAdded, v:created, v:notAttached } -> illegal state
    - { f:notAdded, v:notCreated, v:attached } -> impossible
    - { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
     */

    // { f:notAdded, v:created, v:attached } -> illegal state
    // { f:notAdded, v:created, v:notAttached } -> illegal state
    if (!fragment.isAdded() && view != null) {
        throw new IllegalStateException("Design assumption violated.");
    }

    // { f:added, v:notCreated, v:notAttached} -> schedule callback for when created
    if (fragment.isAdded() && view == null) {
        scheduleViewAttach(fragment, container);
        return;
    }

    // { f:added, v:created, v:attached } -> check if attached to the right container
    if (fragment.isAdded() && view.getParent() != null) {
        if (view.getParent() != container) {
            addViewToContainer(view, container);
        }
        return;
    }

    // { f:added, v:created, v:notAttached} -> attach view to container
    if (fragment.isAdded()) {
        addViewToContainer(view, container);
        return;
    }

    // { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
    if (!shouldDelayFragmentTransactions()) {
        scheduleViewAttach(fragment, container);
        mFragmentManager.beginTransaction()
                .add(fragment, "f" + holder.getItemId())
                .setMaxLifecycle(fragment, STARTED)
                .commitNow();
        mFragmentMaxLifecycleEnforcer.updateFragmentMaxLifecycle(false);
    } else {
        if (mFragmentManager.isDestroyed()) {
            return; // nothing we can do
        }
        mLifecycle.addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                if (shouldDelayFragmentTransactions()) {
                    return;
                }
                source.getLifecycle().removeObserver(this);
                if (ViewCompat.isAttachedToWindow(holder.getContainer())) {
                    placeFragmentInViewHolder(holder);
                }
            }
        });
    }
}

This the tag is "f" + holder.getItemId().

@Override
    public long getItemId(int position) {
        return position;
    }

The getItemId(int postion) returns position by default. but if you override this method.

int postionOfTheFragmentYouWantToGet=0;
Fragment page = getSupportFragmentManager().findFragmentByTag("f" + postionOfTheFragmentYouWantToGet);