IssueTracker

Could be related to the changes you've made in FixedFragmentStateAdapter. Could you share the code, I can have a quick look.

> Could be related to the changes you've made in FixedFragmentStateAdapter. Could you share the code, I can have a quick look.

Sorry, but I believe that is not a culprit. I just added a single line of code to avoid BadParcelableException.

https://issuetracker.google.com/issues/132840199

```
    @Override
    public final void restoreState(@NonNull Parcelable savedState) {
        if (!mSavedStates.isEmpty() || !mFragments.isEmpty()) {
            throw new IllegalStateException(
                    "Expected the adapter to be 'fresh' while restoring state.");
        }

        Bundle bundle = (Bundle) savedState;
        bundle.setClassLoader(getClass().getClassLoader()); // <-- ADDED THIS LINE

        for (String key : bundle.keySet()) {
          ...
    }
```

I investigated the issue and turned out the part of the scenario which causes the problem;

1. RecyclerView.recycleViewHolderInternal()
2. hodler.doesTransientStatePreventRecycling() retuns true
3. FragmentStateAdapter.onFailedToRecycleView() -> onViewRecycled() -> removeFragment()
4. onFailedToRecycleView() retuns false, it means the ViewHolder is not recycled yet.
5. Soon after, FragmentStateAdapter.onViewAttachedToWindow() -> placeFragmentInViewHolder() ->  IllegalStateException("Design assumption violated.")

Hi! I have successfully created a demo project which can reproduce this issue. Please check the following repo and I hope it helps to resolve the issue.

https://github.com/h6ah4i/FragmentStateAdapterOnFailedToRecyclerViewFailure

[UPDATE]
This issue still exists on ViewPager2 v1.0.0-alpha06 with RecyclerView v1.1.0-beta01.

I can confirm I can repro this and will try to provide a workaround / fix soon.
Big thanks for the sample app, repro steps and research info. Super appreciated!

The easiest workaround would involve copying FragmentStateAdapter and FragmentViewHolder into your project, and in FragmentStateAdapter adding ensureFragment(holder.getAdapterPosition()) as the first line of placeFragmentInViewHolder method. Attached as a patch to your sample app.

We'll work on a proper fix. Might unblock you in the meantime though. Thanks again for reporting and a sample app.

Project: platform/frameworks/support
Branch: androidx-master-dev

commit a35a5180df0b98c4af4a4e119388f7d70d27ade4
Author: Jakub Gielzak <jgielzak@google.com>
Date:   Tue Jul 30 17:27:14 2019

    FragmentStateAdapter transient state fix follow-up
   
    Follow-up to Ieb166c6fb48907cbb7aa46ab87a3ae4cc29591a6:
    * Disabled tests on API < 16
    * Updated a comment
   
    Bug: 134246546
    Test: TransientStateFragmentTest
    Test: ./gradlew viewpager2:connectedCheck
   
    Change-Id: Id27c1267d4fc75c41849291322952fc1ea0e8d0d

M       viewpager2/src/androidTest/java/androidx/viewpager2/widget/TransientStateFragmentTest.kt
M       viewpager2/src/main/java/androidx/viewpager2/adapter/FragmentStateAdapter.java

https://android-review.googlesource.com/1093437
https://goto.google.com/android-sha1/a35a5180df0b98c4af4a4e119388f7d70d27ade4

The fix is now in place and should come out in the next release.