IssueTracker

The removal of the nested graphs and using popUpTo the given fragment appears to prevent the crash from happening in 1.0.0-alpha11.

Project: platform/prebuilts/androidx/internal
Branch: androidx-master-dev

commit 63bc1ff1fe7c348138295ee7efb8cdc2999622c0
Author: Ian Lake <ilake@google.com>
Date:   Sun Feb 03 15:18:53 2019

    Update prebuilts for Navigation 1.0.0-beta01
   
    Include fix for b/123803044
   
    Test: ./gradlew bOS
    Change-Id: Icb2a50975541a7bd3327054c72a591c7689c9741

M       android/arch/navigation/navigation-common/1.0.0-beta01/navigation-common-1.0.0-beta01.aar
M       android/arch/navigation/navigation-common/1.0.0-beta01/navigation-common-1.0.0-beta01.aar.md5
M       android/arch/navigation/navigation-common/1.0.0-beta01/navigation-common-1.0.0-beta01.aar.sha1
M       android/arch/navigation/navigation-fragment/1.0.0-beta01/navigation-fragment-1.0.0-beta01-sources.jar
M       android/arch/navigation/navigation-fragment/1.0.0-beta01/navigation-fragment-1.0.0-beta01-sources.jar.md5
M       android/arch/navigation/navigation-fragment/1.0.0-beta01/navigation-fragment-1.0.0-beta01-sources.jar.sha1
M       android/arch/navigation/navigation-fragment/1.0.0-beta01/navigation-fragment-1.0.0-beta01.aar
M       android/arch/navigation/navigation-fragment/1.0.0-beta01/navigation-fragment-1.0.0-beta01.aar.md5
M       android/arch/navigation/navigation-fragment/1.0.0-beta01/navigation-fragment-1.0.0-beta01.aar.sha1
M       android/arch/navigation/navigation-runtime/1.0.0-beta01/navigation-runtime-1.0.0-beta01.aar
M       android/arch/navigation/navigation-runtime/1.0.0-beta01/navigation-runtime-1.0.0-beta01.aar.md5
M       android/arch/navigation/navigation-runtime/1.0.0-beta01/navigation-runtime-1.0.0-beta01.aar.sha1
M       android/arch/navigation/navigation-ui/1.0.0-beta01/navigation-ui-1.0.0-beta01.aar
M       android/arch/navigation/navigation-ui/1.0.0-beta01/navigation-ui-1.0.0-beta01.aar.md5
M       android/arch/navigation/navigation-ui/1.0.0-beta01/navigation-ui-1.0.0-beta01.aar.sha1

https://android-review.googlesource.com/892878
https://goto.google.com/android-sha1/63bc1ff1fe7c348138295ee7efb8cdc2999622c0

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

commit c775e5d1ae2ab4b194ee3c0f18523c603cf4d6ec
Author: Ian Lake <ilake@google.com>
Date:   Sun Feb 03 15:04:21 2019

    Return true when popping from FragmentNavigator
   
    Since NavController only updates its back stack when
    a Navigator returns true from popBackStack(), ensure
    that all updates to the FragmentNavigator's back stack
    (i.e., calling removeLast()) are paired with returning
    true from popBackStack().
   
    This ensures that the FragmentManager state and the
    NavController states don't get out of sync.
   
    Test: Updated FragmentNavigatorTest test
    Test: tested in sample app from bug
    BUG: 123803044
    Change-Id: I8cfc87cba0a0369b5ae5c3ac610e31b5556ba04e

M       navigation/fragment/src/androidTest/java/androidx/navigation/fragment/FragmentNavigatorTest.kt
M       navigation/fragment/src/main/java/androidx/navigation/fragment/FragmentNavigator.java

https://android-review.googlesource.com/892996
https://goto.google.com/android-sha1/c775e5d1ae2ab4b194ee3c0f18523c603cf4d6ec

This is actually an issue where the NavController's back stack state is going out of sync with the FragmentManager / FragmentNavigator's state. This has been fixed and will be available in the next release.

Nice job! A quick response!

I still get this error using the lastest 1.0.0 release :

    implementation 'android.arch.navigation:navigation-fragment-ktx:1.0.0'
    implementation "android.arch.navigation:navigation-ui-ktx:1.0.0"

I'm using nested graphs.

Re #7 - please file a new bug with a sample app that reproduces your issue.

Done.
https://issuetracker.google.com/131733658

I'd rather like to keep the "addToBackStack(..)" to rewind through fragment transactions. Will definitely look further into it also from the above mentioned logs.

Thanks for clarifying, I will investigate further.

Following on the logs above, I suspect that each instance of FragmentStateAdapter gets access to that one BaseFragment in FragmentStateAdapter::restoreState (restore state called on each stack entry).
Making sure each ViewPagerFragment instance only gets access to its own state (so the state is not shared between them) should eliminate this issue.

Not sure where the problem lies yet, whether in ViewPager2's interaction with the state saving stack, or in the sample app, so continuing to investigate.

