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For AndroidX, the Android CL 2366713 seems to trigger an exception Design assumption violated. in FragmentStateAdapter.java (this links to the original CL).

As of 2023-03-06, the code looks like:

    @Override
    public final void onBindViewHolder(final @NonNull FragmentViewHolder holder, int position) {
        final long itemId = holder.getItemId();
        final int viewHolderId = holder.getContainer().getId();
        final Long boundItemId = itemForViewHolder(viewHolderId); // item currently bound to the VH
        if (boundItemId != null && boundItemId != itemId) {
            removeFragment(boundItemId);
            mItemIdToViewHolder.remove(boundItemId);
        }
        mItemIdToViewHolder.put(itemId, viewHolderId); // this might overwrite an existing entry
        ensureFragment(position);
        /** Special case when {@link RecyclerView} decides to keep the {@link container}
         * attached to the window, but not to the view hierarchy (i.e. parent is null) */
        final FrameLayout container = holder.getContainer();
        if (ViewCompat.isAttachedToWindow(container)) {
            if (container.getParent() != null) {
                throw new IllegalStateException("Design assumption violated.");   // <================ FAILS HERE
            }
            container.addOnLayoutChangeListener(new View.OnLayoutChangeListener() {
                @Override
                public void onLayoutChange(View v, int left, int top, int right, int bottom,
                        int oldLeft, int oldTop, int oldRight, int oldBottom) {
                    if (container.getParent() != null) {
                        container.removeOnLayoutChangeListener(this);
                        placeFragmentInViewHolder(holder);
                    }
                }
            });
        }
        gcFragments();
    }

The error has shown up before, but I think this is something new, since Android CL 2366713 directly updates attachment dynamics?

I don't know whether the original AndroidX was making bad assumptions on Android, or that Android CL 2366713 broke something? Looping in jgielzak@.

Note that this is impacting Chrome autoroll for AndroidX (Issue 1421341)

Hmm I cannot CC jgielzak@ (but I can still add comment?!). Pinging him directly instead.

It looks like this code in ViewPager2 is a workaround for the bug that's fixed in this change.

Specifically, it looks to be working around the fact that onBindViewHolder was previously called with temporarily detached views, and using this listener to wait for it to be properly attached so that it can perform operations that rely on it being properly attached.

The previous state was bad precisely because it forced hacky workarounds like this due to the lack of a proper way to listen for changes in temp-detached state (there are listeners, but they don't work, and would likely require changes in both RecyclerView and framework to make them work...it's a very flawed design). Until seeing this workaround, I wasn't aware it was even possible at all to figure it out (and even this has some limitations that probably aren't really relevant to VP2's use-case).

I think the correct solution here is to remove this workaround in ViewPager2 and release new RecyclerView and ViewPager2 versions in parallel to address this, with release notes advising users to upgrade. Jakub, what do you think?

(also, I think I vaguely recall hearing that it's possible to do some sort of Gradle module metadata thing where we can also add the information that VP2 should be at least a certain version if being used with a given RV version? I'm not sure if/how we could do that, though)

Thanks for the explanation!

I'm wondering if there will be versioning issues re. Android platform support vs. AndroidX? (NVM, this is all AndroidX).

The following release(s) address this bug.It is possible this bug has only been partially addressed:

• androidx.recyclerview:recyclerview:1.3.1-rc01

The following release(s) address this bug.It is possible this bug has only been partially addressed:

• androidx.recyclerview:recyclerview:1.4.0-alpha01