ViewPager2异常：java.lang.IllegalStateException：违反设计假设

Question

我自己无法复制这个问题，但我注意到这个问题是在小米米9运行Android10的时候出现的。以下是我使用的viewpager2和recyclerview版本：

androidx.recyclerview:recyclerview:1.1.0 
androidx.viewpager2:viewpager2:1.0.0

下面是StackTrace：

Fatal Exception: java.lang.IllegalStateException: Design assumption violated.
       at androidx.viewpager2.adapter.FragmentStateAdapter.placeFragmentInViewHolder(FragmentStateAdapter.java:287)
       at androidx.viewpager2.adapter.FragmentStateAdapter.onViewAttachedToWindow(FragmentStateAdapter.java:276)
       at androidx.viewpager2.adapter.FragmentStateAdapter.onViewAttachedToWindow(FragmentStateAdapter.java:67)
       at androidx.recyclerview.widget.RecyclerView.dispatchChildAttached(RecyclerView.java:7556)
       at androidx.recyclerview.widget.RecyclerView$5.addView(RecyclerView.java:860)
       at androidx.recyclerview.widget.ChildHelper.addView(ChildHelper.java:107)
       at androidx.recyclerview.widget.RecyclerView$LayoutManager.addViewInt(RecyclerView.java:8601)
       at androidx.recyclerview.widget.RecyclerView$LayoutManager.addView(RecyclerView.java:8559)
       at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1643)
       at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1587)
       at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:675)
       at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:4134)
       at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3851)
       at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4404)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at androidx.viewpager2.widget.ViewPager2.onLayout(ViewPager2.java:527)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at androidx.constraintlayout.widget.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
       at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
       at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
       at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
       at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
       at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
       at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
       at com.android.internal.policy.DecorView.onLayout(DecorView.java:803)
       at android.view.View.layout(View.java:21963)
       at android.view.ViewGroup.layout(ViewGroup.java:6317)
       at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:3144)
       at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2654)
       at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1782)
       at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7785)
       at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1031)
       at android.view.Choreographer.doCallbacks(Choreographer.java:854)
       at android.view.Choreographer.doFrame(Choreographer.java:789)
       at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1016)
       at android.os.Handler.handleCallback(Handler.java:883)
       at android.os.Handler.dispatchMessage(Handler.java:100)
       at android.os.Looper.loop(Looper.java:224)
       at android.app.ActivityThread.main(ActivityThread.java:7520)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

我不确定这是从我的代码中触发的，因为stacktrace不是很清楚。此外，根据Google Issuetracker，这个问题已经在viewpager2:1.0.0-beta03:https://Issuetracker.Google.com/issues/134246546中解决了，这里有提交，但在我的例子中没有解决。这是另一篇文章，也有同样的问题，但没有多少有用的答案：FragmentStateAdapter抛出java.lang.IllegalStateException：配置更改后违反了设计假设。如果有人知道为什么会发生这种情况，或者知道如何解决这种情况，那就太好了。

Answer 1

如果重写getItemid()，请确保也重写containsitem()。