一、错误日志

在使用androidx.viewpager2.adapter.FragmentStateAdapter时遇到了崩溃，报错日志如下：

java.lang.IllegalStateException: Design assumption violated.
	at androidx.viewpager2.adapter.FragmentStateAdapter.placeFragmentInViewHolder(Unknown Source:161)
	at androidx.viewpager2.adapter.FragmentStateAdapter.onViewAttachedToWindow(Unknown Source:0)
	at androidx.viewpager2.adapter.FragmentStateAdapter.onViewAttachedToWindow(Unknown Source:2)
	at androidx.recyclerview.widget.RecyclerView.dispatchChildAttached(Unknown Source:13)
	at androidx.recyclerview.widget.RecyclerView$5.addView(Unknown Source:7)
	at androidx.recyclerview.widget.ChildHelper.addView(Unknown Source:25)
	at androidx.recyclerview.widget.RecyclerView$LayoutManager.addViewInt(Unknown Source:123)
	at androidx.recyclerview.widget.RecyclerView$LayoutManager.addView(Unknown Source:1)
	at androidx.recyclerview.widget.RecyclerView$LayoutManager.addView(Unknown Source:1)
	at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(Unknown Source:34)
	at androidx.recyclerview.widget.LinearLayoutManager.fill(Unknown Source:38)
	at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(Unknown Source:360)
	at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(Unknown Source:66)
	at androidx.recyclerview.widget.RecyclerView.dispatchLayout(Unknown Source:71)
	at androidx.recyclerview.widget.RecyclerView.onLayout(Unknown Source:5)
	at android.view.View.layout(View.java:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	at androidx.viewpager2.widget.ViewPager2.onLayout(Unknown Source:70)
	at android.view.View.layout(View.java:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	at androidx.constraintlayout.widget.ConstraintLayout.onLayout(Unknown Source:70)
	at android.view.View.layout(View.java:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	at androidx.viewpager.widget.ViewPager.onLayout(Unknown Source:256)
	at android.view.View.layout(View.java:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	at androidx.constraintlayout.widget.ConstraintLayout.onLayout(Unknown Source:70)
	at android.view.View.layout(View.java:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
	at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
	at android.view.View.layout(View.java:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	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:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
	at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
	at android.view.View.layout(View.java:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	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:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	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:857)
	at android.view.View.layout(View.java:23561)
	at android.view.ViewGroup.layout(ViewGroup.java:6450)
	at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:3845)
	at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:3295)
	at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:2259)
	at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:8967)
	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1142)
	at android.view.Choreographer.doCallbacks(Choreographer.java:946)
	at android.view.Choreographer.doFrame(Choreographer.java:875)
	at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1127)
	at android.os.Handler.handleCallback(Handler.java:938)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	at android.os.Looper.loopOnce(Looper.java:210)
	at android.os.Looper.loop(Looper.java:299)
	at android.app.ActivityThread.main(ActivityThread.java:8085)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:556)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1045)
Back traces ends.

二、分析步骤

很难复现，一顿狂点，运气好的可能会复现，查看bugly拿到错误日志进行分析，确定方向是ViewPager2的FragmentStateAdapter引起的崩溃

查看FragmentStateAdapter的源码：

    /**
     * Default implementation works for collections that don't add, move, remove items.
     * <p>
     * TODO(b/122670460): add lint rule
     * When overriding, also override {@link #containsItem(long)}.
     * <p>
     * If the item is not a part of the collection, return {@link RecyclerView#NO_ID}.
     *
     * @param position Adapter position
     * @return stable item id {@link RecyclerView.Adapter#hasStableIds()}
     */
    @Override
    public long getItemId(int position) {
        return position;
    }

    /**
     * Default implementation works for collections that don't add, move, remove items.
     * <p>
     * TODO(b/122670460): add lint rule
     * When overriding, also override {@link #getItemId(int)}
     */
    public boolean containsItem(long itemId) {
        return itemId >= 0 && itemId < getItemCount();
    }

得知关键信息：getItemId、containsItem只要有一个重写了就要重写另一个

 When overriding, also override {@link #containsItem(long)}.

 When overriding, also override {@link #getItemId(int)}

寻找项目中使用到FragmentStateAdapter的地方，发现有的地方只重写了getItemId，并未对containsItem进行重写，在此处重写containsItem，问题解决

三、问题结论

FragmentStateAdapter是通过getItemId()来获取唯一标识来缓存fragment的，而默认的getItemId()返回的id是position，并不满足唯一性，实际开发中就有可能自定义一个标识来存储唯一id，

跟进源码得知重写getItemId的话就要重写containsItem

在使用ViewPager2+FragmentStateAdapter的时候，如果Fragment数据是需要动态添加的话，那么一定要重写getItemId和containsItem这两个方法。并且需要给每个Fragment定义一个唯一id，避免崩溃。