Yeah.. I have no clue what's going on, spending too much time debugging. I am planning to attach LifecycleObserver to my ViewHolder's so they act like Fragments.

From the evidence above, I am pretty sure it should not affect ViewHolder pattern but never know. Would love to know what was happening though if you come across a solution.

Looks like the following is happening in the sample:
- the same instance of ViewPagerFragment is being used for various back-stack entries
- multiple instances of FragmentStateAdapter are being created at each ViewPagerFragment view inflation, and each has their post-grace-period gcFragments() scheduled
- the same page Fragment instance reference is passed to all FragmentStateAdapter instances via restoreState, but can only be bound to one ViewHolder (the current visible one)
- once grace-period ends in non-visible FragmentStateAdapter instances, they see nothing is bound to their ViewHolders and garbage-collect the page Fragment

I will look further into the correct solution, potentially cancelling gcFragments() if a FragmentStateAdapter gets detached from a ViewPager2 (e.g. at ViewPagerFragment view destruction).
This would require a code, change, though, so for now, you can probably work around this by not re-using ViewPagerFragment instances, but creating a new one each time you add it to the stack. There might be a need for passing relevant state, e.g. current page.

Raw debugging notes below in case helpful. I reproduced the issue by switching multiple times between buttons 1 and 2.

***

# how many adapter instances exist

️$ cat ff2  | grep -E "FragmentStateAdapter@[^\}]+" -o | sort -u
FragmentStateAdapter@5,446
FragmentStateAdapter@5,474
FragmentStateAdapter@5,484

# how many adapter instances get their state restored

️$ cat ff2 | grep restoreState | grep -E "FragmentStateAdapter@[^\}]+" -o | sort -u
FragmentStateAdapter@5,474
FragmentStateAdapter@5,484

# where are Fragment pages created

️$ cat ff2 | grep createFragment -C1
{androidx.viewpager2.adapter.FragmentStateAdapter@5,446}.mFragments will be accessed at androidx.viewpager2.adapter.FragmentStateAdapter.ensureFragment(FragmentStateAdapter.java:248)
Breakpoint reached at com.test.myapplication.VPAdapter.createFragment(ViewPagerFragment.kt:161)
{androidx.viewpager2.adapter.FragmentStateAdapter@5,446}.mFragments will be accessed at androidx.viewpager2.adapter.FragmentStateAdapter.ensureFragment(FragmentStateAdapter.java:252)

# confirming that instance 446 is the only adapter creating the views; others are getting it from previously restored state
# for reference, FragmentStateAdapter.java:248 = "if (!mFragments.containsKey(itemId)) {"
# for reference, FragmentStateAdapter.java:252 = "mFragments.put(itemId, newFragment);"

️$ cat ff2 | grep ensureFragment | sort
{androidx.viewpager2.adapter.FragmentStateAdapter@5,446}.mFragments will be accessed at androidx.viewpager2.adapter.FragmentStateAdapter.ensureFragment(FragmentStateAdapter.java:248)
{androidx.viewpager2.adapter.FragmentStateAdapter@5,446}.mFragments will be accessed at androidx.viewpager2.adapter.FragmentStateAdapter.ensureFragment(FragmentStateAdapter.java:252)
{androidx.viewpager2.adapter.FragmentStateAdapter@5,474}.mFragments will be accessed at androidx.viewpager2.adapter.FragmentStateAdapter.ensureFragment(FragmentStateAdapter.java:248)
{androidx.viewpager2.adapter.FragmentStateAdapter@5,484}.mFragments will be accessed at androidx.viewpager2.adapter.FragmentStateAdapter.ensureFragment(FragmentStateAdapter.java:248)

# where is the state with a page Fragment reference coming from

Breakpoint reached
          at androidx.viewpager2.adapter.FragmentStateAdapter.restoreState(FragmentStateAdapter.java:515)
          at androidx.viewpager2.widget.ViewPager2.restorePendingState(ViewPager2.java:337)
          at androidx.viewpager2.widget.ViewPager2.dispatchRestoreInstanceState(ViewPager2.java:362)
          at android.view.View.restoreHierarchyState(View.java:17684)
          at androidx.fragment.app.Fragment.restoreViewState(Fragment.java:539)
          at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:907)
          at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
          at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
          at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:439)
          at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2076)
          at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1866)
          at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1821)
          at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
          at androidx.fragment.app.FragmentManagerImpl$2.run(FragmentManagerImpl.java:150)
          at android.os.Handler.handleCallback(Handler.java:790)
          at android.os.Handler.dispatchMessage(Handler.java:99)
          at android.os.Looper.loop(Looper.java:164)
          at android.app.ActivityThread.main(ActivityThread.java:6494)
          at java.lang.reflect.Method.invoke(Method.java:-1)
          at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

