背景#
這週在寫商業化需求 (廣告) 時，為了方便復用，簡單改了下傳入的 Bean。第一次運行時沒有一切正常，第二次之後再運行時，提示空指針異常，異常的堆棧是 RecyclerView，異常信息 (當時沒有保存日誌，找了其他人相同異常的堆棧，原因是同一個)：
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean androidx.recyclerview.widget.RecyclerView$ViewHolder.shouldIgnore()' on a null object reference
at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep3(RecyclerView.java:3951)
at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3652)
at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4194)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at androidx.appcompat.widget.LinearLayoutCompat.setChildFrame(LinearLayoutCompat.java:1645)
at androidx.appcompat.widget.LinearLayoutCompat.layoutVertical(LinearLayoutCompat.java:1499)
at androidx.appcompat.widget.LinearLayoutCompat.onLayout(LinearLayoutCompat.java:1407)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at androidx.recyclerview.widget.RecyclerView$LayoutManager.layoutDecoratedWithMargins(RecyclerView.java:9322)
at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1615)
at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1517)
at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:612)
at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:3924)
at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3641)
at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4194)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at androidx.constraintlayout.widget.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at com.android.internal.policy.DecorView.onLayout(DecorView.java:758)
at android.view.View.layout(View.java:19590)
at android.view.ViewGroup.layout(ViewGroup.java:6053)
at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2484)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2200)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1386)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6733)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
at android.view.Choreographer.doCallbacks(Choreographer.java:723)
2018-10-20 19:24:30.408 20730-20730/com.wynk.basic E/AndroidRuntime: at android.view.Choreographer.doFrame(Choreographer.java:658)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
at android.os.Handler.handleCallback(Handler.java:789)
at android.os.Handler.dispatchMessage(Handler.java:98)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
下意識以為又是 Android Studio 抽風 (公司電腦經常的事)，清除快取 + 重啟 + 重裝一套組合拳下來問題還是沒有解決。終於意識到就是代碼有問題。
改動影響的範圍很小，很容易鎖定。異常對戰中出現 RecyclerView，而且之前沒有出現過，懷疑是寫法有問題。看寫法和之前又沒有什麼差別，開始迷茫懷疑人生....
找問題#
看不出來，找到了拋出異常的代碼ViewHolder.shouldIgnore():
// 只保留了關鍵的兩行代碼
ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i);
if (holder.shouldIgnore()) {
continue;
}
// getChildViewHolderInt
static ViewHolder getChildViewHolderInt(View child) {
if (child == null) {
return null;
}
return ((LayoutParams) child.getLayoutParams()).mViewHolder;
}
其實到這問題已經差不多了，child 為 null 或者 LayoutParams 中的 mViewHolder 為 null 才會有這種情況，代碼中又沒有移除子 View 的操作，只可能 mViewHolder 為 null，但... 沒看出來。
等到把異常信息RecyclerView$ViewHolder.shouldIgnore() NullPointerException甩到 Google 上找到了這，又看了這
回項目裡邊找代碼，果然，代碼裡為了重新計算 View 的寬高，把 ViewHolder#mItemView 的 LayoutParams 用一個新建的對象覆蓋了，類似這樣：
mBinding.getRoot().setLayoutParams(new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT));
LayoutParams 裡的 mViewHolder 肯定也就是 null 了。
解決#
原因就是替換了原來的 LayoutParams，而新的 LayoutParams 中 mViewHolder 又沒有賦值。用 mAdBinding.getRoot ().getLayoutParams () 再改屬性就可以了。像這樣：
ViewGroup.LayoutParams layoutParams = mBinding.getRoot().getLayoutParams();
layoutParams.height = ViewGroup.LayoutParams.WRAP_CONTENT;
layoutParams.width = ViewGroup.LayoutParams.MATCH_PARENT;