# how many Fragment instances go through restoreViewState

️$ cat ff4 | grep "restoreViewState. fragment="
restoreViewState: fragment=ViewPagerFragment{d4490de (a6cfe7d3-5376-4015-8357-2408442116ee) id=0x7f070076 Frag1}
restoreViewState: fragment=BaseFragment{de52066 (f5afa133-40d7-45c1-8193-6ce2d4cb5489) f0}
restoreViewState: fragment=BaseFragment{de52066 (f5afa133-40d7-45c1-8193-6ce2d4cb5489) f0}
restoreViewState: fragment=ViewPagerFragment{d4490de (a6cfe7d3-5376-4015-8357-2408442116ee) id=0x7f070076 Frag1}
restoreViewState: fragment=BaseFragment{de52066 (f5afa133-40d7-45c1-8193-6ce2d4cb5489) f0}
restoreViewState: fragment=ViewPagerFragment{d4490de (a6cfe7d3-5376-4015-8357-2408442116ee) id=0x7f070076 Frag1}

# where are ViewPagerFragment instances created in the sample
    ```
    var fragment: Fragment? = supportFragmentManager.findFragmentByTag("Frag$fragmentToOpen")
    if(fragment == null) {
        fragment = when(fragmentToOpen) {
            1 -> ViewPagerFragment()
            2 -> BlankFragment()
            else -> throw IllegalArgumentException()
        }
    }
    ```

Thanks for the suggestion above, creating a new Fragment every time definitely fixes the problem albeit some very ugly code implementation and hacks I had to write to do some state saving in my code (since ViewModels are attached to a specific Fragment instance), my subsequent fragments also need to do some ugly state saving (playing media).

Looking forward to erasing all of this ugly code on next update!

For anybody who has the same exact problem, page state saving works for me in the following manner:

        binding.gamePagerRoot.apply {
            // Set the Adapter for our Fragment
            adapter = gamePagerAdapter

            // Do not allow user touch for this ViewPager2 to affect its scroll, should only be scrolled programmatically
            isUserInputEnabled = false

            // Set the Page Transformation
            setPageTransformer(genStackedPageTransformer2())

            // Listen to page change calbacks
            registerOnPageChangeCallback(onPageChangeCallback)

            // Customize the RecyclerView child
            val recyclerView = getChildAt(0) as RecyclerView
            // Remove the overscroll shaddow
            recyclerView.overScrollMode = RecyclerView.OVER_SCROLL_NEVER
            // Allow nested scrolling
            recyclerView.isNestedScrollingEnabled = true

            // If we are on a different page as specified by our ViewModel, make sure to scroll to the page instantly (no animation)
            if(gamePagerVM?.gamePageIds?.size ?: 1 > 1) {
                recyclerView.viewTreeObserver.addOnGlobalLayoutListener(object: ViewTreeObserver.OnGlobalLayoutListener{
                    override fun onGlobalLayout() {
                        if(recyclerView.childCount >= gamePagerVM?.currPage ?: 0) {
                            viewTreeObserver.removeOnGlobalLayoutListener(this)
                            // If the current page is not 0, set the current page
                            setCurrentItem(gamePagerVM?.currPage ?: 0, false)
                        }
                    }
                })
            }
        }

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

commit 7166ba571d594baa9016e02c3b5fe967da0b8ec0
Author: Jakub Gielzak <jgielzak@google.com>
Date:   Tue Aug 27 16:20:54 2019

    Fix for FragmentStateAdapter#gcFragments()
   
    Fix for an issue with FragmentStateAdapter garbage collection mechanism.
   
    The bug manifested itself in the following scenario:
    - ViewPager2 host Fragment view recreated 3+ times in a window < 10s,
    - Host Fragment instances added to the back stack,
    - ViewPager2 adapter set to null in onViewDestoyed (too late as view
      state already saved, but affecting PageFragment book-keeping).
   
    The fix adds another criteria to gcFragments() in case a
    PageFragment instance gets taken over by a later created instance of
    FragmentStateAdapter.
   
    Bug: 139095195
    Test: ./gradlew viewpager2:connectedCheck
   
    Change-Id: If42bc4e65e8fe52cc0dcd9bbf67a0c55d3c535b1

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

https://android-review.googlesource.com/1108545
https://goto.google.com/android-sha1/7166ba571d594baa9016e02c3b5fe967da0b8ec0

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

commit 39daf73f75d04b1ba52b007781355595430f8dab
Author: Jakub Gielzak <jgielzak@google.com>
Date:   Tue Aug 27 16:20:16 2019

    Made test FragmentAdapter more flexible.
   
    Needed for the next CL.
   
    Bug: 139095195
    Test: ./gradlew viewpager2:connectedCheck
   
    Change-Id: I3aa4b8a131d3d7bca248f94641c5ee643c6f1819

M       viewpager2/src/androidTest/java/androidx/viewpager2/widget/BaseTest.kt
M       viewpager2/src/androidTest/java/androidx/viewpager2/widget/swipe/FragmentAdapter.kt

https://android-review.googlesource.com/1108544
https://goto.google.com/android-sha1/39daf73f75d04b1ba52b007781355595430f8dab

beta04 with a fix for this was released today